[gdal-dev] Calculating elevation angles from RPCs in NITF files

[Shanasta Daisy]

Hello!

Given a NITF, the RPCs in the NITF, and the center pixel coordinates of the
image, I am trying to determine the elevation angle (center ground point to
satellite). From what I understand, this can be achieved by calculating the
ground point of the image center and another point on the vector looking
towards the satellite by adding a height to the ellipsoid (hae in ECEF
coordinates). So I thought I could use gdal for that:

To get the ground centerpoint in ECEF coordinates (Point B):
gdaltransform -t_srs EPSG:4978 -rpc -to rpc_height=0 test.nitf
1024 1024  (center pixel, Point S)

To get another point on vector looking towards the satellite in ECEF
coordinates (Point A):
gdaltransform -t_srs EPSG:4978 -rpc -to rpc_height=10 test.nitf
1024 1024  (center pixel, Point S)

[image: image.png]

The two points determine the vector, then convert to ENU coordinates and
the angle is calculated (using some test data, these things appear to
work). While the values I'm getting from gdal are different for the
additional height, it is giving results I neither expected nor understand
and always result in extremely low angles - so either my logic is incorrect
or I am misunderstanding how the RPC stuff in gdal works.

Does anyone have any insights?

I appreciate any help!

Thanks and have a nice day!

Charity
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