[Gdal-dev] gdalwarp/gcps and ERROR 1: Failed to compute polynomial equations of desired order

[Ethan Alpert]

Greetings,
 
This is my first post. I tried searching around for examples but didn't
find much and mailman's search capability is non-existant.
 
So what's the secret for using more than 4 gcps with gdalwarp?
 
I have 3 pieces of imagery I'm mosaicing. I know the corners of each
image. I created a single non-projected tif of the three separate
images. I computed where (in pixel space) the top two corners of the
first two images are in the resulting tif and where the bottom two
corners of the last image are. This resulted in 6 GCP points. So I ran
the following:
 
gdal_translate  -gcp 0 0 56.7468972222222 29.4295138888889 -gcp 859 0
56.9820861111111 29.4210638888889 -gcp 0 1363 56.7391 29.1199027777778
-gcp 859 1363 56.9705611111111 29.1071416666667 -gcp 859 2171
56.9677027777778 28.9774388888889 -gcp -gcp 0 2171  56.7381583333333
28.9917333333333 mosaic.tif mosaic_raw.tif
 
Then I ran:
 
 

gdalwarp -co "TFW=YES" mosaic_raw.tif mosaic_new.tif
 
And got:

ERROR 1: Failed to compute polynomial equations of desired order
for provided control points.
 
If I use -order 1 I get a really screwing output.
 
If I remove the middle point and do:
 

gdal_translate  -gcp 0 0 56.7468972222222 29.4295138888889 -gcp 859 0
56.9820861111111 29.4210638888889 -gcp 859 2171 56.9677027777778
28.9774388888889 -gcp 0 2171  56.7381583333333 28.9917333333333
mosaic.tif mosaic_raw.tif
 
Then:

gdalwarp -co "TFW=YES" mosaic_raw.tif mosaic_new.tif
 
I get a projected mosiac of my images but it's not warped as I'd like.
 
So again my question is what needs to be done to use higher order
warping? What are the rules gcps must follow?
 
I'm using gdal1.2.1
 
Thanks in advance,
 
-ethan
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