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Author Topic: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland  (Read 1200271 times)

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1100 on: October 31, 2015, 04:44:56 PM »
The big season finale ... 27 Landsat frames showing calving front movement at 7.5 m resolution during 2015. It takes a click to get going and then some time for it to fully load. I'll put dates on the frames in a bit; here is what I used:

2015    43    0    9   11
2015    50    7   10   11
2015    52    2    8   11
2015    59    7    9   11
2015    75   16    9   11
2015    91   16    9   11
2015   107   16    9   11
2015   114    7   10   11
2015   123    9    9   11
2015   132    9    8   11
2015   146   14   10   11
2015   148    2    8   11
2015   155    7    9   11
2015   162    7   10   11
2015   178   16   10   11
2015   180    2    8   11
2015   187    7    9   11
2015   194    7   10   11
2015   196    2    8   11
2015   210   14   10   11
2015   212    2    8   11
2015   226   14   10   11
2015   228    2    8   11
2015   235    7    9   11
2015   251   16    9   11
2015   267   16    9   11
2015   283   16    9   11
« Last Edit: October 31, 2015, 04:50:01 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1101 on: October 31, 2015, 09:29:12 PM »
The calving front is also available -- for a limited time only -- as a commemorative stamp.
« Last Edit: October 31, 2015, 10:08:34 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1102 on: October 31, 2015, 10:22:18 PM »
The big season finale ... 27 Landsat frames showing calving front movement at 7.5 m resolution during 2015. It takes a click to get going and then some time for it to fully load. I'll put dates on the frames in a bit; here is what I used:

2015    43    0    9   11
2015    50    7   10   11
2015    52    2    8   11
2015    59    7    9   11
2015    75   16    9   11
2015    91   16    9   11
2015   107   16    9   11
2015   114    7   10   11
2015   123    9    9   11
2015   132    9    8   11
2015   146   14   10   11
2015   148    2    8   11
2015   155    7    9   11
2015   162    7   10   11
2015   178   16   10   11
2015   180    2    8   11
2015   187    7    9   11
2015   194    7   10   11
2015   196    2    8   11
2015   210   14   10   11
2015   212    2    8   11
2015   226   14   10   11
2015   228    2    8   11
2015   235    7    9   11
2015   251   16    9   11
2015   267   16    9   11
2015   283   16    9   11


Thank you. The level of detail is amazing and instructive.
I think it would be useful to have a slight pause on the last frame of the season, to help distinguish it from the other frames and to help in trying to mentally extrapolate where all of this is going next.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1103 on: October 31, 2015, 11:45:33 PM »
Quote
useful to have a slight pause on the last frame
End of season pause is a good idea. Gimp has some glitches in this area that cause problems in work-around, especially at this many frames and file size given the restrictions of forum display.

I should have some other products ready tomorrow ... a larger overview but at lower resolution, an extraction of pure calving front line progression, and computed areas between successive frames (of ice calved off, per day average).

I've hardly had time to look at the products yet myself -- there is a lot of fascinating information in them though. In terms of calving front trends, it may be better to replace the 'day of maximum retreat' with the 'average seasonal retreat line position' which is readily determined graphically in Gimp.

While 2013 and 2014 are easy enough to compare, a longer term comparison of mean seasonal retreat position starts to resemble work work.

We might expect farther retreat earlier in a longer season as climate change continues to kick in (eg 2013-14 had their DOMR in late September but for 2015 it was mid-August). However simple-minded trending of the calving front however that is done (resp. ice volume calved) will not be a powerful predictive tool.

In theory, the animation can be updated every 12 days this fall with a co-registered frame from Sentinel-1A IW, ie we switch from apples to oranges. This will be the first winter ever that Jakobshavn can be systematically tracked with open source imagery. (When imagery is purchased, it can't appear at resolution in journal articles.)

However JI is not likely to set any new retreat record; usually in winter the forward inertial motion of the glacier slows down but calving slows even more, resulting in the calving front moving down fjord.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1104 on: November 01, 2015, 02:25:13 AM »
The calving front is also available -- for a limited time only -- as a commemorative stamp.

I am sure they (in Greenland) are thrilled with your creativity so am I ;)
Have a ice day!

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1105 on: November 01, 2015, 12:53:51 PM »
Quote
sure they are thrilled in Greenland with creative commemorative stamp
The commemorative stamp of the future will display ads for dog-sled tours and narwhale-watching?

Here are a couple of 'concept animations' that track the retreat and advance of the calving front over the Landsat year. The dark blue alternates with light blue and the area between them is shaded green for net retreat and red for net advance. It is not always so clear where the calving front is located especially when the north shore is surging.

It has to be kept in mind here that the number of days between scenes varies from 2 to 16 days (per schedule in above post) and that the ice sheet advances 4 pixels per day (at 30 m/d at 7.5 m/pxl resolution) regardless of calving. The daily advance varies with season so to correct the calving areas it would be necessary to measure velocities between each pair of frames.

Note, despite needed adjustments, the August 15th mega-event really stands out given that it was bracketed by a Landsat pair only two days apart. The images are slightly reduced below so that they animate without a click. The day of maximal retreat, 228, is shown as background image.
« Last Edit: November 01, 2015, 01:55:17 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1106 on: November 02, 2015, 12:02:12 PM »
Here is another version of the difference in calving position between successive (non-cloudy) Landsat scenes. Since that interval is quite erratic, varying between 2 and 16 days (average 9.2, std dev 5.5), comparisons of variably smoothed inter-scene intervals lack resolving power.

It would be desirable to add in Sentinel and Modis calving front days. However those have various issues with ground control points and co-registration. There were also near-daily overflights by Air Zufari during most of the season during which each of the five passengers presumably took video; however not a single day of this has become available.

Holland's camera A on the north shore took time-lapse imagery up to late-July but that looks horizontally at the calving front position which is unfavorable for reconstructing nadir view. Camera B looks out at passing floes but those are now heavily influenced by north shore iceberg release.

The second animation looks at the prospects for Landsat 'interferometry'. That is, grayscales for three evenly spaced scenes are combined as RGB, then rotated into HSV with only the hue channel retained. This has some prospects for distinguishing regions of stagnant, slow and fast ice as well as objectively defining intra-scene calving. However because of clouds, it is quite difficult to find 3 scenes that fit the criteria.

The 3rd and 4th animations look at the wake of a submerged rock at the north boundary of the main calving front. Note that the ice immediately above and below does not originate very far up-glacier and is not part of the fast ice stream.

In summary, monitoring of the Jakobshavn calving front is unsatisfactory. However even if we had daily drone coverage, nothing that profound would be learned if calving is indeed 'downstream' of the primary driving forces of upglacier meltwater injection and turbulent ejection a km below the surface at calving front/ocean interface.
« Last Edit: November 02, 2015, 02:32:11 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1107 on: November 03, 2015, 01:24:36 PM »
To check that the above feature represents ice flowing over a submerged rock rather than a meltwater pond perhaps formed in a depression or region of pressure drop, it suffices to look at mid-August natural color imagery that provides internal comparison of embedded ponds elsewhere.

Obviously a huge literature exists on air and fluid flow around obstacles (eg airplane wings and ship hulls). Although here we have a cold brittle ice stream that calves into stable rigid icebergs, it nonetheless flows like a very viscous liquid. We've seen some striking eddies in airflow over Jan Mayen but here not, though presumably there is a sharp drop in pressure once past the obstruction. The ice seems smooth over (or immediately behind) the rock but small bumps can be seen at 5 m resolution. Unfortunately the 1 m Google Earth image here is poor quality.

Flow over this feature here contrasts with the situation on the nunatak dividing north and south channels, which is high enough to repel the slow flow coming at it. It is also quite different from the odd feature on the north bank proper, which we determined last year to be a meltlake depression behind a diverting elevated hump.

Rocks and obstructions that don't have surface manifestations would still be having all sorts of effects on deeper flow. It's a wonder that any are left after 125,000 years of glacier grinding away.

The second image shows some of the main channel geometry. Here I fit a flowline through the elbow with two ellipses and two straight shots. This is a lot of fast massive to be making a 72º turn over a short distance. The animation of 12 days in 2015 helps with flow line identification.

It's also convenient to fit a fifth degree polynomial to the flowline using the online tools below. These are nice in that the points along a Landsat flowline can simply be entered as Gimp x,y pixel coordinates, with the output providing the fit, the goodness of fit and and a graphical overlay.

Given an explicit polynomial, it is then easy to calculate properties of interest at any point along the flowline such as curvature, normals, tangents, 2nd derivatives and line integrals. Note though that nadir view of the satellite has to be slightly corrected for the downward slope of the ice sheet (which can be taken from Joughin 2014) to get a fit to the ice sheet surface.

http://www.xuru.org/rt/pr.asp
http://www.mathopenref.com/graphfunctions.html.
« Last Edit: November 03, 2015, 05:07:15 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1108 on: November 04, 2015, 12:18:11 PM »
Here is the final version of movement of Jakobshavn near and above the elbow region. Two tiles are shown that cover the elbow region and to its east; these should animate without a click. After quite a bit of experimentation, these are fairly consistent in contrast which is tricky with early season and cloud shadows on some dates.

2015    43    0    9   11   LC80090112015043LGN00   12 Feb 15
2015    50    7   10   11   LC80100112015050LGN00   19 Feb 15
2015    52    2    8   11   LC80080112015052LGN00   21 Feb 15
2015    59    7    9   11   LC80090112015059LGN00   28 Feb 15
2015    75   16    9   11   LC80090112015075LGN00   16 Mar 15
2015    91   16    9   11   LC80090112015091LGN00   01 Apr 15
2015   107   16    9   11   LC80090112015107LGN00   17 Apr 15
2015   114    7   10   11   LC80100112015114LGN00   24 Apr 15
2015   123    9    9   11   LC80090112015123LGN00   03 May 15
2015   146   14   10   11   LC80100112015146LGN00   26 May 15
2015   155    7    9   11   LC80090112015155LGN00   04 Jun 15
2015   162    7   10   11   LC80100112015162LGN00   11 Jun 15
2015   178   16   10   11   LC80100112015178LGN00   27 Jun 15
2015   187    7    9   11   LC80090112015187LGN00   06 Jul 15
2015   194    7   10   11   LC80100112015194LGN00   13 Jul 15
2015   210   14   10   11   LC80100112015210LGN00   29 Jul 15
2015   212    2    8   11   LC80080112015212LGN00   31 Jul 15
2015   226   14   10   11   LC80100112015226LGN00   14 Aug 15
2015   235    7    9   11   LC80090112015235LGN00   23 Aug 15
2015   251   16    9   11   LC80090112015251LGN00   08 Sep 15
2015   267   16    9   11   LC80090112015267LGN00   24 Sep 15
2015   283   16    9   11   LC80090112015283LGN00   10 Oct 15
« Last Edit: November 04, 2015, 12:43:44 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1109 on: November 05, 2015, 12:30:03 PM »
It actually works out quite well to have a 700x700 pixel limitation on what the blog will animate directly (without a visitor click) because at the 15 m resolution of Landsat, this amounts to a convenient block 10.5 km x 10.5 km, which means tiling can be done with 250 m overlap on each side providing 10 km of new block.

To animate ice movement for the entire perimeter of Greenland would only take 2-3 blocks at most locations, beyond which the ice is moving slowly and predictably downslope from the central summit ridge. The three prominent exceptions would be Jakobshavn, Petermann and the Zachariae bowl where convergent inflow is fast and significant.

At Jakobshavn, the ice is still moving rapidly enough ~80 km inland that co-registratable Landsats of a single year provide an an adequate number of animation frames for determination of flow lines (forward trajectories of identifiable features).

These flowlines are the primary scientific objective; as discussed earlier by sidd and others, they cannot be reliably determined from theory, even if the surface velocity field and DEM elevation are known.

That's despite the fact that mathematically a unique flowline solution exists for any vector field whose tangents at every point are the velocities; the problem has to do with singularities in the actual flow and wild error propagation in computing line integrals from coarsely gridded data. It's better to extract actual flowlines from fast animations and where these depart significantly from ice stream models, determine the physical basis.

Farther inland, the ice is moving slower. It becomes necessary to switch to multiple years of orbit 08 11 to obtain eastward coverage with retention of rock ground control points and adequate motion between frames, which becomes problematic after becoming commensurable with co-registration error (~5m).

Since Landsat scenes are scheduled at 16 day intervals at best and perhaps two-thirds are too cloudy or poor contrast over the regions of interest, inland limits are reached well short of the summit ridge where 15 years would be needed for a single Landsat pixel of movement. Landsat-7 had only 8-bit panchromatic, not enough for satisfactory contrast rescue in even-toned snowy scenes.

Closer in, inter-year velocity comparisons are feasible for 2013-15. That even allows a measure of acceleration by comparing 2013-14 to 2014-15. However a limitation here is that Landsats of matching dates won't generally be available. Even matching calendar dates wouldn't provide a valid match between variable melt years.

The source code for Earth nullschool is open source. To make the all-Greenland map, it may ultimately be better to abstract Landsat animations over to the nullschool gremlin system. Here, instead of isobar levels, it would make sense to offer velocity levels to allow visualizations over a wide range of icesheet speeds.

I've posted the last of these Jakobshavn animations below. Note farther east in the accumulation zone, contrast issues get worse presumably due to fresh snowfall and wind-blown drifting. This degrades the visual appear of the animation though it is still feasible to track ice movement (which is due east north of the icestream but twisting northeasterly below the ice stream).

The table below shows the suitable Landsats for path,row 08,11 for the last three years for those wishing to schedule a bulk download. The second column is numbered day of year, the third is elapsed days between scenes, and the fourth shows cumulative elapsed days since the first scene of 11 Apr 13.

Thus east of Jakobshavn 857 days are available between the first and last scenes. This provides a maximal velocity detection sensitivity of 6.4 m/yr (for a 15 m or 1 pixel shift in position). That will improve to 5.0 m/yr with a April 2016 Landsat.

2013  101    0    0  11 Apr 13  08  11  LC80090112013101LGN01
2013  142   41   41  22 May 13  08  11  LC80080112013142LGN01
2013  261  119  160  18 Sep 13  08  11  LC80090112013261LGN00
2013  302   41  201  29 Oct 13  08  11  LC80080122013302LGN00

2014  081  144  345  22 Mar 14  08  11  LC80080112014081LGN00
2014  097   16  361  07 Apr 14  08  11  LC80080112014097LGN00
2014  161   64  425  10 Jun 14  08  11  LC80080112014161LGN00
2014  193   32  457  12 Jul 14  08  11  LC80080112014193LGN01
2014  209   16  473  28 Jul 14  08  11  LC80080112014209LGN00
2014  305   96  569  01 Nov 14  08  11  LC80080112014305LGN00

2015  052  112  681  21 Feb 15  08  11  LC80080112015052LGN00
2015  132   80  761  12 May 15  08  11  LC80080112015132LGN00
2015  148   16  777  28 May 15  08  11  LC80080112015148LGN00
2015  180   32  809  29 Jun 15  08  11  LC80080112015180LGN00
2015  196   16  825  15 Jul 15  08  11  LC80080112015196LGN00
2015  212   16  841  31 Jul 15  08  11  LC80080112015212LGN00
2015  228   16  857  16 Aug 15  08  11  LC80080112015228LGN00
« Last Edit: November 06, 2015, 11:38:28 AM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1110 on: November 06, 2015, 01:10:45 PM »
The Landsat season wrapped up in a final cloudy scene on November 2nd. All in all, this is a very under-unanalyzed resource. Was there any speedup at Jakobshavn of the overall icesheet between 2015-14 relative to 2014-2013 (the Landsat-8 era)?

After co-registering the images, a region NE of the elbow can be viewed in RGB (ie interferometrically). Any speedup between the two years is very subtle because the spacing of fringes remained about the same. The main ice stream is moving so fast that a different method would be needed there.

LC80080112015148LGN00_B8
LC80080112014161LGN00_B8
LC80080112013142LGN01_B8

I am moving on to Petermann to write up what is really going on with upheavals (hint: begins with a z). The motion of that glacier is quite interesting as well. The animation below shows the area up-glacier from the grounding zone.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1111 on: November 06, 2015, 04:17:18 PM »
The Landsat season wrapped up in a final cloudy scene on November 2nd. All in all, this is a very under-unanalyzed resource. Was there any speedup at Jakobshavn of the overall icesheet between 2015-14 relative to 2014-2013 (the Landsat-8 era)?


Nope there was another Landsat from Jakobshavn Nov. 4 : LC80080112015308LGN00

But nothing of real interest?

Edit: There are signs of a major calving soon!!!!
« Last Edit: November 06, 2015, 04:59:10 PM by Espen »
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1112 on: November 06, 2015, 05:16:38 PM »
Here are the pre-calving cracks at Jakobshavn:

The area preparing to leave is approx. 6 km2.
« Last Edit: November 06, 2015, 05:49:38 PM by Espen »
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1113 on: November 06, 2015, 07:56:22 PM »
Nice spotting! That image will prove to be a very important addition to our collection of path,row 08 11's -- clear skies and a record low sun angle of 5.0º is very favorable for shadows and topographic mapping. Note this is UTM_ZONE = 23 rather than 22 which probably accounts for residual co-registration issues.

Anybody have a tool that can reproject UTM 23 images into 22? This path,row has its feet in both but USGS tipped it to 23. These cannot be aligned by simple translations and rotation in the way that 09,11 and 10,11 can be.

The contrast is a bit congested but there is still quite a bit of inherent detail, further proof that the higher bit depth of Landsat-8 represents a very significant instrumental improvement over its predecessors.

There are some other puzzling features on the north side in addition to the late-season cracks preconditioning the front to calving that we can follow this year in Sentinel IW. I'll toss this one into the animation series in a bit and take it back to the first in the series, 11 Apr 13.
« Last Edit: November 06, 2015, 08:15:51 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1114 on: November 06, 2015, 08:25:06 PM »
Here is another shot:
(After a few dirty tricks in Photoshop)

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« Last Edit: November 07, 2015, 01:11:46 PM by Espen »
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1115 on: November 07, 2015, 12:10:41 AM »
Just in the shadows?

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1116 on: November 07, 2015, 08:30:54 AM »

Anybody have a tool that can reproject UTM 23 images into 22? This path,row has its feet in both but USGS tipped it to 23. These cannot be aligned by simple translations and rotation in the way that 09,11 and 10,11 can be.


something like:

gdalwarp -t_srs '+proj=utm +zone=22 +datum=WGS84'  source-tiff-file   target-tiff-file

(untested but works nice on Sentinel 1 images)

gdalwarp is of course one of the gdal tools, see www.gdal.org

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1117 on: November 07, 2015, 12:27:23 PM »
Very helpful, Wipneus.

Since the mega calving event is bracketed by day 226 (path,row 10,11 UTM 22) and day 228 (path,row 08,11 UTM 23), we do need to convert the latter to UTM22 for accurate area calved measurements.

That's LC80080112015228LGN00_B8.TIF (cropped to the western south branch) if anyone would like to test out that gdal command line for a comparison to the original.

However there is so much ambiguity already in just drawing the calving front and deciding what to do about bergs still hanging on that a slight error reduction from reprojection won't be significant, especially since over the meagre 5 km scale here the UTMs will hardly differ after low budget warping (translation and -4.45º rotation) to nearby common ground control points.

I wonder too if this is not better done at the level of raw Landsat instrument data. That is, they probably look at the scene center and say 'oh that's in UTM23 so let's just output a Level 1 product for the Little People in that coord system' even though 35% of the scene might be in UTM22. But do they even retain the raw data and the option of direct UTM22?

At this point we have 55 clear Landsat-8 scenes of the Jakobshavn calving front over 2013-15, of which 18 are path,row 08,11. Plus Sentinel IW scenes which nukefix has been reprojecting into UTM22. We expect these to come in every 12 days for the next six months, which amounts to 15.

Higher scene density is important in refining ice stream velocities which represent averages over the intervening time intervals). Here we would like to see if Jakobshavn surged immediately after the big calving event. That involves comparing to before and after averages: days 212-226 vs 226-228 vs 228-235. That amounts to counting pixel displacements of a feature on the central flowline on the final icestream straightaway. This definitely requires reprojection of days 212 and 228.

LC80080112015212LGN00
LC80100112015226LGN00

LC80080112015228LGN00
LC80090112015235LGN00


Including 'gdal' in a google search expression lead me to various tutorials that explain the flags and suggest some others:

http://www.geos.ed.ac.uk/~smudd/TopoTutorials/html/tutorial_raster_conversion.html
http://casoilresource.lawr.ucdavis.edu/software/gdal-and-ogr-geodata-conversion-and-re-projection-tools/
https://waterprogramming.wordpress.com/2014/09/30/interpolation-and-resampling-across-projections-and-spatial-resolutions-with-gdal/

Quote
4.7. Changing raster projections with gdalwarp

The preferred coordinate system is WGS84 UTM coordinates. To convert to this coordinate system you use gdalwarp. The coordinate system of the source raster can be detected by gdal [that's provided in Landsat's bundled metafile MTL.txt and, as part of the .tif specification, probably also sits within the B8.TIF header itself], so you use the flag -t_srs to assign the target source coordinate system:

    $ gdalwarp -t_srs '+proj=utm +zone=XX +datum=WGS84' source.ext target.ext

so one example would be:

    $ gdalwarp -t_srs '+proj=utm +zone=44 +datum=WGS84' diff0715_0612_clip.tif diff0715_0612_clip_UTM44.tif

There are several other flags that could be quite handy; for a complete list see here:

http://www.gdal.org/gdalwarp.html

        -of format: This sets the format to the selected format. This means you can skip the step of changing formats with gdal_translate. The preferred format is:

                ENVI for ENVI rasters.  GDAL deals with these files well and they retain georeferencing. We use the extension bil with these files.

So, for example, you could output the file as:

        $ gdalwarp -t_srs '+proj=utm +zone=44 +datum=WGS84' -of ENVI diff0715_0612_clip.tif diff0715_0612_clip_UTM44.bil

        -tr xres yres: This sets the x and y resolution of the output DEM. It uses nearest neighbour resampling by default. So say you wanted to resample to 4 metres:

        $ gdalwarp -t_srs '+proj=utm +zone=44 +datum=WGS84' -tr 4 4 diff0715_0612_clip.tif diff0715_0612_clip_UTM44_rs4.tif

IMPORTANT: LSDRasters assume square cells so you need both x any y distances to be the same

        -r resampling_method: This allows you to select the resampling method. The options are:
            near: nearest neighbour resampling (default, fastest algorithm, worst interpolation quality).
            bilinear: bilinear resampling.
            cubic: cubic resampling.
            cubicspline: cubic spline resampling.
            lanczos: Lanczos windowed sinc resampling.
            average: average resampling, computes the average of all non-NODATA contributing pixels. (GDAL >= 1.10.0)
            mode: mode resampling, selects the value which appears most often of all the sampled points. (GDAL >= 1.10.0)

So for example you could do a cubic resampling with:

        $ gdalwarp -t_srs '+proj=utm +zone=44 +datum=WGS84' -tr 4 4 -r cubic diff0715_0612_clip.tif diff0715_0612_clip_UTM44_rs4.tif

        -te <x_min> <y_min> <x_max> <y_max>: this clips the raster. You can see more about this above in Clip-using-gdal.

« Last Edit: November 07, 2015, 01:12:45 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1118 on: November 07, 2015, 01:14:47 PM »
In the future I will use twitter for important news about various glaciers in Greenland at http://twitter.com/ecoverycom these messages will be announced after being reported on Arctic Sea Ice Forum.
« Last Edit: November 07, 2015, 04:56:48 PM by Espen »
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1119 on: November 07, 2015, 02:28:10 PM »
First youtube calving videos, now twitter. What's next, a facebook page for each glacier in Greenland?
« Last Edit: November 07, 2015, 02:37:12 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1120 on: November 07, 2015, 02:35:44 PM »
First youtube calving videos, now twitter. What's next, a facebook page for each glacier in Greenland?

No definitely not Facebook, but the other channels are ok, we may reach a broader audience?

But I do have a few domains .com and .net just in case ;)

 
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1121 on: November 07, 2015, 04:23:12 PM »
You can use without "s" if you want like that 
http://forum.arctic-sea-ice.net/index.php?topic=154.new#new

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1122 on: November 07, 2015, 04:29:27 PM »
The problem is people think this site is not safe?

I forgot the details, but the problem with the security certificate apparently is difficult to solve. The easiest solution is to link to an address starting with http (not https).

Good luck with the Twitter handle, Espen! I try to post on the ASIB as soon as something big happens. I wish I was able to translate more of the stuff you do to a more understandable level, but it's just too difficult for me to follow, sorry. But this thread is awesome, perhaps the best on the forum.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1123 on: November 07, 2015, 05:23:26 PM »
Yes we should think about a strategy to get out to a broader audience, I am sure the interest is there.
And I see no reason to be registered to watch images etc. on this site? 

I will let you (A-Team) do the invitation to a new thread exploring ways to get the message out to a wider audience?
« Last Edit: November 07, 2015, 06:03:02 PM by Espen »
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1124 on: November 07, 2015, 06:45:08 PM »
I announced the forum on the CRYOLIST mailing list at the time, inviting scientists to participate. Few did as far as I can see, but of course, the forum has grown since then.
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1125 on: November 07, 2015, 08:22:12 PM »
1)security cert: the warning we see are because Neven dares use a self signed key rather that one signed by a Certificate Authority, which is Frowned upon by the Powers That Be. In spite of the fact that every Certificate Authority in the world has been multiply compromised by those Powers and others, and using their certificates is worse than using nothing.

2)Rather than youtube, or in addition to youtube, please consider bittorrent, vimeo, and coralcache. Youtube does random takedowns, infringes intellectual property rights and is part of the Borg.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1126 on: November 07, 2015, 09:34:33 PM »
Quote
The thing to do might be invite known outreach-inclined scientists to submit signed guest pieces to the appropriate forums. For example it would be very timely to invite commentary -- or even host a round-table -- on the Zwally paper on Antarctica mass balance. We have a forum going already to put it in.

There are lots of scientists who work in this area just itching for a more dignified space and better message control than they get from newspaper quotes but want it one-off with no ongoing commitment.

Here I envision Neven as chief dignitary issuing the email invitation and if interest is expressed, passing them off to a specific forum editor (here AbruptSLR) who in turn might pass off a submission to a graphics, copy-editor and blog-htmler for forum-compatible formatting. This way the scientist need not register for the site nor learn arcane rules of blog syntax.

That would be interesting, but it wouldn't work, I think. First of all, it's the Arctic Sea Ice Forum, so the name isn't right. Second, I'm not the right person. Third, there's places like the RealClimate blog or Skeptical Science website, and they only very occasionally get guest blogs by experts, despite having much more clout than I would ever have.

No, I think the best thing to 'do', is for you guys to keep going like this, do the interesting stuff wrt analysis and image processing here, and when one glacier or other does something spectacular, I will use the blog, Espen uses Twitter, etc. People will know where to get it first (which is why the forum has almost 900 members now).

Like I said a while ago in the donations forum, and The Smiths sang: You should never go to them, let them come to you.  ;) ;D
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1127 on: November 08, 2015, 12:06:05 AM »
Neven,

Yes we have a problem, I have read a few articles on many sites like WaPo, DR, DMI etc. where this site is mentioned, but not directly referred, but then nonsense comments from a Waco professor is added with no relevance!

But that's just how things go, Espen. I don't there's anything we can do to change that, unless we make a really big effort. But you guys are here because it's fun, right? Or you get a kick out of it. And also to raise awareness a bit, of course.

The fact that this forum is even mentioned or linked to, is in itself amazing. A forum, for crying out loud.
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1128 on: November 08, 2015, 05:15:54 AM »
Put me down for the fun column. And the "get a kick out of it" column. And you need a column for "learn sumpn often from the forum" and a column for "i need to be reminded that i'm all wrong more often than i'd like" and a column for ...

On a more serious note, how many gig of image and video are on this site now, anyway ? i'm trying to figger how much storage for a mirror. I might be able to help.

But perhaps take this to a separate thread, rather than add to this one ?

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1129 on: November 08, 2015, 09:35:21 AM »
Yes, somewhere in the The forum category, perhaps here. Dungeonmaster probably knows these things.
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1130 on: November 08, 2015, 06:25:52 PM »
The "insecure site" problem of using https is a result of not having an SSL certificate specifically matching the virtual hostname (forum.arctic-sea-ice.net). Certs cost between $6 and $40 per year. This cost could probably be managed with member sponsorship if allowed by the hosting service.

The other issue of images being broken is a site policy of not serving images to people not logged into the forum. There probably is a reason for this policy but policies can be changed (usually) if desired.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1131 on: November 08, 2015, 06:32:49 PM »
I'm willing to discuss all of this, but not here.
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1132 on: November 08, 2015, 08:15:12 PM »
I went through the cloud-free Landsat-8's for three years 2013-2015 to study the motion of the north branch of the Jakobshavn icestream. Here the issue, in broadening the scope to the entire perimeter of Greenland (or at least the low-hanging fruit), is the boundaries of iceshed drainages.

However as I post these animations, you will note ice swirling around an obstruction to join another flow and hereby creating a prolonged zone of compression.

We know already that Swiss Camp does not drain into JI and the ice there moves so slowly that it is 200 years away from the coast despite being nearby!

Eqip is of considerable interest as it has the southernmost basal upheaval. These have been proposed to arise from basin flow compression and extensional thinning; however ice flowlines have not yet been determined at Eqip. Alternatively upheavals have multiple mechanisms and this one is not applicable at Eqip.

The first animation examines the proposition that UTM23 Landsat images are can be co-registered with very high precision. This shows a bicubic blowup of band 8 down to 3 m resolution around the channel dividing island which is the primary consistently visible ground control point. The 25 frames range from day 058 of 2013 to day 251 of 2015 and do indeed align very satisfactorily.

The second animation shows the same range of dates at 7.5 m resolution just north of the water and melange filling the north bank. At one time this branch contributed significantly to buttressing of the main south channel of Jakobshavn. That does not appear to be the case today. The area is of interest because of complex flows coming off the distant summit ridge and joining a flow to the south.

The drainage basin map adapted from JH Bondzio 2015 shows speeds but not flow lines in 2008-09 in Google Earth projection. The third animation shows the main flow in this northern region at 15 m resolution for the following scenes:

2013 140 10 11 LC80100112013140LGN01
2013 172 10 11 LC80100112013172LGN00
2013 293 09 11 LC80090112013293LGN00

2014 040 09 11 LC80090112014040LGN00
2014 056 09 11 LC80090112014056LGN01
2014 159 10 11 LC80100112014159LGN00
2014 184 09 11 LC80090112014184LGN00
2014 216 09 11 LC80090112014216LGN00

2015 043 09 11 LC80090112015043LGN00
2015 075 09 11 LC80090112015075LGN00
2015 091 09 11 LC80090112015091LGN00
2015 123 09 11 LC80090112015123LGN00
2015 155 09 11 LC80090112015155LGN00
2015 187 09 11 LC80090112015187LGN00
2015 235 09 11 LC80090112015235LGN00
2015 251 09 11 LC80090112015251LGN00

In a time series animation with many frames, the displacement vectors of recognizable features can be computed for each pair of consecutive frames. These give jointed line segment that -- when smoothed with splines -- are excellent analytic approximations to flowlines.
« Last Edit: January 16, 2016, 09:27:48 AM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1133 on: November 13, 2015, 02:30:11 PM »
S-1 IW from 11.11.2015

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1134 on: November 13, 2015, 03:34:52 PM »
I must say Sentinel does an excellent job on auto-delineation of the calving front line; results are maybe similar to hand-tracing but more consistent and probably more objective. Note the discontinuous nature on the north end makes sense given the sliver of intrusive tributary ice coming in from the east.

The image below is simply made off a 3-day IW animation nukefix posted earlier by combining to RGB and normalizing contrast. Thus by rotating in each new date and scripting the process, we can sit back and watch an animation showing the current front flanked by the preceding and following.

The second image shows the same thing for earlier dates. It is simply cropped off the high resolution file version at the ESA web site that wipneus found. It is a montage of three dates around the big calving event  27 July, 13 August, and 19 August 2015. They seem to process Sentinel IW to higher resolution, whether that is real or helpful is up in the air.

http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2015/08/chasing_glacier_retreat/15562607-1-eng-GB/Chasing_glacier_retreat.jpg

However it's not so easy to lift off the calving front from the rest of the image nor to determine the area between them calved. That's as it should be: the calving front of Jakobshavn is rather a mess (3rd image, OO Becker ).

www.nytimes.com/2015/11/15/magazine/the-secrets-in-greenlands-ice-sheets.html
« Last Edit: November 13, 2015, 04:07:00 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1135 on: November 20, 2015, 03:26:29 PM »
The image below shows west-central Greenland ice sheet velocities newly determined by Sentinel. The image is taken from an all-Greenland map at the ESA link below. Note the interior is still very poorly mapped. There is likely higher resolution IW velocity data for Jakobshavn but that has not been processed. Because the velocity changes so much seasonally, the exact date pairs would have to be specified.

The second image examines the ice piracy issue in the vicinity of Jakobshavn. It would take very careful measurements from two years to determine the extent of any trends. Note the image supplied only shows speed, the direction of motion was determined but discarded.

Quote
This map of Greenland ice sheet velocity was created using data from Sentinel-1A in January–March 2015 and complemented by the routine 12-day repeat acquisitions of the margins since June 2015. About 1200 radar scenes from the satellite’s wide-swath mode were used to produce the map, which clearly shows dynamic glacier outlets around the Greenland coast. http://www.esa.int/spaceinimages/Images/2015/11/Ice_sheet_in_motion
« Last Edit: November 20, 2015, 03:34:02 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1136 on: November 24, 2015, 01:38:02 AM »
I met an out-of-town visitor who works for a satellite-image-producing agency who seemed to be interested in both the analyses done on these glacier threads and the discussion (above) about the inconsistent product availability from some agencies.  I was given the impression that "having to join" in order to see images would not be a barrier to scientists interested in the efforts shared on these threads. [edit: I went to the ASIF from a computer I am not logged in - i.e., as a 'guest', and could see the images and gifs on this thread just fine.]
« Last Edit: November 27, 2015, 07:24:34 PM by Tor Bejnar »
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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1137 on: November 27, 2015, 01:07:30 PM »
Movement between 23&11.11.2015

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1138 on: November 27, 2015, 04:54:09 PM »
Nice to see the action this late in the year! Quite feasible to accurately measure the post-elbow velocity ... now we need to figure out how to automate that and over-tint with appropriate colors.

I measured one spot on a central flowline quite a ways back from the calving front, obtaining 26 pixels of displacement which is 32 meters/day (assuming 12 day interval and yr images are 15 m resolution) which is maybe higher than expected for this date. The calving front thins and there the motion averages to 41 m/d, again quite high for this late in the season (3rd image).

The processing produces a nice looking animation but when I saved it out in various ways to various image editors, it always shows as a very posterized historgram, one that makes very little use of what is essentially a grayscale. I don't recall the original Sentinels as having this property (which limits opportunities to despeckle etc.).
« Last Edit: November 27, 2015, 05:57:57 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1139 on: November 27, 2015, 11:38:28 PM »
Oh I reduced the colors to 32 in GIF animator to make the file smaller, will make one with better fidelity.

edit: here it is with 128 shades of gray
« Last Edit: November 29, 2015, 08:50:21 PM by nukefix »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1140 on: November 28, 2015, 04:52:48 PM »
I have been perusing the above animation for the last day. Two of the most prominent features of the ice sheet that lies north of the main channel are the island (pointed out earlier by Espen) that is slowly emerging where the north and south branches split. Another prominent feature is the noticeable dome that lies directly north of the main channel where it turns sharply east and heads towards the interior of Greenland. If you look at the topography map, this dome is clearly the westernmost tip of the large island that is due north of the main channel.

Now if you look closely at the northern wall of the main channel that is moving quickly and calving into the main branch, I believe you can see evidence of a second very small island emerging. It is easiest to see by locating the ice that is projecting from the northern wall into the main channel about midway between the island and the dome. The topography map indicates a small island at this location.

Another noticeable indication is the crescent of whiter ice (grounded below sea level and moving more rapidly) that surrounds this slightly gray projection.
« Last Edit: November 28, 2015, 11:58:39 PM by Shared Humanity »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1141 on: November 29, 2015, 01:18:16 PM »
Quote
edit: here are Sentinels of 11 and 23 Nov 15 with 128 shades of gray
Interesting question overall, the point of diminishing returns with more bit depth. On Sentinel, my understanding the satellite hardware and processing bins the data so that the end user is looking at pixels taking on one of 256 shades of gray (ie 28 or 8-bit), like Landsat 1-7.

Landsat-8 captures 12-bit or 4096 grays but is provided after bicubic interpolation to 16-bit where processing can take place advantageously but ultimately has to drop down to 8-bit, what computer monitors display.

The bit depth isn't the whole story, especially for cryosphere scenes, because the region of interest may only utilize half or less of the available grays. Combined with dropping bit depth (to reduce file size or processing time), this can result in a severely degraded image with posterized histogram and few options for enhancement.

The first animation has cropped out a central fjord scene at the original resolution of #888. Here the the advancing glacier front is displacing individual bergs down-fjord en bloc with no change in relative positions, with the exception of a stationary block in the southern embayment where ice hangs up on a known submerged nunatak.

The left half of the second animation shows the similar use of gray in the two November scenes but with the histogram average shifted towards white by 14 units on the later date. It's not clear what this means in terms of radar reflectivity but the tones could be better matched by adding a constant gray to the lighter image. If hundreds of such scenes were stacked and the sole purpose was to display motion, this would reduce distracting flicker/

The right half of this animation examines the scene to see where the extreme dark and light regions are coming from. Unsurprisingly, exposed rocks on the south contribute most of the dark tones. However some of the bergs are dark too and that could carry significant information about the ice being calved.

Thus an otherwise desirable contrast stretch of the middle tones would have an unfortunate side effect of pushing darker tones towards pure black. The rocks could be masked (protected) from the stretch but it still wouldn't be very beneficial to the mid tones because these have already lost resolution from being posterized to 128 grays = 27 or 7-bits. Note rock cracks do not move at all -- the two scenes are perfectly co-registered.

The extremely light regions (right half of 2nd frame of 2nd animation) is mostly contributed by smaller bergs and is physical detail that probably needs to be retained.

In summary, had the original image been kept at 8-bits, greater tonal detail could have been obtained by masking out the rocks and stretching mid-tones over a wider range of grays at the expense of compressing the low end. Visually this would result in a barely perceptible improvement over 7-bit.

However if the scientific objective is measurement of velocities, the improvements brought by 8-bit and contrast optimization are hugely important to the accuracy of automated feature-tracking. It will also be argued that the billions spent on getting the satellite instrument up to 8-bit should not be squandered to save pennies in file processing and storage. (The forum does have issues however with too-large files.)

The flip side is what would have been gained had Sentinel been built to 10-, 12- 16- or 32-bit sensitivity? Probably something at 10-bit, less at 12-bit and not very much after that for the vast majority of end user communities.

Landsat is a different story. The third image shows how little of the grayscale space is actually utilized originally in snowy scenes. However given the higher bit depth, the contrast can be improved without losing tonality by the time it is (inevitably) re-displayed as 8-bit. Contrast is what can be distinguished at a given resolution; resolution is how many sq meters are contributing to an individual pixel.

The final animation shows that the 7-bit definitely has some advantages over the 5-bit posted earlier in #1140. Here the contrast has been adjusted to bring out features in the final km of the calving front, at the expense of fjord image quality. However the latter is still adequate to show there has been no significant calving over the last two weeks, only the glacier advancing, pushing older icebergs down the fjord.
« Last Edit: November 29, 2015, 03:44:27 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1142 on: November 29, 2015, 02:11:41 PM »
Quote
been perusing the above animation for the last day
Good use of time. We can expect more of the landscape to become exposed over the next several years. It is a little baffling why there has not been more erosion over the last 125 kyears. The ice stream has largely been content to stay in its winding channel (which some people say reflects the course of paleo rivers draining a pre-isostatic compression gradient).

Now that we have both Landsat and Sentinel in 15 m UTM, it would be good to prepare a matched set of animation dates. That is possible at the lcm of 16 day and 12 day orbital repeats, namely 48 days since we did have such a match in October.

Even though Sentinel records at a wavelength far far beyond human visual capabilities, it has been dropped down to where we can see it as intensities at the same range of wavelengths as Landsat via rod rhodopsin (since both are effectively grayscale despite Sentinel polarizations and 11 Landsat bands).

At this point, the synergy between these different ways of viewing the same landscape on the same dates has not been explored to any extent. I have seen zero guidance from ESA or the scientific literature on Sentinel image interpretation of cryosphere scenes. Combining polarizations into a color scheme has to be informative but so far that's been opaque.

So by viewing the two animations side by side over and over (in conjunction with bedrock, surface, and velocity maps), we might be able to make a worthwhile contribution in this area. It's better though to make one animation by combining Landsat and Sentinel channels.

Rather than RGB, it might be worthwhile to try HSV with Hue one polarization, Saturation the other and Value the Landsat. A more attractive alternative -- as it's built around actual visual perception -- might be CIELab color space, with L coming from Landsat and a and b opposing channels coming from polarizations. These color spaces only make sense if there's pixel perfect registration (in the meeting ground, UTM24).

There's another visualization option here that we haven't used on these forums, decorrelation. That is, from two Sentinel polarizations and band 8 of Landsat of the same day, take the first principle component from PCA analysis. This will be a grayscale. Repeat over three dates obtain three grayscales that can be combined as a single color image.

You ask if you can scroll through all the myriad possibilities, pre-computed? Yes, there is a script for that. It's widely used to find optimal viewing conditions for petroglyphs. The ImageJ plugin was discussed back at #449.

Changes seen in the animation are really about the time evolution of the glacial system as seen through these peculiar but powerful sets of lenses.

https://en.wikipedia.org/wiki/Lab_color_space
« Last Edit: November 29, 2015, 03:23:31 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1143 on: November 29, 2015, 02:53:18 PM »
Quote
been perusing the above animation for the last day
Good use of time. We can expect more of the landscape to become exposed over the next several years. It is a little baffling why there has not been more erosion over the last 125 kyears. The ice stream has largely been content to stay in its winding channel (which some people say reflects the course of paleo rivers draining a pre-isostatic compression gradient).

https://en.wikipedia.org/wiki/Lab_color_space

For myself, finally being able to locate this small island under the ice helps me to realize how quickly the north wall, grounded below sea level, is calving and retreating. If I had any doubt that this receding was unpinning the main stream of ice, locating this island has eliminated it. I actually think that Jacobshavn is entering into an entirely new regime as a result. Couldn't the northern portion of the main stream be much more prone to calving as a result? Would the recession of  the northern wall also trigger an increase in the speed of the glacier on a more or less permanent basis?
« Last Edit: November 29, 2015, 02:59:47 PM by Shared Humanity »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1144 on: November 29, 2015, 03:32:41 PM »
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Would the recession of  the northern wall also trigger an increase in the speed of the glacier on a more or less permanent basis?
No. The Jakobshavn ice stream runs down a perfectly straight channel, there is no 114º bend, it is completely uniform with depth under any flow line, the walls are smooth U-shaped parabolas, friction from the non-idealized channel does not warm the ice and enhance creep, no ice comes in from the sides now or ever, only its personal melange provides buttressing today, no hydrated glacial till sits in bedrock overdeepenings, upstream tributary confluences can be ignored, and ice piracy plays no future role.*

*See any peer-reviewed paper from the last 30 years.
« Last Edit: November 29, 2015, 03:54:35 PM by A-Team »

oren

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1145 on: November 29, 2015, 08:32:26 PM »
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Would the recession of  the northern wall also trigger an increase in the speed of the glacier on a more or less permanent basis?
No. The Jakobshavn ice stream runs down a perfectly straight channel, there is no 114º bend, it is completely uniform with depth under any flow line, the walls are smooth U-shaped parabolas, friction from the non-idealized channel does not warm the ice and enhance creep, no ice comes in from the sides now or ever, only its personal melange provides buttressing today, no hydrated glacial till sits in bedrock overdeepenings, upstream tributary confluences can be ignored, and ice piracy plays no future role.*

*See any peer-reviewed paper from the last 30 years.

You actually made me laugh out loud  ;D

oren

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1146 on: November 29, 2015, 08:36:40 PM »
For myself, finally being able to locate this small island under the ice helps me to realize how quickly the north wall, grounded below sea level, is calving and retreating. If I had any doubt that this receding was unpinning the main stream of ice, locating this island has eliminated it. I actually think that Jacobshavn is entering into an entirely new regime as a result. Couldn't the northern portion of the main stream be much more prone to calving as a result? Would the recession of  the northern wall also trigger an increase in the speed of the glacier on a more or less permanent basis?

I too believe this retreat of the north wall is significant. Yes the main stream should be more prone to calving, yes the glacier might speed up. Plus the total flux should increase with some ice flowing/calving from the direction of the north wall.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1147 on: December 14, 2015, 06:30:45 PM »
Just an update via the 05 Dec 15 Sentinel IW ... same old icebergs visible as in 11 and 23 Nov but a rather unusual slow motion calving event seemingly in progress (arrows).

The second image, upstream at the confluence, suggests radar has some interesting prospects for tracking the status of meltlakes during the winter. (Of course there is nothing to compare it with as Landsat won't be on the job again until early February.)

The third image is an oblique photo of the island divider between the north and south branches, date and credit unkown. Note how the back section, which we know moves rapidly from right to left, is deeply crevassed, whereas we know from multi-year animations that the front section is stable enough to host an elder hostel. There are no exposed rocks and none coming here from dynamic thinning. The crevassed slow motion ice fall in the very front formed as flat ice tries to drape over domed rock. Depth to bedrock is not known accurately but is probably tens of meters.

The fourth image shows a deeply shadowed image of the calving front, apparently by M Studinger on IceBridge. I could not locate anything of higher resolution. This is unfortunate because there is a lot of good information in oblique low sun angle shots. However they are wrongly perceived as having little scientific value and so are taken and distributed haphazardly as artwork.

Methods in the new paper resolving bathymetry from AirGrav [Boghosian 2015] proved ineffectual at Jakobshavn due to lack of a nearby reference density column and poor performance of radar at delineating bedrock and sediment. The best model is still said to be Gogineni 2014 though the actual bedmap dataset from that exercise is not actually provided by the paper in a usable form. The main fjord sill appears to be down by Illulisat, with a significant gap in the NW corner, though bathymetry of the fjord itself remains very spotty.
« Last Edit: December 16, 2015, 11:54:26 AM by A-Team »

oren

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1148 on: December 15, 2015, 07:29:54 AM »
Regarding the second image, wow, the radar shows local hills and valleys perfectly, much better than Landsat at this. And the melt lakes are also shown very clearly. I guess it's because the local reflections come from planes at different angles, and ice of different characteristics. So an animation of multiple radar images at this resolution will be able to show not just whether melt lakes are stationary, but also details about their underlying topography and how it changes over time. For example, do "hills" and "valleys" move with the stream or remain in place.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1149 on: December 15, 2015, 05:29:37 PM »
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do "hills" and "valleys" move with the stream or remain in place?
Good post. I am not aware of any comprehensive analysis of stationary features on the western Greenland ice sheet. Of course we don't want a list so much as the physical explanations and implications, which would be both historic and futuristic.

There's a distinction between literally immobile ice (eg over the branch dividing island) and stationary resonances (standing waves). Even if the underlying bedrock were a perfectly even plane with fixed friction and constant geothermal flux, there can still be surface features (such as pancake dough rippling: northern Greenland's Jungersen glacier #493 Pine Island forum photo). However unseen basal topography, stick-slip movement, pressure changes, and variable viscosity etc are more typically implicated in features such as stationary meltlakes (ie stationary surface depressions which collect meltwater).

The faux stationary features are in a steady state of change coupled with re-creation as dyed ice would be seen moving horizontally with the entire ice sheet as it slumps towards the coasts. The ice is moving vertically as well -- thinning, melting, sublimation, snowfall accumulation, snowdrift, subglacial meltlake fill/drain events etc but that's not apparent from downward-looking satellites like Landsat lacking radar ranging.

Landsat looks straight down and sees only sun shadows; the sun's angle of elevation varies helpfully with season yet the scene azimuth (local noon) hardly varies, limiting inference. There are no overhangs in western Greenland. Sentinel's incidence angle ranges from 20o to 45o depending on operational mode and so sees topographic blockage of radar returns as shadows but again there is the limitation of repeated orbits. Interferometric mode measures motion along the line of sight.

For sure we could update this nicely written November 2009 paper of Bindshadler, Scambos et al to Landsat-8 and Sentinel 1A:

Quote
Differencing of digital satellite image pairs highlights subtle changes in near-identical scenes of Earth surfaces. Using the mathematical relationships relevant to photoclinometry, we examine the effectiveness of this method for the study of localized ice sheet surface topography changes using numerical experiments.

We then test these results by differencing images of several regions in West Antarctica, including some where changes have previously been identified in altimeter profiles. The technique works well with coregistered images having low noise, high radiometric sensitivity, and near-identical solar illumination geometry. Clouds and frosts detract from resolving surface features. The ETM+ sensor on Landsat-7, ALI sensor on EO-1, and MODIS sensor on the Aqua and Terra satellite platforms all have potential for detecting localized topographic changes such as shifting dunes, surface inflation and deflation features associated with sub-glacial lake fill-drain events, or grounding line changes.
http://tinyurl.com/nlfgmnd
We have not done a lot of coverage here of recent papers by O Sergienko and coworkers that speak to this very topic. The image below shows predicted surface ribs at Jakobshavn induced by basal shear stress distributions at the ice/bedrock interface. It's not clear how such theories are tested.

http://www.sciencemag.org/content/342/6162/1086.short (Pine Island and Thwaites)
http://onlinelibrary.wiley.com/doi/10.1002/2014GL059976/full (organized surface patterns)
http://adsabs.harvard.edu/abs/2015EGUGA..17.5867H
http://rspa.royalsocietypublishing.org/content/470/2171/20140185.full
http://onlinelibrary.wiley.com/doi/10.1002/2014GL062248/pdf
http://www.ingentaconnect.com/content/igsoc/agl/2011/00000052/00000058/art00003
http://geosci.uchicago.edu/pdfs/macayeal/2007GL031775.pdf
« Last Edit: December 16, 2015, 11:43:51 AM by A-Team »