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Topics - A-Team

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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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