Measurements of the Satellite’s position and pose are often not perfect. Those minor mistakes can cause gross offsets when producing DEMs from cameras with such long focal lengths. Correcting this is achieved with bundle adjustment. I’ve discussed this before, but I’d like to expand my previous article by adding ground control points.

Ground Control points are special measurements in an image that have well known geodetic position. On earth, this might be achieved with a large target (an expedition’s tent) that can be observed with a satellite and has a GPS beacon. We don’t currently have such an analog on the Moon or Mars. The best bet is to instead compare an image against a good world map and then pick out individual features. For Mars, I think the best map to register against is the THEMIS global Day IR 100 m map. For the Moon, I would pick the 100 m LRO-WAC DTM and Image mosaic.

In the following sections, I’m going to run through an example for how to register a pair of stereo CTX observations to the THEMIS global map. For those who are interested, my example images are of Hrad Vallis and use P17_007738_2162_XN_36N218W and P18_008028_2149_XN_34N218W.

Getting a THEMIS Global Map Tile into ISIS

The 11^th version of the THEMIS Day IR map is available from the following ASU link:

http://www.mars.asu.edu/data/thm_dir_100m/

Unfortunately the data you download is just a simple PGM file that mentions nothing about how it was georeferenced. That information can only be obtained through careful reading from the website. You however just need to know that the data is in Simple Cylindrical projection, it’s Geocentric, it’s sampled at 592.75 PPD, and that the files are indexed by their lower left corner.

I’m going to leaving it up to you to read the documentation to see what the following commands in ISIS do. That documentation is available here from USGS’s website. The tile I’m processing is “lat30_lon120.pgm” and I choose it specifically because it contains my example CTX images. I had to use an intermediate GDAL command because std2isis couldn’t read a pgm file.

gdal_translate -of GTiff lat30_lon120.pgm lat30_lon120.tif
std2isis from=lat30_lon120.tif to=lat30_lon120.cub
maptemplate map=map.template projection=SIMPLECYLINDRICAL resopt=ppd resolution=592.75 clon=0 targetname=Mars targopt=user lattype=planetocentric eqradius=3396190.0
maplab sample=0 line=0 coordinates=latlon lat=60 lon=120 from=lat30_lon120.cub map=map.template

Creating Image to Image Measurements

I’ve discussed this process before, and I’m not going to re-describe it. (I’m quite lazy). However, the following is a quick recap of the commands I used to do it.

Autoseed.def:

Group = PolygonSeederAlgorithm
      Name = Grid
      MinimumThickness = 0.01
      MinimumArea = 1
      XSpacing = 4000
      YSpacing = 4000
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 = 75
     Lines   = 75
   EndGroup
 EndObject

The ISIS Commands:

parallel spiceinit from={} ::: *cub
parallel footprintinit from={} ::: *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=ctx pointid=???? description=mars
pointreg fromlist=cube.lis cnet=control.net onet=control_pointreg.net deffile=autoRegTemplate.def
qnet

Then you’ll need to do clean up in ISIS’s Qnet after you’ve completed the above commands. In my experience, pointreg will only correlate maybe 75% of your control measures correctly. To clean these up quickly, just go into qnet and select that you want to filter your control points to show only those that have been “ignored”. Then go and save. I had an excess of control points, and decided to just delete a lot of my ignore points. You can see a distribution of my control points in the following Qnet screenshot.

Creating Ground Control Measurements

Finally, we are breaking new ground here. We are going to measure some ground control points against the THEMIS map tile we downloaded earlier. You could do this in Qnet, however in practice I find this segfaults a lot (I’m using ISIS 3.3.1). So instead we are going to do this process with Qtie.

Open Qtie by simply just typing “qtie” into your terminal. Then click open button or command “O”. It will ask you for a basemap and you should select your THEMIS tile that is in cube format. It will then ask you for a “cube to tie to base”, select your first CTX image of the stereo pair. When it asks you for a control network, click cancel. If you actually give it your pointreg’d control network, it will go into an infinite loop because that control network has no GCPs.

You’ll then want to use the ‘tie’ tool to create a bunch of GCP. If you don’t remember, you have to right click on your CTX image to create a new measurement. You’ll want to capture at least 3 GCPs if you are working with a frame camera. Do about 10 evenly distributed GCPs if you have a linescan camera. CTX is a linescan camera, so have fun clicking.

When matching a low-resolution image to a high-resolution image, I find it very helpful to turn on the affine alignment option in the “Tie Point Tool”. That is the “Geom” radio button on the right side of the window. I also find it very helpful to have the left window constantly flipping between the two input images at a high rate. You can achieve this by clicking the play button in the lower left and then lowering the number next to the play button. You’ll also want to zoom in on your basemap.

On the left is an example of a match I found. This process is difficult and takes some patience. You’ll want to look at your match from different scales just to make sure that you haven’t aligned to some pareidolia.

Once you finish, you oddly have to click the diskette in the “Tie Point Tool” to save your control network. I’ve saved mine as “gcp.net”. If you were selecting ground control points against the LRO-WAC Global Image mosaic (for registering images of the moon), you might want to take a detour here and use the “cnetnewradii” command. That way you can have your GCPs use radius measurements from the LRO-WAC Global DTM. Otherwise your radius measurements will come from ISIS’s default sources of an interpolated LOLA or MOLA for Mars.

We now want to merge our GCP control network to our control network we created with pointreg. This is done with the following:

cnetmerge inputtype=cnets cnet=control_pointreg.net cnet2=gcp.net onet=control_gcp.net networkid=CTX_with_gcp description="against themis 100m"
qnet

Back in qnet, you’ll want to tie the other CTX image to your new GCPs. You can do this by open each GCP point up by double clicking it, and then selecting “Add Measure(s) to Point” in the Qnet Tool window. A screenshot is shown left. You’ll then need to manually align the control measures. Be sure not to move your reference control measure from the first CTX image. You’ll also want to change your GCP’s PointType from “Fixed” to “Constrained”. This will allow your bundle adjustment session to move your GCP to reduce its reprojection error. However it will be rubber banded to the measurement you made in Qtie. This is helpful because your basemap source will usually have mistakes.

After having registered all your GCPs to the other image, and also having set their type to “constrained”, you’ll want to do one last thing. Select all your ground control points in the Control Network Navigator and then press the “Set Apriori/Sigmas” button. I have a screenshot below. The sigmas are your uncertainty measurement for your GCP and basically tell bundle adjustment just how much it can ignore or trust your GCP measurements. Since our measurements were all made from the same source, we’re using the same sigma for all our GCPs. In my opinion a good sigma value is two times the pixel resolution of your basemap. So in this example, I’m putting the GCPs as being accurate to 200 meters in longitude, latitude, and radius directions.

Bundle Adjusting

Alright, it’s time to bundle adjust. This almost exactly the same command we ran before, however we can turn on a new option we couldn’t before:

jigsaw fromlist=cube.lis radius=yes twist=no cnet=control_gcp.net onet=control_jigsaw.net spsolve=position

This time we can solve for the spacecraft’s position since we have GCPs! Do a test run without updating the spice. If it converges to a sigma0 less than 2 px, then you can go ahead an add the “update=yes” option. If you are working with LRO-NAC images, you can likely converge to less than 1 px sigma0. If you unfortunately didn’t converge or your sigma0 has a high value, you likely have a messed up measurement in your control network. This probably came from pointreg, where it managed to match to the incorrect repeated texture. You can identify the mistaken control point by looking at the output residuals.csv file that was created by jigsaw. Just go an manually align the control points in Qnet that show a high residual error. The error should be obvious in Qnet. If it is not, then likely jigsaw fitted to the outliers. If that happened, the control points with low residual errors will have the obvious misregistration in Qnet.

My final DEM results

Creating the above measurements and performing jigsaw took me about 2 hours. Creating the DEMs in Ames Stereo Pipeline took another 3 hours. Here are my results with and without my jigsaw solution rendered in Google Earth. I’ve overlayed a hillshaded render of my DEMs at 50% opacity on top of the THEMIS Map I registered to. My DEMs also look a little crummy since I just used parabola subpixel for speed in processing.

Bundle Adjusted DEM

Non Bundle Adjusted DEM