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I use QGIS when it's needed, but primarily stick in the AutoCAD world where I have a bit more experience. I have an orthophoto in Ground coordinates for a project site, and all the survey data that was collected from this stereo orthophoto is ALSO in Ground coordinates. I wanted to create a CRS in QGIS that is essentially "New Mexico State Plane Central FT, Project" so that I could project other, larger orthophotos and maps that are naturally in "NM State Plane Central FT" into my local coordinate system and have them be accurate locally (i.e., be exportable as GeoTIFF's and importable into my CAD drawings)

Right now, the NMSP data is off by ~250', and I know that my scale factor is 0.999633840 to go from ground to grid, but I can't figure out the appropriate adjustment to my CRS (or an appropriate means to scale a GRID RGB color orthophoto) to get it to line up with my Ground orthophoto.

  • As your project is in metres (or feet) you could use an existing UTM coordinate system and adjust the false easting and northing to match your project origin. This is quite easy in ArcGis but I'm not sure on the mechanics in QGIS.. you will need to create your custom CRS and use that for the canvas, unknown CRS data should be unprojected. Normally I'd georeference the CAD file and work in a well known CRS but as your ortho is in project coordinates as well that's a bit more work. – Michael Stimson Oct 12 '16 at 23:50
  • It would be a custom coordinate system. Similar to ArcGIS, apply grid-to-ground scale factor to the existing NM state plane definition AND to the false easting and northing values. NM state plane uses transverse Mercator, so it's easy to apply the adjustment. – mkennedy Oct 13 '16 at 21:04

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