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Does anybody know which CRS should be selected when georeferencing a Falklands map made in 1961? The map itself says only:

Grid: UTM Zone 21
Projection: Transverse Mercator
Spheroid: International
Meridian of Origin: 57° West of Greenwich
Latitude of Origin: Equator
Scale Factor at Origin: 0.9996
False Co-Ords of Origin: 500,000 m. Easting, 10,000,000 m. Northing

Another question is, what coordinate values are more suitable to georef this map? It has both 6/7-digit UTM coordinates and lat/lon values. However when I create control points with lat/lon values from the map corners I get 10 to 20 pixels error in every point, and the resulting map does not exactly match the overlaid OSM layer. Could this be due to incorrectly selected CRS? if not, what else could be tweaked here?

  • The correct projected CRS would be WGS 84 / UTM Zone 21s which is EPSG:32721 – AWGIS Aug 2 at 10:52
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    It could be also EPSG:29221: epsg.io/29221 – TomazicM Aug 2 at 11:01
  • I thought that perhaps I shouldn't use WGS 84 as it was obviously designed after the map was created. I will give both 32721 and 29221 a try. – Nebehr Gudahtt Aug 2 at 11:06
  • Another vote for 29221 (original reference, see epsg-registry.org) which uses Sapper Hill 1943 which is based on the International 1924 ellipsoid. – mkennedy Aug 6 at 23:48
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Georeference it to UTM Zone 21, using the UTM coordinates.

That should be a good fit.

You would need to select a different model like polynomial for reprojecting.

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EPSG:29221 turned out to be the right option. I also had to use polynomial model as suggested (I had stupidly thought those models only determine the quality, not the actual transformations allowed).

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