Steep boundaries being "doubled" - WV02 imagery

[Elaine Collins]

Hi all, 

I’m attempting to create a DEM using WV02LV1B imagery (captured 2024-07-14). My workflow is the following: 1) bundle_adjust images, 2) mapproject each image onto a reference DEM (2m lidar in UTM10N), 3) run parallel_stereo with asp_mgm and subpixel-mode 9, and 4) use point2dem.

My issue is that the final DEM seems to blur sharp/steep contrasts (see figure below). This is pretty consistent throughout the ASP DEM, where steep boundaries appear to be “doubled” with a ~12-15m offset. I’ve tried adjusting the reference DEM resolution, running parallel_stereo with subpixel-mode 2, not bundle adjusting, running on a smaller region, and still no luck…

Is this just a limitation of ASP for steep areas or is there something obvious that I’m missing?

Thanks!

-Elaine

[Oleg Alexandrov]

These are likely phantom edges due to the DEM you mapproject onto. It is suggested to apply a big blur to your input DEM. We have a small writeup here: https://stereopipeline.readthedocs.io/en/latest/next_steps.html#hole-filling-and-smoothing-the-input-dem

Since your input DEM is at quite a high resolution, perhaps it could be resampled first to something notably coarser before blurring it, or apply even more blur than suggested in that doc. The idea here is that those steep areas should be shallow or smooth enough to not deform the images much during mapprojection and then show up in subsequent stereo.

This can be tried first on a small area, as likely will work the same way everywhere.

The 12-15 m of offset likely also suggests notable misalignment between the DEM you will get later and the reference one. If your reference DEM is not relative to the ellipsoid, but rather to the geoid, one may correct it first. That is also covered at the link above (a little higher up). In any case you will need alignment later between the ASP DEM and your reference one.

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[Elaine Collins]

Thank you for the suggestions! It seems like the misalignment was the issue - correcting the reference DEM to the ellipsoid using dem_geoid fixed the phantom edges.