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I have Python code that will automatically georeference drone images regardless of overlap and without SfM software. The code runs without error when I set my transformation type to a first order polynomial but I am unhappy with the georeferencing. I can alter my code to use a thin plate spline transformation and this also runs without error. I would like to test the output of a second order polynomial. When I change the arguments to gdal_warp to apply a second order polynomial I get the following error:

Error Info:
Failed to compute GCP transform: Transform is not solvable

Here is the relevant code for the working TPS transformation:

kwargs = {'format': 'GTiff', 'tps': True, 'dstSRS':'EPSG:4326'}

and here is the relevant code for the working first order polynomial transformation:

kwargs = {'format': 'GTiff', 'polynomialOrder':1,'dstSRS':'EPSG:4326'}

And here is the relevant code for the failed second order transformation:

kwargs = {'format': 'GTiff', 'polynomialOrder':2,'dstSRS':'EPSG:4326'}

If I try a second order transformation in QGIS with less than 6 GCPs QGIS complains and returns an error message that I need 6 GCPs for this transformation. I am feeding 6 GCP into GDAL. See this link for some past trouble shooting issues and details on the project.

Why is GDAL/Python not allowing me to do a second order polynomial transformation?

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  • Can you use second order if you add your ground control points into GeoTIFF manually with gdal_translate and then run gdalwarp with -order 2?
    – user30184
    Commented Sep 16, 2021 at 6:53
  • Yes, that is how I am doing it. I write the GCPs to a list, then iterate the list ap add gdal.GCP. After the GCP are added to the geoTiff I use gdal.Warp with my transformations.
    – GBG
    Commented Nov 1, 2021 at 22:41

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As per the suggestion from @user30184 feeding 8 gcps into the process solved the issue and allowed me to do a second order transformation.

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