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I have been given some utility plant maps for a project my company is working on but they don't come with a projection listed (they are in PDF format, but I will strip them down, make them transparent, probably change to png or tiff.). I'd like to add them as a raster layer in QGIS 3.4 for reference and planning, but I am running into some problems.

I have tried using the georeferencer and cannot get any of the methods to work without grossly distorting the maps, or not giving me accurate enough results.

What I would like to do is: 1. Find out what projection these maps are in. 2. Set my project crs to the same. 3. Import the rasters (undistorted). 4. Set my project CRS back to what I am typically using.

Is there a good method for doing this? Because of the source of these files, I am presuming that they are in an actual projection, however, they may have been intentionally skewed.

Project CRS is EPSG:104199

Sample map to import:Sample Map

EDIT: Added more images.

Linear Input Linear Linear Output Linear output Polynomial 3 Input Polynomial 3 Polynomial 3 output Polynomial 3 output

Michael Stranks
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  • If you can't get in touch with whoever exported the plans to PDF to ask for the projection, my first idea would be to try and find what projection is used for cadastral plans in that area. The utilities seem to be overlaid on parcel boundaries. – Gabriel Feb 13 '19 at 14:50
  • What is the CRS of the base layer (Layer -> Properties -> Information)? Can you share the result of the Thin Plate Spline method with some transparency for the georeferenced layer? – Gabriel De Luca Feb 13 '19 at 20:52

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Since your map is in PDF format, it do not have any CRS.

When you convert it to a image format, it will have a very custom and strange CRS. You will not find a well known CRS for the image with any method (brute force included).

All you need to do is georeference the images.

Also if the image is supposed to be an accurate representation (of an accurate survey) in a good scale of a plant, when you georefence it to your project CRS which is a geographic (longlat) coordinate system of a sphere, you will see the image nicely deformed, but that not means necessarily an unexpected behaviour.

Gabriel De Luca
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  • I know that the map doesn't have a CRS attached to it. I want a way to find which projection/CRS is closest to it, in a way similar to the georeferencer but with the steps reversed. Instead of distorting the image as it does now, I want it to tell me the closest CRS so that I can set my project to that for the import. – Michael Stranks Feb 13 '19 at 16:56
  • Any image has the following coordinate system: The center of the upperleft pixel has coordinates (0, 0); The center of the first pixel at the right has coordinates (1, 0); and The center of the first pixel at the bottom has coordinates (0, 1). You will not find a Coordinate Reference System or projection that establish a relation between the coordinates of your image with the real world, if you don't know how the pixels and the world are related. If you know it, write the world file for the image and assign a CRS for it. – Gabriel De Luca Feb 13 '19 at 18:13