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I've created a virtual raster using gdalbuildvrt, which covers a large extent (England) at a high resolution. The data is fairly sparse, focused around the coastlines with no files covering inland areas. I'm trying to read the data in Python using rasterio.

When I try to invert the affine transformation (to get world coordinates converted to pixel coordinates) I getting the following error:

>>> layer.affine
Affine(0.0002777778000000001, 0.0, -5.5000000044,
       0.0, -0.0002777778000000001, 55.0000000836)
>>> layer.affine.__invert__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\Python27\lib\site-packages\affine-1.0-py2.7.egg\affine\__init__.py", line 407, in __invert__
    "Cannot invert degenerate transform")
affine.TransformNotInvertibleError: Cannot invert degenerate transform

I don't get this error accessing the individual raster files. I have some idea of what it means, but not how to work around it. Perhaps it needs approaching in a different way? How do I access this data without having to worry about which file it's stored in?

  • I cannot find docs for __invert__ but I suspect it is a general matrix inversion procedure. The 2 by 3 matrix returned by layer.affine cannot be inverted as a matrix (because it is not square). If I am correct, you would need to apply a procedure intended specifically for this shorthand representation of affine transformations, perhaps invertAffineTransform. – whuber Aug 4 '14 at 12:41
3

Inverting the transform isn't the way to go. Try the dataset's index() method or its code: https://github.com/mapbox/rasterio/blob/master/rasterio/io.py#L62.

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