More georeferencing notes

I updated Quantum GIS to 1.6.0. It claims some improvements for georeferencing. I also learned better how it works.

Transformation Types are documented in the Quantum GIS manual. The docs suggest Polynomial 3 or Helmert are the best choices. Polynomial 3 is a nonstarter since it requires at least 10 GCPs and Helmert produced an oddly rotated image that was not correct.  Linear, Polynomial 1, Projective, and Thin Plate Spline all produced very similar results. Linear isn’t even a proper transform, really, but it seems close enough for my purposes. I’m shooting in the dark here, not knowing how the source NACO plates are projected, but at this scale and for my purposes any errors are not too big a deal.

The Target SRS you set in the Georeferencer is just an annotation, it doesn’t magically cause some warping. I’d tried setting it to EPSG:900913 for the Google (scale: 1 = 1m or something) but was typing latitude and longitudes in the tool (scale: 1 = 1 degree). Works better if I set the Target SRS to EPSG:4326 for WGS 84, produce a GeoTIFF, then later warp the image to Google mercator.

Raster / Warp will let you do the warping to Google. All it’s doing is calling gdalwarp, something like gdalwarp -s_srs EPSG:4326 -t_srs EPSG:900913 -r cubicspline -of GTiff input.tif output.tif. I wonder if this is really even warping anything or it’s just rescaling?

Bottom line:

  1. Draw 4 or so georeferencing control points on the chart. Type latitude and longitude.
  2. Set Transformation Settings to Polynomial 1, Cubic Spline, LZW, EPSG:4326.
  3. Warp the image to Google mercator