[GIS] Generating a GDAL script through the QGIS Georeferencer

gdalgdal-translategdalwarpgeoreferencingqgis

For several months, I have been georeferencing files manually using the Georeferencer plugin within QGIS. I am now attempting to automate this process using GDAL. The Georeferencer provides a "Generate GDAL script" option, which creates the following commands:

gdal_translate -of GTiff -gcp 774.73 -338 -88.52 34.37 -gcp 1012.17 -215.136 -81.22 27.48 -gcp 972.709 -486.055 -77.76 36.67 asciiMAE20121001.asc testoutput1
gdalwarp -r near -order 1 -co COMPRESS=NONE testoutput1 testoutput2

When I run this script, however, the georeferenced raster is placed about 40 degrees further west than where it should be, and 7 degrees further south. Manually using the Georeferencer plugin continues to georeference the raster correctly. I think the problem is with the gdalwarp, because the raster is just shifted too far from where it should be. The placement of the values within it appears to be correct, so it looks like the gdal_translate worked correctly.

Does anyone know why the GDAL script produced by the Georeferencer would produce a raster displaced from the raster produced manually by the plugin?

Best Answer

Assuming you have saved the output of your GCP points, can you compare the values in this file with the auto-generated script? I've noticed that the script rounds values; I wonder if this could be introducing a source of error.

Related Solutions

QGIS – Georeferencing Mars Images: From ISIS to GDAL to QGIS

Make a custom CRS using the following parameter string:

+proj=longlat +a=3396190 +b=3396190 +no_defs

You might call this CRS (say) "Mars_Geo". Then load the *.cub raster, set the project CRS to "Mars_geo" and enable on-the-fly CRS transformation. Doing this will distort the raster to some extent but you will get lon/lat coordinates reported as you move the pointer around the screen.

When checking the corner coordinates of the raster bear in mind that the corners are not the corners of the data. The image is an upright rectangle having the extents shown in the attached image.

Please bear in mind that I know nothing about Mars and next to nothing about Sinusoidal CRSs, so be careful!

N.

enter image description here

[GIS] Georeferencing an image with GDAL, based on QGIS Georeferencer Plugin

The coordinates in your gdal_translate command should be all positive.

The GCP list is, so I think this could be a bug of the GDAL script creator.

EDIT

I could replicate your error, and think that your use case is not the intended use of GCP.

In "normal" georeferencing, the GCP points map pixel coordinates from an unreferenced picture to projected or geographical coordinates. The pixel coordinates are relative to the upper left corner.

In your use case, you take projected coordinates from a georeferenced tif, and compare them to "correct" coordinates. The coordinates on the left side are not relative to the upper left corner, but to the origin of the projection. If you start the reprojection process, this information will be distroyed and replaced by the GCP information. This does not seem to work.

In the GDAL batch file, you can add -of VRT to get vrt instead of tif files. You can open those vrt in a text editor and see that the matching goes terribly wrong.

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