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Messages - A-Team

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I looked further into the Jakobshavn Isbrae upchannel P3 flight track of 2014. The primary purpose may have been to measure ice surface for purposes of determining thinning relative to previous years; the particular radar configuration did not seem to detect the bedrock layer (or at least that was not 'picked' as it was in other series).

The data comes in the form of an overview map at three levels of resolution, the ice-penetrating radar scan itself with and without a dashed purple surface line, an excel-type file that has elevations for the surface (and mysteriously for bedrock) and so a column for total ice thickness, and a kml file that overlays the track on Google Earth satellite imagery of Nov 2010.

I sought to tile consecutive segments of the flight path. Each segment covers 50 km so only two were needed to get 100 km up the channel (in curved channel coordinates). I soon discovered the radar scans did not use the same vertical scale. Neither did the flight line depictions, which ill-advisedly used a poor quality satellite image from the 1990's at too low a scale.

The image below has rescaled all this to match plus patched in yesterday's Landsat under the flight path and brought way down to 700 px width to fit on this page. The correspondence is not perfect because of the difficulty in stretching the satellite imagery out to linearize the track -- though this can be done in gimp using a piecewise linear cage transform using the 1 km mileposts I marked out in an earlier image.

The alternative is to use the lat,lon scale (which is not flat, as a coord system for the globe) but as noted above Landsat imagery uses mercator UTM zone 22 W meters which conveniently scale to 15 or 30 m Landsat pixels.

The ice-penetrating radar shows some deeper structures just beyond the calving front and stratification layers below the ice surface way upchannel that probably continue to the summit ridge (and probably improve along the way). I have not yet located the flight scenes that would allow tiling to the summit.


I got this from an open access article in J. Geod. Sci. 2014; 4:8–18 DOI 10.2478/jogs-2014-0001
M. Herceg, C. C. Tscherning, and J. F. Levinsen
Sensitivity of Goce gradients on Greenland mass variation and changes in ice topography

"...Establishing the presence of an acceleration on the order of magnitude found in the Greenland Ice Sheet requires more than 5 years of data, and we find that the GRACE time series available are now long enough to establish the presence of such an acceleration. Maximally four years of reprocessed GOCE gradient observations will to be available by mid-2014, which may add the supplement GRACE derived solution for mass changes..."

"...The largest accelerations have been observed by the Helheim glacier, Kangerdlugssuaq and Jakobshavn Isbræ"

"...There is hope for future GOCE data acquisitions to be able to further our understanding of mass changes using gravity gradients and gradient anomalies. We expect maximally four years of GOCE gradient observations to be available at the middle of 2014. Furthermore, lowering of the GOCE satellite down to an orbit of 235 km will increase the accuracy and spatial resolution of the measurements. This may then lead to the accuracy needed in order to ob- serve ice mass changes using the GOCE gravity gradients."

"A new GOCE-type mission, with improved accuracy, will undoubtedly provide the possibility to detect areas of mass gain or losses over short time periods."

Sounds like a 'new' mission would indeed be needed if the old one went kaput before mission accomplished.

Yes and no. LC80 84 232 2014198LGN00 normally contains an oblique slice of about the SW third of the calving front, not enough really. It's obscured by clouds anyway. It looks to me like the next Landsats are not due until July 24 and July 26th (two). Modis has some clear recent days and is not showing anything dramatic.

It looks like NASA/USGS had some colossal processing pipeline error, possibly involving many terabytes of imagery that had to be re-worked, hence the weeklong snafu.

One of the images today from 11 July had to be rotated 5.6º, probably a sign that it is still in stereographic pj rather than UTM W zone 22. I will write them to get this fixed.

From the metadata files, it appears that path 8, row 11 and path 8, row 12 are taken a mere 24 seconds apart (center of scene time). From the lat,lon distance calculation on the nadirs, the satellite had moved 162 km over the ground. It is 709 km up. A hasty calculation,  2DEGREES(ASIN(82/709)) in the flat earth approximation gives an angular separation of 13.3º for stereo viewing. However there's very little relief on that landscape.


Three new Landsat images were just posted. I'm not sure what caused the five day holdup on the 12 July pair nor for that matter why the 17 July arrived on 17 July. It takes a while to download these giant packages and process them into 8 bit but I have high hopes that something interesting can be done with the near-simultaneous stereo pair. If not, ponder why annual net calving front retreat ~1 km is slightly less than annual glacial advance ~12 km.

Scene   #Day   Day   Mon   Year   Path   Row   Clarity

LC80842322014198LGN00   198   17   Jul   14   84   232   dark + cutoff
LC80080122014193LGN01   193   12   Jul   14   8   12   excellent
LC80080112014193LGN01   193   12   Jul   14   8   11   excellent

The EarthExplorer interface to Landsat-8 images presents its preview images in polar stereographic coordinates north of 63º latitude (ie for Jakobshavn Isbrae). However the main GeoTiff download package has converted the images into Universal Transverse Mercator projection using zone 22 W and WGS 84 for earth shape and cubic convolution.

In the meta data file, the four corners of the image are provided in both systems for example:

CORNER_UL_LAT_PRODUCT......................70.81070......(latitude of upper left corner)
CORNER_LR_PROJECTION_X_PRODUCT......739800.000...(easting of lower right corner)

Note the resolution of the images in Band 8 panchromatic is 15 m yet the northing and easting UTM data is provided to the nearest millimeter. This is puzzling -- a factor of 15000 too much precision -- when the dimensions of each pixel represents 15 x 15 m on the ground.

Fixed points on the rocks to the immediate SW of the calving front are important in co-registering data that often arrives without a scale. For this, a grid of latitude and longitude points can be very precisely placed on the high resolution Digiglobe image used at Google Earth via the coordinate converting utilitiy

Note incremental bumps in lat,lon do not correspond to integral pixel value differences whereas this holds for UTM coordinates. It is more convenient to resample the 15 m imagery to 10 m as then 100 pixels amounts to 1 km (rather than 66.6667. Landsat-8 images can be usefully upsampled to 7.5 m in the Jakobshavn calving region.

Anyone see any interferometric SAR images on the internet for Jakobshavn? Or for that matter any ESA Sentinel-1 radar images besides the one we have for Epiq?

They have fringes like the one for Petermann glacier below. To some extent on Jakobshavn, the neighboring ice sheet has been entrained by the main high velocity icestream. The crevasse fields (second image, wavelet transform) indicate strain to the extent they depart from gravitational flow (orthogonality to DEM contours). Note SAR velocity fields are projected onto the x,y plane whereas actual movement includes a vertical (z) component.

The Grace satellite pair is way beyond its design lifetime and had a power outage for a chunk of last summer. Some say they are flying too high for ideal resolution at the scale of Greenland but that Goce can fix that (after a couple more years of data).

Those altimeter measurements are useful for sure. But what exactly are they measuring, surface height of the snow? That would be problematic for mass balance, because the density profile of snow through firn to ice needs to be known. This density could vary significantly depending on depositional and compaction history and water content from melt or rain-on-snow events. Modelling it however might reduce the error to insignificance.

Maps like the one above go beyond determination of overall Greenland mass balance loss -- they seek
to map where that is occurring. That is not needed for sea level rise contribution.

Errors in localization have recently been refined by replacing Grace spherical harmonics expansion to order 60 with Slepian functions. I reviewed this somewhere on the ASIF last year, proposing cylindrical coords for Greenland plus  cylindrical Slepians, the counterpart to Bessel functions, to maybe refine mapping resolution.

That Dec 2012 PNAS paper has free access. I have not had a chance to follow up on the 12 subsequent articles that cite it. In terms of extrapolating mass loss forward in time, that could be done by simple-minded trending or by separately modelling identified main contributors such as Jakobshavn and 3-4 others.

Isostatic rebound comes up on the forum from time to time; I came across this recent account specific to the south branch of Jakobshavn Isbrae. The figure shows glacial thinning (Hoehenrate) is on the order of -5 m per year whereas mantle rebound is less than 20 mm per year, centered of course right under the area of mass balance loss. That's a factor of 250.

Upper left shows glacial velocity near the calving front in 2010 was markedly slower towards the sides but pushing 50 m/day towards the center. Upper right shows the region lifted up by tides; at that time, the glacier had a floating tongue.

I also found some excellent videos of the fjord and calving front -- take the helicopter tour to save yourself a bucketful of euros:

August, 2012 Jakobshavn calving event (different  from Chasing Ice)

‪Helikoptertur fra Sermeq Kujalleq til Illulissat‬

‪Jakobshavn Glacier POV Helicopter Ride‬

Here is the 15 m panchromatic Landsat image with an accurate grid overlay on 11 Jul 14 (LC80812332014192LGN00_B8, available at EarthExplorer). The Jakobshavn forum explains how fixed reference rocks and very high resolution imagery are used to anchor and scale the grid.

For digital latitude, longitude over a small area it is more convenient to use milli-degrees (mD). In those terms, kittycorner latitude changes are 10 mD while those of longitude are 20 mD. By counting over from the labelled reference cell, the coordinates of other points are easily determined without obscuring the image with numberings.

Here is the latitude, longitude grid for the area at 30 m resolution in Landsat-8 projection. This is necessary in order to accurately geo-locate ice penetrating radar transects and up-channel flight tracks. I'll post the 15 m channel-only coords over on the Landsat/bedrock forum.

There is plenty of opportunity for error in doing this so the first image outlines how this is done -- pick two fixed rocks whose coordinates can be accurately determined from google earth's very high resolution digiglobe photo. These rocks must also have good vertical and horizontal separation.

Next, choose a grid cell that has easy-to-remember digital lat,lon coordinates and grid spacing favorable to the overall map scale. Note the latitude increments by 0.01 abut the longitude by 0.02. These can be converted into x,y pixel coordinates by considerations of proportionality. The grid itself is drawn in gimp using a deeply buried command, Filters -> Render -> Pattern -> Grid.

Next up: geo-locate some Cresis stratification profiles on our preferred surface imagery and bedrock DEM.

Not yet, Espen. I'm hoping to get more people engaged in these files now that the football tournament is over (though there is always some other sport to replace it).

I'll post a couple rounds of Cresis profile analysis specific to Jakobshavn Isbrae here but if there's interest, start a separate forum section for ice-penetrating radar, a huge deal in Greenland and Antarctic and critical to Arctic Ocean ice thickness measurement validation (ie volume).

These profiles, in my view, are seriously under-interpreted and in some cases like Petermann Glacier, demonstrably mis-interpreted. The intermediate stratifications and their deformation are exceedingly important to ice sheet history and flow properties; it is not enough simply to read off surface elevations and bedrock depths if the goal is estimating future sea level rise attributable to Greenland by prioritizing to the fastest marine outlet glaciers.

After visiting numerous dead '404' urls, I did eventually find a 'FAQ' among the read-me Cresis files. It did not have any examples of annotated profile features but did explain their rather redundant data storage system and contained some interesting factoids about ice-penetrating radar.


The first thing to understand is Cresis uses MatLab formats but you probably do not: $2250 per individual license, $149 for home. The home use license does not include government, academic, commercial, or other organizational use (blog?).

There'd be a learning curve: "MATLAB is a high-level language and interactive environment for numerical computation, visualization, and programming. Using MATLAB, you can analyze data, develop algorithms, and create models and applications." <url></url>

However, the data is also available as desktop-readable triples of track location maps (file names ending in _0maps.jpg), radar return profiles (_1echo.jpg), and interpreted tracks (_2echo_picks.jpg). The latter are 'manually driven processes' where a trained individual marks up surface and bottom reflections with purple and red lines respectively. (However bedrock cannot always be located for the Jakobshavn gorge.)

Those depths are captured into excel-readable cvs numerical format. Of the 9 columns, 5 are useful (lat, lon, surface, bottom, elevation), 1 is easily derived (thick = surface - bottom), and 4 can be deleted. Cresis data is carried to excessive precision -- surely the bedrock is not really measured to centimeter accuracy as numbers like 2087.12 suggest. And surely latitude is not usefully measured to 6 decimal points (0.11 m) when profiles show 5 m precision (see below and <url></url>

The 15 years of flight have generated a lot of files. After drilling down to Greenland, look for the Jakobshavn Isbrae specific folders such as 09_01 (transects) and 09_02 (tracks along the icestream) for the given date, here Apr 14. Each profile covers 50 km, so just two profiles suffice for the Jakobshavn gorge.

The internal dimension of profile jpegs is 931 x 734 pixels (in the example examined). The width is variable but for a 49.91 km transect, the 1 x 734 vertical slices are spaced at 53.6 meters which represents a resolution of 75 points to characterize bedrock topography for a 4 km wide gorge. A buried nunatak or pothole of smaller dimension might still be recognized by combining data from different years as the bedrock doesn't change year to year and the flight lines would be slightly different.

The depth is not measured directly but rather return time of a radar pulse (microseconds of propagation delay). Under the assumption of 3.15 dielectric  for ice -- not applicable to snow, firn, englacial pockets, or wet temperate ice -- the depth is then calculated using the speed of electromagnetic radiation in a medium of refractive index sq rt (3.15), or 168,913,914 m/s instead of the usual 299,792,458 m/s.

The depth scale range has to vary from scene to scene to accommodate top elevation and bedrock depth. In the example I looked at, 2000 vertical meters was represented by 369 pixels for a resolution of 5.4 meters. This scale seems to be consistent, only modified by offsets.

Cresis also offers kml (Google Earth) files for the track segments. I found these convenient for precisely co-registering track profiles via their lat,lon coordinates to Landsat images. Simply mouse along the track to find the exact lat,lon of a radar reflection column. This is otherwise problematic because the icestream channel curves quite a bit. I found a way to lay down a precise grid in Gimp and will post that shortly.

For the two along-gorge, west to east flight lines of April 2014 (which may overlap slightly rather than butt up end-to-front), go to, open 2014_Greenland_P3/images_csarp-combined/ and append:



For the seven cross-gorge transects (some require a north/south pair) in west to east order of April 2014, append:

20140409_01/20140409_01_022_0maps.jpg N
20140409_01/20140409_01_021_0maps.jpg S
20140409_01/20140409_01_017_0maps.jpg N
20140409_01/20140409_01_016_0maps.jpg S
20140409_01/20140409_01_012_0maps.jpg N
20140409_01/20140409_01_011_0maps.jpg S

The data go back quite a few years (to 1993 for Jakobshavn) and involve other aircraft and other radars. It is not trivial to commingle older with newer data but there may be some value to it.

I've attached the 2014 flight line that goes under the calving front -- note the intriguing structure approximately at the first sill.

"It doesn't get any better than this" --  the guys are running low at beer camp but just then the swedish bikini team drops by.

Oh yes it does, a lot better. Despite an incomprehensible file navigation system, I eventually stumbled across the Cresis folder containing the P3 radar echoes overflights of Jakobshavn Isbrae for 04 Apr 13. And, make my day, they allowed anonymous guest/password ftp download (201 files).

The map in the upper right shows the flight lines: 6-7 transects plus one straight up the channel! These flight lines are also provided as a satellite image overlay.

The flight lines are slightly kattywumpus (not perpendicular to flow lines) so rotation and rescaling of the sat photo is necessary to see what the echogram refers to on the ground (though it also gives coords). The dotted orange rectangle shows this correspondence more or less worked out for the one I tried.

Cresis provides both a raw radar return and an annotated version called echo_picks where someone competent to interpret the data has colored in certain boundary lines. (Actually there is phenomenal software that can do this; I took a pass at this myself with PovRay relief, bottom.)

The example below has two such lines (purple and red) plus an intermediate line that for some unexplained reason is not deemed of interest. I'm guessing that the lines show the ice surface (omitting snow cover), water pockets, a Wisconsonan/Holocene transition, temperate ice, bedrock and side scatter from hills and walls.

The stratifications are rather minimal compared to the deformation drama up at Petermann. However the radar return is measuring dielectric permittivity which can amount to water content which can amount to critical rheological properties of intermediate ice filling the Jabobshavn channel.

So if you came for the bedrock, stay for the stratifications. They provide the natural coordinate system for any serious modeling of this glacier's behavior.

Continuing, if that slice of ice took 100 years to move 100 km, that represents 100*365*24*60*60 = 3.2 x 10^9 seconds, the divisor necessary to convert the 6.3 x 10^13 joules above to watts (energy to power, or rate of energy delivered), here 20 kw. Not a lot of light bulbs shining on a slice of ice thick enough to fill the Grand Canyon.

Had all the potential energy been converted to kinetic, then mgh =  0.5 mv^2 implies a glacier velocity today of v = sq rt (2gh)= 1.4*9.8*3200, a very respectable 0.177 km/sec (396 miles/hr) compared to the maximal velocity observed at the calving front of 17 km/year (0.0005 m/s), a ratio of 354:1.

The earth's geothermal gradient (from radioactive decay) provides another very substantial source of energy, especially given the steepness of this gradient in view of overdeepening to a depth of 1500 m below sea level by a hundred thousand years of previous glacier grinding of bedrock. (The actual history of the Jakobshavn channel has not been reconstructed outside o Holocene moraine dating).

The time-invariant geothermal gradient and fixed boundary condition provided by the ice above set the stage for a steady-state solution of the heat equation, further constrained by the observed temperature profile in drill holes (none of these reached bedrock in the channel itself).

The drainage of this glacier acts a solar collector. Sunlight warming the ice surface has little direct influence on deep rheology; it is drainage of melt lakes to subglacial channels that puts this energy where it has, arguably, potential to create more 'temperate' ice lubricating the bed and so speed up glacier deformation and discharge.

Two AGU posters take a more serious look at heating issues. Dating from 2010-11, these have not yet emerged (for unknown reasons) as peer-reviewed journal papers.

The latter poster asks if softening ice at the channel side walls influences glacier speed, concluding that ice shear could be enough to raise to raise temperatures by 9º C. The stress field is deduced from surface velocity transects and assumption of a weak bed unable to absorb basal shear.

While ice rheology is indeed significantly affected by temperature, the putative 9º C change needs evaluation in a full-fledged model to see if it is enough to account (or even over-account) for observed icestream velocity changes. The speedup is sudden and recent suggesting a threshold being crossed, yet marginal wall warming effects are gradual and continuous.

Rather than a threshold, a run-away positive feedback could provide an explanation. The first poster considers bed friction leading to more temperate ice (at the melting point) on the bed, leading to more deformation, more speedup and yet more temperate ice. The poster concludes that while this is definitely going on, it can only account for 1% of observed speedup.

If the kinetic energy of the glacier seem low, as estimated above, how much 'should' there have been? Here mgh is the gravitational potential energy that would have been acquired by the icestream during its 'fall' from interior to coast.

Here m, the mass of the glacier is a very large number (enough islandwide to displace the viscoelastic mantle), g the gravitational constant at the surface of the earth is not small at 9.8 m/sec^2, and the height h is substantial for a 3.2% grade over 100 km (3200 meters).

For comparison the Grand Canyon, a serious whitewater river, has an average gradient of 7.0% over 386 km This implies Jakobshavn Isbrae would be quite a fast-flowing river were it liquid water.

Recalling the definition of joule, under mgh a cubic meter (917 kg) of ice falling a km amounts to 917 x 9.8 x 1000 = 9x10^6 joules. Thus a meter-wide slice of ice 1400 m thick and 5000 m across making its way downstream to the calving front acquires (9x10^6)(7 x 10^6) = 6.3 x 10^13 joules.

However the speed of glacier is so slow in meters per second that very little of this arrives in the form of kinetic energy. The slice has lost almost all this energy in friction with the bedrock and walls (and internally, as deformation).

The lost energy could go into heating ice or cold rock at their interface (recalling the reverse slope on much of the channel) with the long time frame allowing (per heat equation) for ample dissipation and conductive loss.

There is very likely some liquid water at the bottom despite a large latent heat of transition. However this may originate from surface meltwater. Pressures are enormous so the phase diagram needs to be considered, along with freeze-up and the existence of an ice-water state distinct from either ice or water seen in drill cores.

In summary, potential energy considerations alone will not go very far towards understanding the state of the ice and how that might account for glacier motion (and especially recent speedup). For that we are largely dependent on the interpretation of ice-penetrating radar profiles and modeling of surface observables.

Here is the shadow situation on the north branch. It sheds light on the melt lake/tarn/nunatak feature we discussed previously.

Most unexpectedly, LC80 81 233 2014 192 LGN00_B8 arrived today at EarthExplorer -- and it is a crisp cloud-free beauty. We have not seen anything from satellite path 81 before.

Perhaps the reason is Jakobshavn is usually in the dark at the time of flyover; this photo was taken close to midnight (23:53:59) with the sun only 8.4º above the horizon at an azimuth of -46.8º (ccw from due north).

The illumination gives fantastic shadow relief to an otherwise flat landscape. In fact, with a cosine or two, you could figure out surface heights at the edge of the north channel (which is really an icefall at this point; its depression is more like a tarn than a melt lake).

There has been some calving along the NE edge of the front and it looks like this will continue. I won't belabor the comparison to earlier dates since we just did that yesterday for the 10 July and earlier images. To spare people the gigabyte download and 16 bit processing, I have attached the calving front from the 15 m panchromatic.

The two animations below illustrate (imperfectly) certain aspects of surface ice flow on Jakobshavn Isbrae.

The first imagines lines of red and yellow traffic cones laid out in 20 lines perpendicular to main channel by an unpaid intern. These lines progress over a calendar year towards the calving front in proportion to the velocity of their spot.

The velocity increases monotonically closer to the calving front, so from the perspective of a line (lagrangian coords), the faster ones in front are getting farther away whereas the slower ones upstream are falling farther back. The animation does capture that but not very noticeably.

The second animation is similar but also displays the effect of slow-down along the edges. This has the effect of making an initial line of ice more and more convex over time because its center moves more rapidly. The motion illustrated is only heuristic -- not tied quantitatively to the actual data of Joachin et al -- though I did bake in some acceleration using milliseconds of frame delay.

Neither animation captures the seasonality of motion nor the year-to-year acceleration of this motion. While the glacier is fast throughout the year, it moves even faster during the summer months. It would be possible to display the motion accurately but not by the methods I have been using.

Arctic sea ice / Re: Latest PIOMAS update (July)
« on: July 11, 2014, 07:55:12 PM »
Wipneus, I notice sometimes your palettes are measured gray scale steps with a hue overtint. Other times, not. In the first case, I can drop them into PovRay freeware for a 3D perspective view (for what that is worth). Script:

include ""
    location <-15,20,-31>
    look_at 0
    angle 35
  light_source{ <1000,1000,-1000> White }
  height_field {
    png "t2gray.png"
    pigment { White }
    translate <-.5, -.5, -.5>
    scale <17, 2, 17>
                     gradient y
                     color_map {
   [ 0.00000 rgb < 1.00000, 0.01111, 0.05829> ]
   [ 0.05000 rgb < 0.99728, 0.68188, 0.62282> ]
   [ 0.10000 rgb < 0.99457, 0.43569, 0.12356> ]
   [ 0.20000 rgb < 0.99457, 0.65239, 0.12521> ]
   [ 0.30000 rgb < 0.99457, 0.87889, 0.06625> ]
   [ 0.40000 rgb < 0.83409, 0.99457, 0.02736> ]
   [ 0.50000 rgb < 0.38010, 0.99457, 0.07204> ]
   [ 0.60000 rgb < 0.05287, 0.99457, 0.64586> ]
   [ 0.65000 rgb < 0.46341, 0.97697, 0.89598> ]
   [ 0.70000 rgb < 0.04840, 0.95938, 0.99457> ]
   [ 0.75000 rgb < 0.43931, 0.82767, 0.99457> ]
   [ 0.80000 rgb < 0.09890, 0.49815, 0.99457> ]
   [ 0.90000 rgb < 0.16623, 0.08811, 0.99457> ]
   [ 1.00000 rgb < 0.47403, 0.02386, 0.99457> ]
} }}  }

Yes, clouds can look so much like calving front, it takes two very clear days to compare Modis. 250 m vs 15 m resolution means 16.7 pixels in Landsat for the 1 in Modis. Still, it gives a heads-up in most cases. However even with Landsat, it is a struggle to measure ice stream velocities.

It appears that 10 July - 03 July is a pure surge situation, no calving. The same bergs can be seen in both. That provides an opportunity to compare the positional shift of micro-feature pairs.

The largest reliable pixel-pair movement I found was 43 pixels at 7.5 m resolution or 322.5 meters in 7.004 days (subtracting the SCENE_CENTER_TIME time stamps in the metadata files MTL.txt). That works out to 46.1 m per day or
16.8 km per year which is a near-record surge velocity. However, upstream values were less and somewhat variable.

We have an ~ stereo pair of Landsats coming in tomorrow 8,11 and 8,12. Might give us a better sense of the surface, depending on sun heights, azimuths and time differences. The 10 July was SUN_ELEVATION = 42.6, SUN_AZIMUTH = 173.0.

With better imagery, Joachin 2014 did this much more accurately as far as 37 km (M43 in their coords) up the south branch for 2009-13. They see unmistakable acceleration as far as as 20 km from the calving front (M26).

Various explanations have been put forward but I expect the physics here to be resolved by September (which will trickle down to us via a Feb 2015 journal article). That will allow future discharge of Jakobshavn Isbrae to be modeled better than today.

However that model won't be applicable to any other glacier in Greenland as the others lack the overdeepened bed channel. In some ways, GRACE trending (mass balance from gravity) may provide the best overall view of Greenland's total contribution to sea level rise.

The two expected Landsats came in today, LC80100112014191LGN00, a mostly clear path 10, row 11 and
LC80832332014191LGN00, a path 83, row 233 totally obscured by pretty clouds.

We last had a clear 10,11 on 08 Jun 14 if you are trying to do something along photogrammetric lines. the lat,lon photo centers are 69.60648, -50.86015 and 69.6066,-50.86458 respectively, so very favorable geometrically as those numbers correspond to a nadir displacement of 172 m

The slides compare the 10 July 14 calving front with 03 Jul 14 (LC80 09 011 2014184LGN00). Both are processed up through cropping and contrast normalization in ImageJ as 16 bit Tiffs, then exported to Gimp for alignment and resolution bumping from 15 m to 7.5 m using Lancsoz3 (sinc).

Note the strong surge of the ice stream -- faster than calving front retreat this week -- moving it 315 m further out in the fjord. Note the sharp shear line in the lower left. The giant iceberg in the upper left is just over a km in width -- this is a good photo pair for measuring berg displacements.

Click on image if it doesn't animate

"hard to measure a 100 m dash on a moving surface"

Right. The relationship between the volume of ice discharged (contribution to sea level rise), position of the calving front, glacier movement and terminus thinning is far from simple. For example, if the velocity of the glacier happened to exactly match the rate of calving, the front would be stationary yet record amounts of ice might calve into the fjord.

The first image below paces off 1 km steps along the main ice stream and shows how the July 2013 velocity spead up towards the terminus. This glacier moves so fast that ice 12 ticks out the blue line will be all calved off within a year.

I sought to find markers on the Landsat time series to see how fast the icestream is moving this year. However it is fairly featureless at 15-30 m resolution so not feasible to measure marker movement. This is a job for Digiglobe imagery -- the zoom from 30 m to google map resolution (24 Jun 12 photo) shows what we are missing.

As we brace for tomorrow's hi-res Landsat (which might not be online until Friday -- and even then might be clouded over), we might set some rules governing 'records'.

It is not enough for the calving line to retreat past the all-time record set on 20 Sep 13. Although mildly remarkable with 70 more days to go in the retreat season, just going slightly beyond last year is not even keeping up with the multi-year trend: an additional km of retreat is the new normal.

Indeed, slightly more would still be business as usual since the rate of retreat may be accelerating. Thus we shouldn't trouble our colleagues on the ASIB with news on this little glacier until the retreat is a second km beyond the first km expected.

Determining the pixel scale on Fig.3 of Joachin 2014 as 23.3 px per km, measuring peak retreats, and finding the increments gives, noting a measurement error of 1 pixel amounts to 35 m:

Since color Landsat is 30m per pixel as it comes (caution: Espen uses a rougher scale), I've moved the goal line to the 1 (yellow) and 2 km (red) distances from the lines of retreat for 03 Jul 14 (image) and 20 Sep 13 (blue).

Ok ... more or less have elevation color scale figured out ... a little garish but can fix shortly. Povray scene descriptor looks like this:

include ""
    location <-25,18,-2>
    look_at 0
    angle 35
  light_source{ <1000,1000,-1000> White }
  height_field {
    png "jakobshavn_bedmap-HSVinvert.png"
    pigment { White }
    translate <-.5, -.5, -.5>
    scale <17, 2, 17>
                     gradient y
                     color_map {
   [ 0.00000 rgb < 1.00000, 0.01111, 0.05829> ]
   [ 0.10000 rgb < 0.99457, 0.43569, 0.12356> ]
   [ 0.20000 rgb < 0.99457, 0.65239, 0.12521> ]
   [ 0.30000 rgb < 0.99457, 0.87889, 0.06625> ]
   [ 0.40000 rgb < 0.83409, 0.99457, 0.02736> ]
   [ 0.50000 rgb < 0.38010, 0.99457, 0.07204> ]
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   [ 0.80000 rgb < 0.09890, 0.49815, 0.99457> ]
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} }}  }

This is a very fine hand-held from an airplane, maybe IceBridge, on an unknown late summer date. It looks just east upstream on the south branch of the calving front of Jakobshavn Isbrae at the Big Curve. This is some seriously jumbled ice -- we have to applaud the Swiss for venturing out there to drill holes for temperature profiles and bedrock hydraulic connectivity.

The insets in lower left show rendered bedrock DEM under the photo and (logarithmic) ice stream velocities of 2008. The scripts for making these yourself are given over on the Landsat/bedrock forum. The right is a tweaked July 2014 overhead Landsat.

Here someone would need to chase down and post a DEM for each glacial valley of interest before I could process them. A DEM consists of a grayscale images in which the tone of gray indicates the z height above (or below) sea level for a region than includes the valley.

These are often found inside journal articles or their supplements. Often you would need to use something like to get the best resolution the authors had. I think the minimum size should be something like w,h = 400x300 pixels as postage stamp size just will not render details well

It is not good practice just to take a screenshot of a pdf as this will likely be crudely upsampled in hardware. In some cases, I can decompose a colored DEM in HSV space to find the underlying elevation grayscale.

I was also looking for a good map of Arctic Ocean ice thickness (wipneus makes these, but where). If the map is not 'too busy', it may be possible to display that thickness directly and do away with the color scale.

Here is one that contours glacier velocity. Promising, not there yet. So far I can tint uniformly by any RGB but haven't come across a scheme yet that really lays on a height color scheme. (Note to self: maybe actually read height map tutorials.)

Ok, that can be done by inverting the grayscale in Gimp so lows <-->highs. But, monkey at the keyboard again, here is a view looking west up past the calving front, 4x vertical exaggeration.

Here is my initial foray into displaying the Jakobshavn Isbrae bedrock as rendered 3D. I am using PovRay 3.7, a free ray tracing program that is capable (if the operator knows what they're doing, not applicable here) of very exquisite imagery. The most common scientific applications of PovRay is to crystallographic structures of proteins (see PDB or to NASA-type visualizations.

It has a conventional interface but as a practical matter is driven by easily edited scripts. However it takes quite a bit of experimentation to get the parameters to do what you want.

Below, I used a grayscale DEM and eventually got an instructive display though it is upside-down, ie looking upwards at a mold of topography. This wasn't what I intended but it does bring out the troughs and sills that lie just upstream of the calving front. The scale <26,7,26> means 7x vertical exaggeration.

Here are some tutorials on drawing these heightmaps:

camera{location <-7, 12, -7> look_at <0,0,2>  translate <-6,-1,0>}
light_source {<70, 80, -40> rgb 1}
light_source {<150, 50, 40> rgb 0.7 shadowless}

#declare selectSize = 1;
    #declare SourceImage = "jakobshavn_bedmap-HSV.png";
    #declare SourceImage = "jakobshavn_bedmap-HSVinvert.png";

height_field {
  png SourceImage
  translate -0.5
  scale <26,7,26> 
  pigment{color rgb <1,1,1> }

Calving looks dramatic, Espen ... the Landsat schedule is shown below, unfortunately the next high resolution image is not due until July 10.

I located the experimental data on temperature profile down the bore holes -- quite interesting. There's been more recent followup on the significance of bottom 'temperate ice' (near melting point) and its effect on creep: surprisingly, not that much.

Very interesting post, VaughnAn, one that makes clear the scale at which melting can occur over a historic time frame.

Glacier Bay is an absolutely awesome place -- we went up there last Sept on the Norwegian Princess. Didn't see any calving in a couple of hours parked in front of the glacier but it was making a lot of growling noises. A few glaciers in the western basin are still in balance because of heavy precip being so close to the north Pacific.

I've been pondering whether there is a significant difference between these mountain glaciers and the Greenland ice sheet where there's no underlying mountain.

Jabkoshavn Isbrae is atypical in that it is the only icestream in Greenland that flows over/in a deep gorge, one that averages 1167 m depth below sea level over its last 30 km, not to mention 402 m of ice thickness above for a total of 1569. (I have these numbers handy from calculating total kinetic energy of the icestream today at the Landsat/bedrock forum.)

I've been wondering now about the total kinetic energy of this icestream, the ice sheet that feeds it, and indeed that of all of Greenland's ice. And how to display the energy density over a map of the calving front.

Surely this conversion of gravitational potential energy to motion will all end up somewhere as heat. Will this heat provide a runaway feedback loop (melting or making temperate ice at the interface with bedrock, further accelerating motion) or 'just' go into warming Baffin Bay as it calves?

If the icestream has speeded up 40% in the last few years, then its kinetic energy has doubled as it goes as the square of velocity, 0.5m(1.4v)*(1.4v) = 2(0.5mv*v).

The glacier is moving rather slowly but there is a lot of it. Suppose a cubic meter of ice at the surface is moving at 10 km per year. That is a mass of 916.7 kg moving at 0.00032 m/s which is 94 µjoules.

This cubic meter of surface ice is sitting on top of a column of ice moving with it. On average, the depth of the ice stream is 1167 m below sea level and 402 m above for a total of 1569 meters (calculated from fig.3 of Joaghin 2014 by counting pixels, see figure). So this column has 147 mjoules of energy. Suppose the calving front is 5 km wide. The kinetic energy carried by this 1x5000 m vertical slice is then 735 joules. Enlarging this to include ice up to a kilometer upstream gives 7.3 x 10^5.

It takes 21,080 joules to bring a kilogram of ice at -10 C to 0 C and a further 334,000 joules to melt it to 0 C water, so 3.5 10^5 joules.

Thus the kinetic energy of this giant block of ice converted to heat is barely enough to melt two kg (liter) of cold ice. Even if some gross mistake was made in the above calculation, the outcome will still be trivial. The explanation: the ice is moving very slowly so when squared, an exceedingly small number overwhelms the enormous mass term.

The good news is the velocity map above which is logarithmic in its scale, can double as the energy density map (for what little that is worth). That is, a given color corresponds to a fixed velocity and so to a fixed velocity squared which is consistently proportional to pixel kinetic energy (approximating thickness as constant). So all that is needed is a second very different scale.

Here is another contour map, this time bedrock elevations rather than icestream speed. As you can see, the 'Bristol' map had really crummy resolution and could not produce a good outcome. I then tried Bamber 2013 but they only made public a badly colored 100x100 pixel postage stamp.

Today I wrote JPL to see if they could release their newest tomographic DEM. That is described in an abstract from last July:

It sounds like very satisfactory resolution despite some radar return voids. These arise from surface and shallow sub-surface pockets saturated with water. Highly reflective, they greatly reduce energy reaching and reflected back from the bedrock surface.

If the resolution is 100x80 km at 50,50,10 meters resolution, that amounts to 2000 x 2000 data points. Since the z value ranges from 500 to -1500 m, it has 200 possible values at 10 m resolution. So the 255 possible values of 8 bit grayscale seems adequate. So I just asked for that as a tif or png image.

The main channel is ~ 5 km wide north to south so there would be 100 data points in a transect, with adjacent transects separated by 50m. That would be sufficient to make a nice colored contour map of any under-ice bumps, holes, ridges, sills and troughs.

"The northern branch might become an ice fall and a new fascinating tourist attraction."

Right now, people mostly go to Epiq calving front where there is a hotel with direct views. For that, you would fly to Ilulissat and take a 5 hour boat. (Be careful landing at that airport because I heard the guy who built it took some shortcuts.)

Seattle to Reykjavik is $1783, 12 hours. Reykjavik to Ilulissat $992 but sold out already for August. The Epiq hotel is something like $225 a night.

However we want to go to the calving front at Jakobshavn Isbrae. That is far too dangerous for chartered boat or kayak, though you could go overland to Swiss Camp from Ilulissat with a pack raft for small crossings.

However web cam A shows this is not really not close enough. I suppose there are helicopter or fixed wing charters. Might be better just stay home and just watch 'Chasing Ice' a few more times.

The Joughin group has described what they consider the physical limits to acceleration of retreat. It has about 80 km of channel to go; they estimate a few decades for that. In other words, not an abrupt collapse a la Antarctic ice sheets.

Nice imagery, Espen. I found some more recent historic images at the Nasa Visualization Lab, 2001-2006, from an unnamed satellite that has 6x the resolution of Modis Terra in a 28º ccw rotation.

The character of glacier flow on the north branch has really changed. Formerly, a lot of moraine material was coming down. There are also various melt lakes and other features on both channels. The last image in the slide show is a murky Modis from 2014.

I'm going with freshwater melt lake.

The TanDEM-X DEMs provide 5 m/px resolution; DigitalGlobe WorldView 0.5 m/px stereo pairs. Surface rock observation IceBridge ATM/LVIS, ICESat GLAS, and GPS give absolute control on elevation data. A forthcoming paper from the Joughin group has a time series of sub-meter horizontal and vertical absolute accuracy, fully rectified photogrammetry. This feature is well above the error bars for sea level.

Jakobshavn has not been a floating ice shelf situation cf Antarctica since 1998 or so (except ephemerally in winter). The ice here is thousands of feet thick; it goes all the way down to the bedrock (modulo basal meltwater) and retains stratifications formed during the early and mid Holocene down 1500 m below sea level.

On ice-penetrating radar (bedrock), far fewer flight lines exist for the north branch. Here you want to look at adjacent raw radar profiles, look at their spacing and ask yourself how the bedrock surface in between is filled in.

In the late 1980's, there was quite a bit of discussion of a pinning point out in the main south bay of the main fjord. Although way below sea level, it had a very noticeable effect on movement of  ice (which was grounded out there then). It might better be called a seamount than a nunatak.

I don't recall any experimental data on the geothermal gradient for the Jakobshavn region. The expectation is way lower than central Greenland where the weight of the ice has thinned the viscous mantle, thickening it under the island margins and so slowing heat flow upward. There's only been some shallow steam drilling on the icestream and three bore holes (from the 1990's) way up the main branch that did not go to bedrock.

Now the Disko Bugt suture could change the geothermal picture. Although ancient -- archaean micro-continent pushed against paleoproterozoic -- anomalous heat flow could occur along the suture or a re-activated fault. It could also explain why the main Jakobshavn channel is located where it is. However the geology map is showing the suture well to the north, more like Epiq.

Here we go, perspective DEM from TerraSar. May or may not be some degree of vertical exaggeration. Not sure of the date either. North branch is clear locatable; more extreme topography than i would have guessed from the nadir view of Landsat. It's not so easy to be sure the melt lake is in the pocket though.

Right, strange flow pattern in the vicinity, topography is a bit of a puzzle. However ordinary google map has a great series of zooms here (once you're sure you're zooming on the right object, the north fork has retreated a lot since this imagery. The melt lake is very black in Landsat today but by clicking on it with the color wand tool, other lakes prove just as black (while most are classic blue). Click if the animation doesn't start.

Good observation. The north branch is quite interesting in its own right. In the late 1980's, it joined the south fork in an extended down-fjord feature called 'the zipper' (the thicknesses were markedly different). Now that the south fork has retreated up its own channel, there is less back pressure on the north and middle fork calving fronts.

The black streaking on discharged ice below the nunatak probably represents rock on the bottom of calving pieces (as in 'Chasing Ice') rather than moraine streaking from the nunatak.

Note the north fork does not overlie a deep channel nor does it have a large drainage. Thus it does not have potential to be a major contributor to sea level rise.

Below is the panchromatic 7.5m from yesterday of this nunatak. We won't be seeing similar pinning points emerge on the south fork because its channel is thousands of feet below sea level for 125 km inland.

In terms of developing a time series, I looked through the Landsat-8 imagery database posted on the other forum,909.0.html, both for other path 9, row 11 cloud-free imagery (which will have the most similar platform geometry if you want a stereo pair or count interometric movement fringes) and also for the most recent regardless of path, row. There is no counterpart to July 3 imagery in the 2013 archive.

This Landsat-8 is a path,row 9,11 which is always followed by a 10,11 the next of which should appear on July 7th, or if not then July 11. It has a few puffies but otherwise is an exceedingly sharp image.

I noticed a peculiar thing about Band 8 panchromatic. It looks a lot better with its grayscale inverted (x --> 255-x). This could have something to do with the lighting. According to the metadata file:

SCENE_CENTER_TIME = 15:00:24.4130118Z
SUN_AZIMUTH = 173.30855079
SUN_ELEVATION = 43.33909208

It looks like a couple of bergs at the two extremes of the calving front will be calving off sideways. Otherwise, lots of crevices (and even patterns to them) but nothing that indicates a next calving front.

Landsat imagery is now available for download at EarthExplorer. The big one is LC80090112014184LGN00_B8. I adjusted all the contrasts within the 16 bit framework of ImageJ as tests show it does make difference.

The most curious thing about this calving event is its edge at the fast-moving main ice stream. The slower ice coming off the hill to the south has not yet participated. The image below is 30m. I upsampled the 15m panchromatic to 7.5 m to see what the details of the calving front looked like ... a work in progress it appears.

For some reason, the animation above is not working today -- preview here does not include images. Have to click on it, works fine. Here is the final frame:

Thanks! Now if only I could wrap up all the loose ends above.

I did a couple more versions of surface ice speed contours in the Jakobshavn Isbrae drainage. It's probably ok to muddle the distinction between speed and velocity because the direction of motion is clear (downhill).

The first image is regional. It shows that motion is dramatically faster -- but not uniformly -- in the main icestream channel. The ice is moving fastest along the central flowline and the speed falls off towards the walls. This region markedly accelerates as it nears the calving front.

The north branch is also moving but not as rapidly. It drains a much more limited iceshed. Note too a 'middle' branch that we see on 30 m imagery as an icefall. It has a larger drainage than the north branch and may speed up its discharge when the calving front retreats out of its way.

The second image shows more detail near the calving front and big bend. I've faded this in to help co-locate the underlying icestream. From this, you could figure out what ice will arrive at the calving front a year from now (remembering that this particular DEM dates from 2008).

That is, the velocity vectors have to be parallel along any straightaway or the ice would become impossibly jumbled. Integrating instantaneous velocity as a function of position yields the time to arrive at the calving front.

Ice arriving a year from now does not correspond to a channel transect at all but rather is bent like the contours. It is rather reminiscent of Hawaiian lava tube flows to the ocean.

These contour maps are easily made by overlaying a grayscale gradient, picking a gray, extending it over the whole image, and replacing it with some palette off the internet. The ideal palette (haven't located!) makes sensible use of color, both in distinguishable but related incremental local steps and in an overall fade in saturation towards pastel.

Jim, that would be great contribution if you could figure out how to dish out something more interactive in terms of raster GIS layers... this glacier is its own news site the way imagery and flight line keep rolling in.

A couple more posts and I'll have this particular example built out starting with adam and eve. To de-mystify the process.

We've got quite a few people posting raster base layers (photo time series animations) but only a few (Wipneus) doing actual GIS analysis on the stacks (ice thickness DEM).

Photoshop/Gimp/ImageJ/ImageMagick are actually GIS software in disguise, glitzy visual front ends for an underlying spreadsheet (one pixel per cell, one color per sheet) so you could ditch them and do it all in Excel or by command-line on coupled arrays. Including extruding a grayscale as DEM heights in perspective view.

Dedicated GIS software today has unbelievable capabilities. That's great in terms of producing eye-popping illustrations but problematic when limitations in the raw experimental data are forgotten. That's what I'm checking here: the channel is 5 km wide but how many radar bedrock soundings do we have over that 5 km?

I looking at the beyond-fantastic to see if it could be tweaked to show glacier flow lines instead of weather over wind.

Actually David Podrasky already did a nice time series of speed, slope, and solar aspect for the Jakobshavn icestream. And Cindy Starr over at NASA Visualizations did a low resolution version of nullschool gremlins for both all-Greenland and just Jakobshaven flows.

This amounts to first computing the gradient (downhill, direction of steepest descent) of a high resolution digital elevation map and then drawing the flowlines.

Contours and flowlines then provide locally orthogonal coordinates for the surface (or for that matter, bedrock under the ice) respectful of the gravity that ultimately drives ice sheet flow and meltwater drainage.

Crevasse fields being the tangents to contours, we would like to isolate them directly from the imagery (via wavelet or fourier decomposition) because that's the test of whether computed flow lines are really consistent with observation.

We need to produce catchy, intuitive illustrations to communicate global warming to the world outside this blog -- and somehow retain scientific accuracy.

Looking at Podrasky's overlays, I would say logarithmic color (for speed) is hard for the average citizen to grok and circularly permuted color (for slope angle) is not immediately intuitive either. To get the photo-realistic effect, the DEM is shown as shaded relief. The final images have flattened the layers but fortunately they can be resurrected by decomposition in hue, saturation, grayscale (RGB --> HSV color space).

I did that for surface glacier speed over the Jakobshavn gorge, contouring by grayscale select and coloring with a recent Wipneus palette that distinguishes 'adjacent' colors rather than more 'logical' spectral stepping. This came out quite well if you believe the ice stream slows from wall friction along its edges.

The 01 Jul 14 Landsat-8 image was released today. Skies were clear over the calving front. Unfortunately LC80842322014182LGN00 being orbital path 84, row 232 just nicks the calving zone, providing the triangular overlap below. Image quality is not the best at the edge of its frame either.

Modis Aqua/Terra on 30 Jun 13 show more of the event but they are a couple of bricks short of a wall in terms of adequate resolution.

The bottom image in the 15 m slide show below has been painted to display different origins of ice arriving at the calving front, based on shear lines. This ice likely has different properties (temperature, thickness, velocity) depending on its origin which presumably correlate with where the calving front calves.

I'm very skeptical of journal displays indicating uniform velocity transects. We have an event here each fall where the students line up on a bridge at 1 m intervals, dropping rubber duckies into the river. As you can imagine, these hardly arrive at the next bridge downstream at the same time. I will put this matter to rest for Jakoshavn Isbrae the day we get our first pair of supersite radar images this fall.

It's very difficult to get a sense of volume lost, which after all is the main interest (increasing contribution to sea level rise). Rather than modeling the glacier, I think it would be better just to make accurate lidar measurements of the downstream fjord (noting tide level). Heights there imply depths since this is mainly ungrounded freshwater ice. Volume is then proportional to the surface integral of the DEM.

Next we would like to automate the download, cropping and alignment of the entire series. To do that, fix one date (say 24 June 14). Then in a spreadsheet, subtract from its lat, lon center from those of the other cloud-free dates.

This gives the translation vectors necessary to move the centers so the photos would align on their overlap, which by construction includes the calving front area. Really we just want to use this vector to get crop coordinates for the 34 images and not move large files into a stack until file size is seriously reduced.

Normally the crop box rectangle is specified by (w,h) pixel coordinates in the fixed image. So it boils down to converting decimal lat,lon coordinates provided by EarthExplorer into pixel coordinates.

For 30 m Landsat-8 images of Jakobshavn Isbrae, the dimensions are always the same, 8591 x 8641. That puts the central pixel at (4296,4321) which for the 24 June 14 center (lat,lon) is (69.60667,-51.29052). Entering the upper left and lower right pixel coordinates of a chosen crop view into a spreadsheet, convert those into lat, lon.

Now add the translation vectors to get lat,lon crop corners for the 34 images and convert them back into pixel coordinates. This gives the specifications necessary to crop them without opening the files. (As Wipneus notes elsewhere, these are sometimes called the offsets.

Open them all in ImageJ rather than Gimp and apply the two contrast options because this will process the whole stack to a uniform standard making full use of 12 bit resolution. Save and open in Gimp for further processing -- 8-bits per channel at this point will not lose information.

In Gimp, view the stack in a fast animation to see if the images are properly aligned. If not, the move tool can nudge them into register with the fixed date.

In choosing the crop box, it is a very good idea to include fiducial points. For Jakobshavn Isbrae calving front, a lot of the scene is snow-covered moving ice sheet and ice stream. So no reference points there. However, I found two well-separated fixed points in exposed rocks that are locatable in radar as well as visual imagery.

Their lat,lon can be determined with great precision because EarthExplorer zooms down to Digital Globe imagery of much higher resolution than LandSat-8 or TerraSar. The first picture below shows the two features as they appear in a variety of imagery along with assorted scale conversion factors.

Recall Cresis DEM files come in lon,lat,z format. To view depth slices along a latitudinal gradient, these have to be located on imagery via pixel coordinates. Or vice versa, we would like to look at bedrock depth around the calving front, sills and troughs and need to find our location within the DEM. In the path up the main icestream channel used by Joaghin, those coordinates are provided over the TerraSar peak retreat image of 20 Sep 13 in the upper right corner of the second image.

The DEM data only allows vertical or horizontal depth profiles and sometimes these cut obliquely across the icestream, distorting the profile. Thus the image also shows the angles needed to rotate the latitudinal depth profile to be orthogonal to flowlines of the calving front, the two troughs and the first still (the second is well served by a vertical slice of the DEM).

Here picture two vertical profiles flanking the oblique cut (ie for which the cut is the diagonal). To interpolate the DEM onto the diagonal cut, simply weight the z values  of the two flanking vertical profiles point by point in accordance to distance.

For everyone's convenience, I looked at 4"x4" thumbnails of all Landsat-8 coverage of the current calving region of Jakobshavn Isbrae for cloud coverage (rather lack thereof). Of the 70 available scenes, 38 appear useful. EarthExplorer accepts these scene identifiers under Additional Criteria searching.

There are 6-8 stereo pairs (same day, consecutive scenes, overlap) but no exact matches of dates between 2013 and 2014. However, the best matched date pairs can easily be found sorting on the Match column in the tab-delimited database below. Use Lsort and Csort to restore the current (very special!) order of rows.

Lsort   Landsat Scene Identifier   Match   #Day   Day   Mon   Year   Path   Row   clarity   Csort   Cloud   PaRow   Center Lat   Center Lon
3   LC80832332014175LGN00   2   175   24   06   14   83   233   good   1   33   83233   69.60667   -51.29052
6   LC80842322014166LGN00   2   166   15   06   14   84   232   cutoff   1   16   84232   68.27978   -51.15327
7   LC80080122014161LGN00   2   161   10   06   14   8   12   good SP   1   2   08012   68.27988   -49.46752
8   LC80080112014161LGN00   2   161   10   06   14   8   11   good SP   1   1   08011   69.6066   -47.7732
10   LC80100112014159LGN00   2   159   08   06   14   10   11   good SP   1   22   10011   69.6066   -50.86458
11   LC80090112014152LGN00   2   152   01   06   14   9   11   good SP   1   6   09011   69.60662   -49.31691
12   LC80842322014150LGN00   2   150   30   05   14   84   232   cutoff   1   4   84232   68.27948   -51.15202
17   LC80080122014129LGN00   2   129   09   05   14   8   12   good SP   1   12   08012   68.27992   -49.60601
18   LC80080112014129LGN00   2   129   09   05   14   8   11   good SP   1   4   08011   69.60666   -47.9235
19   LC80100112014127LGN00   2   127   07   05   14   10   11   good   1   28   10011   69.60623   -51.0104
21   LC80100112014111LGN00   2   111   21   04   14   10   11   good   1   10   10011   69.60624   -51.0072
22   LC80090112014104LGN00   2   104   14   04   14   9   11   good   1   10   09011   69.60643   -49.45147
23   LC80080122014097LGN00   2   097   07   04   14   8   12   good   1   17   08012   68.27955   -49.59887
24   LC80080112014097LGN00   2   097   07   04   14   8   11   good   1   22   08011   69.60652   -47.91558
27   LC80080122014081LGN00   2   081   22   03   14   8   12   good   1   4   08012   68.27951   -49.59312
28   LC80080112014081LGN00   2   081   22   03   14   8   11   good   1   4   08011   69.6065   -47.9094
34   LC80090112014056LGN01   2   056   25   02   14   9   11   good   1   6   09011   69.60626   -49.43474
37   LC80090112014040LGN00   2   040   09   02   14   9   11   good   1   12   09011   69.60632   -49.42945
38   LC80080122013302LGN00   0   302   29   10   13   8   12   goodSP   1   9   08012   68.27983   -49.51267
39   LC80080112013302LGN00   0   302   29   10   13   8   11   goodSP   1   11   08011   69.60661   -47.82377
41   LC80090112013293LGN00   0   293   20   10   13   9   11   good   1   5   09011   69.60646   -49.38038
42   LC80080122013286LGN00   0   286   13   10   13   8   12   so-soSP   1   12   08012   68.27961   -49.52841
43   LC80080112013286LGN00   0   286   13   10   13   8   11   so-soSP   1   32   08011   69.60642   -47.83909
44   LC80100112013284LGN00   0   284   11   10   13   10   11   so-so   1   23   10011   69.60661   -50.93055
46   LC80080122013270LGN00   0   270   27   09   13   8   12   goodSP   1   61   08012   68.27948   -49.5069
47   LC80080112013270LGN00   0   270   27   09   13   8   11   goodSP   1   39   08011   69.6064   -47.81722
49   LC80090112013261LGN00   0   261   18   09   13   9   11   so-so   1   9   09011   69.60647   -49.36928
54   LC80080122013238LGN00   0   238   26   08   13   8   12   so-soSP   1   29   08012   68.2798   -49.52364
55   LC80080112013238LGN00   0   238   26   08   13   8   11   so-soSP   1   22   08011   69.60668   -47.83321
58   LC80100112013172LGN00   1   172   21   06   13   10   11   good   1   30   10011   69.60634   -50.87524
59   LC80100112013156LGN00   1   156   05   06   13   10   11   so-so   1   40   10011   69.60661   -50.91349
60   LC80822332013149LGN00   1   149   29   05   13   82   233   good   1   7   82233   69.60659   -49.75361
62   LC80080122013142LGN01   1   142   22   05   13   8   12   goodSP   1   9   08012   68.27962   -49.52037
63   LC80080112013142LGN01   1   142   22   05   13   8   11   goodSP   1   13   08011   69.60664   -47.82795
64   LC80100112013140LGN01   1   140   20   05   13   10   11   good   1   45   10011   69.60623   -50.91778
65   LC80100112013124LGN01   1   124   04   05   13   10   11   so-so   1   25   10011   69.60624   -50.87119
69   LC80100112013108LGN01   1   108   18   04   13   10   11   good   1   27   10011   69.60654   -50.88116
70   LC80090112013101LGN01   1   101   11   04   13   9   11   good   1   12   09011   69.60663   -49.54324
1   LC80080122014177LGN00   0   177   26   06   14   8   12   useless   2   67   08012   68.27959   -49.44788
2   LC80080112014177LGN00   0   177   26   06   14   8   11   useless   2   27   08011   69.60633   -47.75372
4   LC80100112014175LGN00   0   175   24   06   14   10   11   useless   2   72   10011   69.60629   -50.84766
5   LC80090112014168LGN00   0   168   17   06   14   9   11   useless   2   34   09011   69.60667   -49.31732
9   LC80832332014159LGN00   0   159   08   06   14   83   233   useless   2   28   83233   69.60634   -51.30726
13   LC80080122014145LGN00   0   145   25   05   14   8   12   useless   2   30   08012   68.27973   -49.45283
14   LC80080112014145LGN00   0   145   25   05   14   8   11   useless   2   27   08011   69.60661   -47.75806
15   LC80100112014143LGN00   0   143   23   05   14   10   11   useless   2   66   10011   69.60656   -50.8458
16   LC80090112014136LGN00   0   136   16   05   14   9   11   useless   2   60   09011   69.60651   -49.48273
20   LC80080122014113LGN00   0   113   23   04   14   8   12   useless   2   51   08012   68.27967   -49.60016
25   LC80100112014095LGN00   0   095   05   04   14   10   11   useless   2   34   10011   69.6063   -51.00663
26   LC80090112014088LGN00   0   088   29   03   14   9   11   useless   2   39   09011   69.60654   -49.44342
29   LC80100112014079LGN00   0   079   20   03   14   10   11   useless   2   52   10011   69.60654   -51.00212
30   LC80090112014072LGN00   0   072   13   03   14   9   11   useless   2   6   09011   69.60658   -49.45018
31   LC80080122014065LGN00   0   065   06   03   14   8   12   useless   2   37   08012   68.27984   -49.57592
32   LC80080112014065LGN00   0   065   06   03   14   8   11   useless   2   11   08011   69.60628   -47.89226
33   LC80100112014063LGN00   0   063   04   03   14   10   11   useless   2   30   10011   69.60624   -50.97462
35   LC80080122014049LGN00   0   049   18   02   14   8   12   useless   2   15   08012   68.27988   -49.57734
36   LC80100112014047LGN00   0   047   16   02   14   10   11   useless   2   11   10011   69.60666   -50.9785
40   LC80100112013300LGN00   0   300   27   10   13   10   11   useless   2   13   10011   69.60653   -50.91439
45   LC80090112013277LGN00   0   277   04   10   13   9   11   useless   2   74   09011   69.6065   -49.37297
48   LC80100112013268LGN00   0   268   25   09   13   10   11   useless   2   27   10011   69.60643   -50.90873
50   LC80080122013254LGN00   0   254   11   09   13   8   12   useless   2   53   08012   68.27957   -49.52488
51   LC80080112013254LGN00   0   254   11   09   13   8   11   useless   2   54   08011   69.60667   -47.83454
52   LC80100112013252LGN00   0   252   09   09   13   10   11   useless   2   82   10011   69.60663   -50.92559
53   LC80090112013245LGN00   0   245   02   09   13   9   11   useless   2   79   09011   69.60641   -49.37792
56   LC80100112013236LGN00   0   236   24   08   13   10   11   useless   2   63   10011   69.60625   -50.92145
57   LC80090112013229LGN00   0   229   17   08   13   9   11   useless   2   76   09011   69.60639   -49.36156
61   LC80842322013147LGN00   0   147   27   05   13   84   232   useless   2   17   84232   68.27988   -51.15388
66   LC80090112013117LGN01   0   117   27   04   13   9   11   useless   2   29   09011   69.60627   -49.3174
67   LC80080122013110LGN01   0   110   20   04   13   8   12   useless   2   26   08012   68.27985   -49.47715
68   LC80080112013110LGN01   0   110   20   04   13   8   11   useless   2   7   08011   69.60659   -47.78475

Sidd, that link to the x,y,z raw DEM data at Cresis is then then jakobshavn_bedmap_pts.txt after unzipping.

Wipneus and Espen, here's the variation in Landsat-8 orbital repeats -- the animation below shows all fifteen path 8 row 12's from 22 May 13 to 25 May 14, many cloudy.

As Nukefix noted, the satellite only repeats its orbit up to within some solid tube, so different scenes don't quite repeat perfectly in either ground footprint or geometry. There's definitely some deformation towards the sides, as well as a quantitatible shift of center (lat,lon) coordinate (assuming EarthExplorer implemented these faithfully).

Various online tools calculate distances between two (lat,lon)'s,  though if latitudes are the same, you could get by with  longitudes using a French meter (defined as one ten-millionth of the distance between the North Pole and equator).

Certain other (paths, rows) like the clear (83,233) 24 Jun 14 ascending repeat only once over the last year. And comparing (83,233)'s with (8,12)'s will probably have bigger issues, though life is better when calving front sits in the center of the image. (Almost all of these row,paths show it off in a corner.)

I just noticed that (8,11) and 8,12) not only tile -- that overlap sits over the calving front. So a stereo pair and more ... if only the weather would be clear. (Click second icon: 'Browse Overlay' to display footprints in EarthExplorer.)

It's worth noting that Gogineni (and others) lumped depth classes rather drastically in making their shaded reliefs. That plus jpging disconnect the color key from the image. The raw data is instructive -- sort on z depth for example.

I can't decide between a fly-through or a mountain bike ride up the Jakobshavn Isbrae bedrock channel. I did manage to embed a moving yellow transect line over a Landsat scene so you can tell where the current bedrock DEM transect is sitting.

Cresis also provides nice grayscales of their actual radar returns. However I found it slow drilling given their many many  tracks and never did find the 2014 of Jakobshavn. Ditto Icebridge. (I'm animating 2008, there's been a big push to get these tight deep fast icestreams better.)

Nice group of people here! Crowdsourcing knowledge really works ... I learn something from every page.

Today I took apart the Cresis ice-penetrating radar file for Jakobshavn Isbrae. It is in csv format (lon,lat,elevation), the first two in excessive decimal degrees and the last in meters. The DEM data is an array of size (1340,181) but linearized to 644,540 lines in the form (lon,lat,z). It comes sorted by decreasing latitude.

A whole lot of numbing numbers. I found that EarthExplorer will  display the coverage rectangle translucently over the high res Digital Globe image (google map default) -- in Search Criteria, just paste into the lat,lon popup from Add Coordinates.

-49.7104  69.1870 NW corner
-49.1171  69.0710 SE corner
-49.5943  69.1530 calving front (approx)

However I wanted to make a flythru showing just the bedrock channel with the ice gone. So to chop the file down, I deleted the two blocks of irrelevant latitudes from top and bottom of the file, then re-sorted on longitude and discarded top and bottom there. This got the file down to a more manageable 23034 rows or an 349 x 66 array.

A frame of the flythru then consists of a fixed longitude line (starting to the west of the calving front), with the z values associated with each latitude visualized (and smoothly interpolated) by a simple spreadsheet graphic that is then exported to a Gimp layer where 2x horizontal exaggeration (this fjord is really narrow), rock and stratified ice texture, etc are easily added.

By popular demand ...  I'm posting now to the forum thanks to a memorable new password from Neven.

I took a look into what is involved in precise co-registration of Jakobshavn Isbrae imagery. Wipneus and Chris have gone round and round with this previously, relating the goofy coordinate system of Piomas to conventional Arctic projections.

For the calving front of Jakobshavn Isbrae, the scale is much smaller, just a few tens of km covers the calving front and descending ice. We'd like to have data co-registered to 15 meters or so when possible.

We have Landsat-8  and Modis (nadir), satellite radar (whiskbroom vs pushbroom), Operation IceBridge airplane radar transects (trailing antenna), projections such as mercator and stereographic, and coordinate manipulations done during image processing by satellite centers and later by journal authors, changes over time in archive coordinates, satellite drift, image resolution, and so forth.

All this data needs to be brought into a common coordinate system so it can be co-registered (stacked in pixel-perfect co-registered image layers) for purposes of arcGis-type analysis, for example overlaying the ice surface digital elevation map, bedrock DEM, surface velocity field, and a time series of images.

One problem I've encountered is the lack of fixed reference points on the ground. Historically a pattern of quarter circles would be emplaced on the ground along with a corner radar reflector. With 3-4 of these to anchor alignment, any imagery can be forced into pretty good registration.

While rock shows in most imagery, it can be obscured by clouds, snow-covered, have inland lakes frozen vs calving and melting, with shorelines alternately clear or obscured. The rest of the scene is worse with snow and ice blanketing features and the ice stream -- and indeed the whole Greenland ice sheet -- in motion.

In short, to tie everything onto a common lat, lon coordinate system, we are forced to root around in metadata for each data source. For example, the bedrock elevation transacts of Gogineni comes in x,y,z format, with the best DEM file for Jakobshavn an excel-busting 524,288 array. When sorted for depth in meters (z), the deepest parts look like this:

digital lat          digital lon      depth of bedrock below mean sea level
-48.49456221  69.20637076  -1512.200
-48.49656494  69.20637076  -1512.200
-48.46251853  69.20837349  -1508.124

I'll stop here. The idea is to get the precise lat,lon of major glacier overdeepenings and sills so they can be co-located precisely with lat,lon of the calving front and so brought into a predictive environment.

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