Creating a registered LRO-NAC DEM without GCPs

I’ve previously written a tutorial on performing a bundle adjustment, jigsaw in ISIS lingo, using CTX. Let’s try something a little bit more complex with LRO-NAC. If you are not previously aware, LRO-NAC is actually two separate optical assemblies that are joined by the spacecraft bus. Ideally this spacecraft hardware would be solid as a rock and wouldn’t bend. Unfortunately that’s not the case, so what would have been a bundle adjustment between 2 cameras is now 4 in the case of LRO-NAC.

LRO-NAC Data Preparation

Picking a stereo pair has always been the most difficult part of the job for me. I honestly don’t know how to find stereo pairs. So instead I’m just going to recreate something ASU did in SOCET SET. If you are not aware, ASU has processed about 100 DEMs with LRO-NAC. I’m going to recreate their Moore F Crater DEM using M125713813 and M125720601. You can download the raw data via PDS.

I picked this stereo pair blindly and it ended up being extremely difficult to register correctly. In order to help out, I added a fifth image to the bundle adjustment to give another orbit’s worth of spacecraft ephemeris into the measurements. That image was M110383422LE and I picked it because it overlapped 2 of my previous images and had the same lighting conditions as my previous images.

Once you have downloaded all the images, prep the files in ISIS with the following:

parallel "lronac2isis from={} to={.}.cub; lronaccal from={.}.cub to={.}.cal.cub; spiceinit from={.}.cal.cub; rm {.}.cub" ::: *IMG

* The ‘parallel’ command is GNU Parallel, a non-standard system utility that I love dearly.

Attaching a custom Elevation Source

From a previous article I wrote about, we saw that map projecting against the WAC Global DTM gave much better results than working against the sparse LOLA that is the default for map projecting by in ISIS. So I’m going to do that now. Once we have a successful jigsaw result, then we’ll map-project. If you forgot how to attach the WAC Global DTM, here are the commands I used. My image pair was at 37.3N and 185E. So the tile WAC_GLD100_E300N2250_100M.IMG from this link was what I needed.

pds2isis from=WAC_GLD100_E300N2250_100M.IMG to=WAC_GLD100_E300N2250_100M.cub
algebra from=WAC_GLD100_E300N2250_100M.cub to=WAC_GLD100_E300N2250_100M.lvl1.cub operator=unary A=1 C=1737400
demprep from=WAC_GLD100_E300N2250_100M.lvl1.cub to=WAC_GLD100_E300N2250_100M.lvl2.cub
rm WAC_GLD100_E300N2250_100M.cub WAC_GLD100_E300N2250_100M.lvl1.cub

parallel spiceinit from={} shape=USER model=$PATH_TO/WAC_GLD100_E300N2250_100M.lvl2.cub ::: *cal.cub

Making the control network

Making the control network is pretty straightforward and is just time consuming. I performed the commands I previously outlined here on this blog.

Autoseed.def:

Group = PolygonSeederAlgorithm
      Name = Grid
      MinimumThickness = 0.01
      MinimumArea = 1
      XSpacing = 600
      YSpacing = 1000
End_Group

AutoRegTemplate.def:

Object = AutoRegistration
   Group = Algorithm
     Name         = MaximumCorrelation
     Tolerance    = 0.7
   EndGroup

   Group = PatternChip
     Samples = 19
     Lines   = 19
     MinimumZScore = 1.5
     ValidPercent = 80
   EndGroup

   Group = SearchChip
     Samples = 120
     Lines   = 100
   EndGroup
 EndObject

Commands

parallel footprintinit from={} ::: *cal.cub
echo *.cub | xargs -n1 echo > cube.lis
findimageoverlaps from=cube.lis overlaplist=overlap.lis
autoseed fromlist=cube.lis overlaplist=overlap.lis onet=control.net deffile=autoseed.def networkid=lronac pointid=???? description=moon
pointreg fromlist=cube.lis cnet=control.net onet=control_pointreg.net deffile=autoRegTemplate.def
qnet (editting and fixing control_pointreg.net)

I had to spend a lot of time in Qnet for this stereo pair. About half of my automatic matches didn’t work. (Possibly my search range was too small?) In order to correct this, I had to then manually edit the output control network in Qnet. I then filter down all the control points to the ones marked ‘Ignored’. Then I just manually align the measurements. You just need to get the measures close to the right spot by clicking on the same feature. Afterwards, you can then click the ‘Register’ button to get subpixel accurate alignment.

Autoseed and AutoReg also have a bad habit of only matching LE images to LE, and RE images to RE. It is a good idea to run along the edge of an LE image and try to find common points that are seen in both RE images. These are going to be the few control points that have more than 2 control measures. I also had to do this between my 5th image and the other 2 images that it had overlap with. This problem was likely because the overlap between these 3 images didn’t meet the requirement of MinimumThickness in Autoseed.def.

Normally at this point I would then recommend to you to start picking out ground control points. I tried that. Then I went a little insane. I’m better now. Instead we’re going to do crazy stuff in the next section.

The reason we’re not using GCPs, is that the only source to align against is the WAC Image mosaic. I tried this and I could easily get a bundle adjustment result that aligned well with the image mosaic. However the output DEM would always show a massive error between my height results and the results of LOLA and the WAC Global DTM. After adding the 5th image and clamping the trust of my GCP to 50 meters in radius, I got a result that matched LOLA and WAC DTM in an okay manner but didn’t really align at all to the WAC image mosaic. Things made sense once I compared the image mosaic to a hillshade of WAC DTM.

WAC’s Image mosaic doesn’t agree with WAC’s Global DTM! Ghaaaa! Madness! I was so incredibly disappointed. I didn’t know what to do at this point, so I just dropped the ground control points.

Bundle Adjusting

Alrighty then, time to start bundle adjusting with jigsaw. Likely on the first run things are going to blow up because bad measurements still exist in the control network. My solution to this was to first have jigsaw solve for only a few camera parameters and then check out the output control network for features that had the highest error. In Qnet you can filter control measures that have errors higher than a certain amount of pixels. After I fixed those measurements manually, I resave the control network as my input network and then redo the jigsaw.

Each time I redo jigsaw, I let jigsaw solve for more parameters. My pattern was:

jigsaw fromlist=cube.lis radius=no twist=no cnet= control_pointreg_gcp.net onet=control_jigsaw.net update=no
qnet
jigsaw fromlist=cube.lis radius=no twist=yes cnet= control_pointreg.net onet=control_jigsaw.net observations=no update=no
qnet
jigsaw fromlist=cube.lis radius=yes twist=yes cnet= control_pointreg.net onet=control_jigsaw.net observations=no update=no
qnet
jigsaw fromlist=cube.lis radius=yes twist=yes cnet= control_pointreg.net onet=control_jigsaw.net observations=no update=no camsolve=velocities
qnet

After iterating many times, I eventually produced a control network and jigsaw result that had a sigma0 of .34 pixels. However, if we’re to write this camera solution to file and then make a DEM, we would see that I have worse alignment with LOLA than if we hadn’t bundle adjusted at all.

So how can we get a good bundle adjustment result that aligns well with LOLA? One idea would be to pick GCPs directly against LOLA using their gridded data product. I know folks in the USGS who have done exactly this. However I don’t even see how they can determine correct alignment, so I’m going to ‘nix that idea. Another idea would be to have Jigsaw not solve for radius (radius=no), but instead use only the radius values provided by LOLA. This will keep problem in the ball park, but the resulting jigsaw solution will have a sigma0 around 15 pixels! Not solving for radius essentially ties our hands behind our back in the case of LRO-NAC when our radius source is so incredibly low res.

What we need instead is to simply constrain / rubber-band our Control Points to have radius values that are similar to LOLA. Unfortunately, how do we determine what correct Lat Long to sample the LOLA radius for each control measure? My solution was to not define that. I did two things: (1) I changed all my control measures from ‘free’ to ‘constrained’ with a sigma of 200,200,50. This can be performed with cneteditor. (2) I then wrote and ran the following bash script.

#/bin/env bash

input_control=control_pointreg_all_constrained.net
radius_source=/Users/zmoratto/Data/Moon/LROWAC/DTM/WAC_GLD100_E300N2250_100M.lvl2.cub

cp $input_control control_loop.net

for i in {0..100}; do
    echo Iteration $i
    cnetnewradii cnet= control_loop.net onet= output.net model= $radius_source getlatlon= adjusted
    mv output.net control_loop.net
    jigsaw fromlist=cube.lis radius=yes twist=yes cnet= control_loop.net  onet= output.net update=yes spsolve= position camsolve= velocities
    mv output.net control_loop.net

done

Your mind should be blown at this point. I’ve essentially written an iterative closest point algorithm using the ISIS utilities jigsaw and cnetnewradii. This process is only able to work because we have a perfect control network that we vetted with the previous jigsaw runs. This also works because our LOLA data here is not flat, and has plenty of texture. Finally, LRO-NAC was close to the correct solution. Imagine images with horrible attitude data, like Apollo, would just blow up with this technique.

On each iteration, the sigma0 shouldn’t change. Or if it changes, it should be extremely small. That’s because a translation or scale change of the control points and camera location has no effect on the reprojection error. What should be reducing on each iteration is the standard deviation of the radius correction for each control point as listed in bundleout.txt. The sparse 3D model of the problem in Jigsaw is slowly fitting itself to the LOLA DEM but constrained only by the camera geometry. The standard deviation of the radius correction will never go to zero and will eventually plateau. This is because there will always be some error against LOLA since large portions of the measurements are interpolations between orbits.

Disclaimer: I’ve only tried this technique for this LRO-NAC stereo pair. I don’t know if it will work for others. However this was the only technique I could come up with that would produce results that best aligned with LOLA. I couldn’t get anything good to come out of using WAC Image Mosaic GCPs.

Map projecting for Stereo

Now that we have an updated ephemeris, we’re ready to perform a map projection to speed up stereo. Be sure to not accidentally spiceinit your files, which would wipe your updated ephemeris. Notice that I’m map projecting everything at a low resolution. This is because I’m in a hurry for this demo. In a production environment for LRO-NAC, we would likely map project at 1 MPP or 0.5 MPP. This step and ASP’s triangulation are usually the longest part of this entire process.

parallel cam2map pixres=mpp resolution=5 from={} to={/.}.map.cub ::: ../*cal.cub

Running ASP

Nothing special here, I’m just processing the LE-LE, RE-RE, and LE-RE combination. RE-LE stereo pair wouldn’t resolve anything at 5 m/px. Afterwards, I gdalbuildvrt the outputs together. The massive lines separating the individual DEMs that make up this stereo pair may concern you. However that effect will diminish if I had map projected my inputs at a higher resolution. The reason being that, correlation is much like convolution, in that we erode the input image by a half kernel size along the image border. That border gets perceivably smaller if the image is of higher resolution.

parallel -n 2 stereo M125713813{1}.cal.map.cub M125720601{2}.cal.map.cub {1}{2}/{1}{2} ::: LE LE RE RE LE RE
parallel -n2 point2dem -r moon --t_srs \"+proj=eqc +lat_ts=37 +lat_0=0 +lon_0=180 +x_0=0 +y_0=0 +a=1737400 +b=1737400 +units=m +no_defs\" --tr 5 --orthoimage {1}{2}/{1}{2}-L.tif {1}{2}/{1}{2}-PC.tif --nodata -32767 ::: LE LE RE RE LE RE
gdalbuildvrt aspba_drg.vrt */*-DRG.tif
gdalbuildvrt aspba_dem.vrt */*-DEM.tif
parallel gdal_translate -co TILED=yes -co COMPRESS=LZW -of GTiff {} {.}.tif ::: *.vrt

Conclusion and Comparison

Now the moment you’ve been waiting for. The point where I have to prove I’m not crazy. I have three versions of this stereo pair. There is the result produced by ASU using SOCET SET and then there’s ASP with and without Bundle Adjustment. The look of ASP’s result will be a little shoddy because I map projected my input at 5 m/px which prevent me from get the most detail out. It did however save me a lot processing time while I experimented this method out.

Here are the hillshades for those 3 datasets overlaid a hillshade of the LRO-WAC Global DTM which is 100 m/px. There’s not much to tell, but you can at least see that everyone seems to have decent horizontal alignment.

Here are the difference maps between those 3 datasets and the LRO-WAC Global DTM.

Here are the difference maps between those 3 datasets and the LOLA Gridded Data Product, which I resampled to 100 m/px. I think this shows straight forward the improvement bundle adjustment made to the output DEM.

Just for giggles, I also did a difference between the ASU DEM and the bundle adjusted ASP DEM. They’re pretty close to each other. There seems to just be a simple tilt between the two or that the ASP DEM is just shifted to a slightly lower latitude. Who’s more correct is anyone’s guess for now.

 

Ideally at this point I would then compare the ASU DEM and the BA’d ASP DEM against the raw LOLA shot points. This is where curiosity got me and where I move on to the next personal project. Maybe someone else would be interested in a more rigorous review.

Also, I’m not sure if the 5th image is required.