[Gdal-dev] GCP's, Warping and Source Data Projections

Ethan Alpert ealpert at digitalglobe.com
Thu Sep 16 22:04:35 EDT 2004

Well I've done some good GDAL documentation reading in the past couple days trying to get a handle on what I'm trying to do with respect to GDAL's data model and I'm left with more questions than answers.
I've boiled them down to one which if I can get a handle on I might be to conceptually work my way through the rest.
Being pretty ignorant I figured that gdal's GCP's support could infer a projection.  Which means I believed that by providing geographic coordinates and pixel/line coordinates that GDAL would some how infer a custom projection for my input data.
Is this truly a misconception?
The documentation for GDALCreateGenImgProjTransformer seems to suggest this but I can't really find any references to how GCPs need to be related spatially for it to work or whether my source needs to be in a known projection.
My data is reduced resolution raw satellite imagery. By raw I mean there's been no ortho-rectification. In real world coordinates my pixel size can vary a lot. 
I do have lots of useful information about my data. I have ephemeris, attitude, bearing angle and off nadir angle data. I also have anywhere from 4 to 40 geographic coordinates that define the polygon of the strip (my GCP's) depending on the length of the strip. Each strip can often bend and stretch through out it's scan.
So anyway it seems to me I have enough to define a unique projection for each strip of satellite imagery. My question is do I really need to go through all of this? I'm not looking for the most accurate projection but I'm looking for a better way to do it than I am currently.
So the utility gdalwarp doesn't like my GCP's for higher order warping and produces an awkwardly placed warped image when using -order 1.  The only thing I can think of is that I have some misconception of how to classify my data's coordinate system,  I'm doing something wrong with preparing it or I need to look into the utility that initially generated the 4-40 GCP's of this data.
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