On Tue, 23 Mar 2021, Bullinger, Sebastian wrote:
Dear gdal community,
I'm currently working with a set of satellite images that are geo-registered
with a set of GCPs. To compute a geo-transform from the GCPs, I've been using
gdalwarp with:
gdalwarp -of GTiff path/to/original/file path/to/warped/file
While "gdalwarp" correctly computes a transformation from the GCPs, at the same
it also performs some kind of rectification / north-up image transformation. I'm not sure
about the applied operation.
Presumably, the reason for this is that the original GCPs reflect a
transformation with skew factors.
See the following transformations of the original and the warped image.
import gdal
dataset = gdal.Open("path/to/original/file")
print(gdal.GCPsToGeoTransform(dataset.GetGCPs()))
(-58.57294039342205, -3.991598267026281e-06, 0.0, -34.451175177186684,
-3.6492767328575985e-07, 3.1987810722132996e-06)
import gdal
dataset = gdal.Open("path/to/warped/file")
print(dataset.GetGeoTransform())
(-58.577917916461026, 3.435846284510554e-06, 0.0, -34.44693359348493, 0.0,
-3.435846284510554e-06)
The "rectification" result is very convenient, since it allows to use the
images with an existing satellite image segmentation pipeline (which tiles the satellite
images using geo-tiles)
However, in order to overlay the segmentations (performed on the "rectified" images) with
the original images, I need some information about the transformation applied during
"gdalwarp".
I've searched the documentation for more information - unfortunately without
success.
For example, parameters like "-novshiftgrid" do not affect this transformation.
It would be very helpful, if someone could shed some light on the used
"rectification".
What kind of operation is applied? What would be the correct term to search for?
Is it "only" a decomposition of the transform? Or does "gdalwarp" something
else too?
Is there a possibility to access this information, while using "gdalwarp"? (Or
can this operation also be done by hand?)
Does GDAL contain any functions to invert this "rectification" step - given the
corresponding transformation used by "gdalwarp"?
I don't know the answers.
I *guess* that the rectification is not deliberate, but simply that the
output rectangle is aligned with the (target) spatial reference system
(SRS).
Have you noticed whether the output image has no-data pixels at the
edges, or crops the original ? I would expect it to do a little of each.
--
Andrew C. Aitchison Kendal, UK
and...@aitchison.me.uk
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