8.26. SPOT 6/7

SPOT 6 (launched 2012) and SPOT 7 (launched 2014) are Airbus Earth observation satellites with 1.5-meter panchromatic resolution. They are part of the same SPOT family as SPOT 5 (Section 8.25), but use the DIMAP V2 XML format and a linescan camera model that closely follows the Pleiades sensor (Section 8.24).

ASP expects raw (non-orthorectified) images. The data have both an exact linescan camera model and an approximate RPC model (Section 8.22), stored in separate XML files whose names start with “DIM” and “RPC”, respectively. ASP supports both. The USGS CSM library (Section 8.12) is used for linescan models.

The session type is -t spot (Section 16.51.8). If the -t option is not specified, it will be auto-detected from the camera files.

8.26.1. Bundle adjustment and stereo with raw images

Bundle adjustment (Section 16.5) is suggested before stereo:

bundle_adjust -t spot            \
  --camera-weight 0              \
  --tri-weight 0.1               \
  left.tif right.tif             \
  left_exact.xml right_exact.xml \
  -o ba/run

With the exact models, the stereo command, with bundle-adjusted cameras, is:

parallel_stereo -t spot           \
  --stereo-algorithm asp_mgm      \
  --subpixel-mode 9               \
  --bundle-adjust-prefix ba/run   \
  left.tif right.tif              \
  left_exact.xml right_exact.xml  \
  results/run

Then, a DEM is created with point2dem (Section 16.56):

point2dem results/run-PC.tif

For steep terrain, it is suggested to run stereo with mapprojected images (Section 8.26.2).

See Section 6 for a discussion about various speed-vs-quality choices for stereo.

See Section 16.38.10 for an example of solving for jitter with these cameras. Note the limitations of the jitter solver in Section 16.38.1. This is available as of build 2026/03 (Section 2.1).

For the RPC model (Section 8.22), the option -t rpc should be used and the RPC camera files should be passed in. If the -t option is not specified, it will be auto-guessed based on the content of the camera files provided as inputs.

8.26.2. Stereo with mapprojected images

ASP supports running stereo with mapprojected SPOT 6/7 images (Section 6.1.7).

All input images must be mapprojected at the same resolution (which is comparable with the ground sample distance, GSD). The same camera models must be used for mapprojection as for stereo, so one should not mix the exact and RPC cameras.

Ensure the input DEM used for mapprojection is relative to the ellipsoid (Section 6.1.7.2).

Example:

proj="+proj=utm +zone=13 +datum=WGS84 +units=m +no_defs"

mapproject -t spot               \
  --tr 1.5                       \
  --t_srs "$proj"                \
  --bundle-adjust-prefix ba/run  \
  ref_dem.tif                    \
  left.tif                       \
  left_exact.xml                 \
  left_map.tif

mapproject -t spot               \
  --tr 1.5                       \
  --t_srs "$proj"                \
  --bundle-adjust-prefix ba/run  \
  ref_dem.tif                    \
  right.tif                      \
  right_exact.xml                \
  right_map.tif

parallel_stereo -t spot           \
  --stereo-algorithm asp_mgm      \
  --subpixel-mode 9               \
  --bundle-adjust-prefix ba/run   \
  left_map.tif right_map.tif      \
  left_exact.xml right_exact.xml  \
  run_map/run                     \
  ref_dem.tif

point2dem run_map/run-PC.tif

The projection string above needs to be modified for your area of interest. It is strongly suggested to use an auto-determined UTM or polar stereographic projection (Section 16.56.1).

The value of the --tr option is the ground sample distance. It is normally 1.5 meters for SPOT 6/7 PAN images. The XML files should have the GSD value.

To not use bundle-adjusted cameras, remove the option --bundle-adjust-prefix from all mapproject and parallel_stereo commands above.

8.26.3. Exact and RPC cameras

To compare the linescan (exact) and RPC models, run cam_test (Section 16.9) as:

cam_test --image img.tif        \
  --cam1 cam_exact.xml          \
  --cam2 cam_rpc.xml            \
  --session1 spot --session2 rpc

With the ESA La Crau sample (PAN band) and the additional option --height-above-datum 200 (given the validity range in the RPC model), the pixel difference between exact and RPC models was max 0.002 pixels.

The camera centers computed by the two methods won’t agree, because the RPC camera model does not store the camera center. ASP then substitutes it with an estimated point on the ray from the camera center to the ground. This disagreement is not an issue in practice.

Note that SPOT 6/7 RPCs use 0-based pixel offsets, while Pleiades RPCs use 1-based offsets. ASP handles both conventions automatically.