Parallel_stereo with mapprojected images over glaciers

[Arnab Sarkar]

Hi, I had been using normal stereo and parallel_stereo, but as I used parallel_stereo with mapprojected images, I found that the steep slopes get captured better with fewer voids in the DEM. 

I was wondering about an issue, as I am using Copernicus or SRTM as a reference DEM (by first converting its datum to ellipsoid). My DEM contains hilly slopes and glaciers. So, when I am mapprojecting my current images with a DEM which is old enough (of 2000 or 2015), will my glacier elevation get biased by the reference dem elevation at the glacier? or will it not be impacted by the reference? 

As a glacier's elevation changes with time, so does using mapprojected images with the reference DEM of a different time a correct approach?

[Oleg Alexandrov]

>will my glacier elevation get biased by the reference dem elevation at the glacier? or will it not be impacted by the reference? 

That is quite possible, especially if your glacier does not have much texture. Then stereo correlation, especially with the asp_mgm algorithm, could perhaps find some textures that comes from the underlying mapprojection DEM, which would be incorrect.

It is suggested to blur the mapprojection DEM quite a lot, and also try this experiment with different DEMs or at least different level of blur. 

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[Arnab Sarkar]

Thanks for your quick response.

I will definitely try with a more blurred and multiple levels of blurring. I had a connecting query to this. Do the elevation of the changed surface for my case, the glacier be shifted to the reference DEM one? I mean a vertical shift or bias, or will it be calculated based on the given new fore and aft mapprojected tifs and RPC files?

Appreciate your thoughts on this.

Regards,

Arnab

[Oleg Alexandrov]

>Do the elevation of the changed surface for my case, the glacier be shifted to the reference DEM one? I mean a vertical shift or bias, or will it be calculated based on the given new fore and aft mapprojected tifs and RPC files?

Ideally both of these should be true. So yes, your glacier elevation should not depend on reference DEM, and it should be calculated based on new mapprojected tif files.

So mapprojection DEM is just a helper. But in practice, as before, this one can bleed over into the solution, which is undersriable. 

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