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Open a project which has a state plane coordinate system attached to it.
In the View/Edit screen, click on the background and drag a box around the points you want to use in the computation. They will turn blue as shown in the figure below:

With the points selected, right-click on the background and select Compute Ave. Combined Factor... as seen below:

This will bring up a dialog box showing the average scale factors computed using the selected points. You can enter in a value to subtract from the northings and eastings so that you won't confuse these ground coordinates with the state plane coordinates. (I recommend that you do this).

When you click OK, you will be able to see your modified northings and eastings in your point list as shown:

NOTE: This computation is only one-way. To be able to fully utilize these ground coordinates, you will have to export the point IDs, modified northings, modified eastings, and ortho heights to an ASCII file and import this file into a new project. Then you will have to create a transformation and coordinate system using the existing project and the new project.
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