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So I have a client with a site datum, which is to say they have an arbitrary coordinate reference system with coordinates between -1000 and 1000 in X and Y.

I have a list of ~20 coordinates that maps between a UTM projection and these coordinates, so I know I can calculate an affine transformation between them without too much difficulty.

My question is, can I handle this case with custom CRS, so that using the site datum is as simple as selecting it as the on-the-fly projection, or exporting data with the site datum transforms the data automatically?

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You can do both:

At Using customized Coordinate System in ArcGIS Desktop? I have explained how to set up a local CRS based on the origin of the local system. The data in the local CRS remains untouched, and can still be edited.

Or you can use the affine transformation plugin to shift the data into real world coordinates. This has the disadvantage that new data coming in has to be rectified every time.

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  • That's awesome! How do you identify the parameters for the custom datum?
    – Alex Leith
    Jul 23, 2016 at 7:49
  • Actually, I see you're mentioning it later in your comment. I guess I just need to get stuck into it and see how it goes!
    – Alex Leith
    Jul 23, 2016 at 7:49

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