Saturday, August 19, 2017

QGIS Georeferencer : recommendation for transformation type

Just making a note on a geo-referencing task I was doing with QGIS.

I had an outer boundary shape of a village, and had to geo-reference a PDF map of that village which has some internal data that I wanted. (won't go into more details here, but see this and this to get your shape and map.)

And chose the "Polynomial 2" tranformation type.

My georeferencing points (GCP) were points along the outer boundary of the village only : nothing from inside / center.

Ran the georeferencer : it got done instantly, and lo and behold, the image/raster map appeared perfectly on my shapefile map. It's a perfect fit, and the original map doesn't have any major distortions.. it has a slight rotation.

The output .TIF file is also much smaller than what I've had earlier : only 6.5MB compared to 405MB for an earlier run.

Previously based on some other article I had been using the setting "thin plate spline". It was extremely slow and created very distorted outputs and much larger .tif's . So for my kind of task (outer boundaries pinning), the "Polynomial 2" transformation has proved to be much better on all accounts.

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