Strategy for comparing DEMs

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christina Toldbo

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Sep 8, 2022, 4:59:22 PMSep 8
to Ames Stereo Pipeline Support
Hello again

I am trying to understand how asp works by playing with all the settings. I have an image pair with a resolution of 64 m/pix and a steep crater in the frame. I would like to generate different DEMs using different settings (and later other stereo pairs from the area) and compare these DEMs by subtracting them. As I understand it these DEMs can be different either due to how they are generated (as a result of the images, settings etc.) but also because a lack of control when comparing them.

I am a bit confused about all the options for alignment and control and how they influence the solution (ie. PC_align, bundle_adjust, map project using ISIS cam2map and map project using ASP). Should I perform all of them? is there an order that is preferred?
In short, how can I assure that the DEMs are aligned for comparison in the best possible way that introduces the least bias. I have no 'truth' DEM or independent measurements to compare against. 

I hope this question makes sense, and sorry for it being somewhat vague.

best,
Christina



Oleg Alexandrov

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Sep 8, 2022, 5:21:06 PMSep 8
to christina Toldbo, Ames Stereo Pipeline Support
Bundle adjustment makes the cameras more self-consistent, so rays emanating from matching points intersect better. You can start with no bundle adjustment at all as it may not be necessary.

pc_align is used to bring your DEM closer to a reference DEM. With or without bundle adjustment.

Mapprojection helps when you have very steep areas and you see obviously unresolved regions. On terrain which is not too steep and camera perspectives are not too different, it may not be necessary.

The order, if you use all of them, is bundle_adjust, mapproject, stereo, pc_align. 

I will suggest experimenting on small clips, as this can greatly reduce the amount of time for trying things out. The stereo_gui program, the ISIS crop command, and the gdal_translate tools can create crops (for various purposes).

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christina Toldbo

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Sep 8, 2022, 5:59:24 PMSep 8
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Thank you for your always fast answers :)

A follow up question: Would you determine if it is necessary to do bundle_adjustment based on the intersection error? Or is there another parameter?

- Christina




Oleg Alexandrov

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Sep 8, 2022, 6:08:57 PMSep 8
to christina Toldbo, Ames Stereo Pipeline Support
Yes, you can tell if you need bundle adjustment based on the intersection error, as computed with point2dem --errorimage. Also, during stereo_pprc, there's a message about triangulation error, based on interest points, which correlates with what you'll get with point2dem. Triangulation errors are in units of meters, so you need to know your pixel size on the ground (ground sample distance, computed by mapproject) to be able to do a fair comparison.


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