Second Thought on LRO-NAC NoProj

Mark Rosiek from USGS Astrogeology expressed some doubt in my noproj recipe for LRO-NAC. This is completely reasonable because if everything were perfect, there would be no offset after the noproj step between the LE and RE CCDs. He requested 2 DEM samples of Tsiolkovsky Crater so he could compare to USGS work. I decided I’d try the same during free time through out the day. Unforunately I couldn’t find a trusted reference DEM to compare against. I can’t find ASU’s result of this area on their RDR webpage and it is impossible to use LMMP Portal. Can you even download the actual elevation values from LMMP? No I don’t want your color render or greyscale! 

Next best idea is to just difference the two DEMs I created and compare them. I didn’t bundle adjust these cameras at all so their placement to each other is off by some ~150 meters. After ICPing them together and then differencing them I can produce an error map. The gradient or shape of the error can help clue into how bad my fiddling in the ideal camera was. Below is the result between the stereo pairs M143778723-M143785506 and M167363261- M167370048.

You can definitely see the CCD boundary in this error map. You can see the CCD boundary in the hillshade. It’s about a 1 meter jump when looking at a single DEM. Error map seems to agree with this and shows a peak error at 4 meters in the CCD boundary location.

So what is the cause of these errors? Well we have two sources, (1) the projection into the ideal camera model and (2) bad spacecraft ephemeris. I’d argue that most of the difference between these two DEMs is the spacecraft ephemeris. That’s top priority in my book to correct. However when I look at the disparity map for these stereo pairs, there is a definitive jump at the CCD boundary. The cause of that would be an improper model of the angle between LE and RE cameras in ISIS. This should be expected. They’re currently not modeling the change in camera angles with respect to temperature. Also when looking at the vertical disparity, it seems one side is always brighter than the other. This suggests that I didn’t quite have the correct values for input to handmos.

Trying to fix all of this now might be a mistake on my part. I know that ASU will eventually produce new SPICE data that contains corrections from LOLA and a USGS study. I also imagine that eventually the work of E. J. Speyerer et al. will be implemented in ISIS.

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  1. Pingback: Advances in LRO-NAC processing in ASP | LUNOKHOD

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