Help aligning DEM: Rotation/Translation issues

[forwork]

Hello,

I am working with Lunar South Pole stereo data and have successfully generated a DEM that is structurally very sound. However, I am facing A issue with its absolute placement relative to my reference DEM (LOLA: LDEM_87s_5mpp.tif).

My generated DEM sits a few kilometers away from where it should actually be. The rotation (angle) is slightly off, and it needs a translation (forward/backward shift) to sit perfectly over the reference DEM.

When I attempt to align it, the DEM seems to sit around the main target position across different iterations, but it refuses to lock onto the exact coordinates.

The pipeline I followed is:

• Bundle adjust, parallel_stereo, point2dem

• Multiple iterations of pc_align after the parallel_stereo's point2dem, hoping it would eventually settle.

• Using pc_align with a specifically calculated --max-displacement based on the measured distance difference.

• Applying the --highest-accuracy, algorithm flags in pc_align.

• Running map projection, gcp_gen followed by bundle_adjust and then mapproject and many other pipelines and commands i tried.

Despite these efforts, the DEM keeps shifting around the true position rather than snapping into place.

Help me with:

1. What is the recommended flags to handle a shift of a few kilometers or make a little rotation using pc_align command and it's iterations?

2. If I want to strictly correct the angle and bring the DEM forward/backward manually, what pipeline can be followed?

Thank you so much.

[Oleg Alexandrov]

A few km of translation can be a lot. Likely the ICP algorithm got confused. Large max displacement also means many outliers. 

Try to resample your DEM to same projection and resolution (grid size) as the LDEM_87s_5mpp.tif and crop them to a big enough box having both of them. You can hillshade these results. If they look similar enough try 

sparse-features hillshading-based alignment https://stereopipeline.readthedocs.io/en/latest/tools/pc_align.html#feature-based-alignment

corelation-based alignment https://stereopipeline.readthedocs.io/en/latest/tools/pc_align.html#correlation-based-alignment

It is suggested hillshading with gdaldem hillshade. The doc has more info. 

These should bring the clouds in the ballpark enough for ICP then to refine the remaining transform, perhaps. If you resume pc_align later with the initial transform from this, can set a notably smaller max displacement, as that is applied after the big transform is applied.

For the first of these alignment methods, you should also be able to inspect the match file between the hillshaded DEMs. It shold also print the observed shift, which will hopefully be on the order of what you expect.

Hope it works. Let me know if not.

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