1) Make a builder brush large enough to cover the entire play area in increments of 1024
2) Maximize a viewport (or tear one off, whichever) and set it to TOP view (ALT-G) in LIT mode
3) Zoom the view so that the builder brush is framing the majority of the viewport. ** DO NOT MOVE THE VIEWPORT **
4) Press and HOLD F9 and click in the viewport. That takes a screen shot.
5) In the viewport, Press G to put it into game rendering mode
6) Press and HOLD F9 and click in the viewport. That will take another screenshot without the builder brush nor any of the world actors visible
7) Open the screenshot with the builder brush in your favorite image editing software. I use GiMP cuz it's the shizzit and free.
8) Use the guidelines to build a square along the builder brush
9) Open the other overhead image, select all, and paste it in the builder brush version of the image then flatten the image
10) Use the rectangle tool to select the image area using the guidelines
11) Crop the image to selection
12) Scale image to 1024x1024
13) Export it as a TGA to teh desktop
14) Import the image back into the editor and put it into the world properties
15) Use the existing builder brush from step 1 to create a ROVolumeMapBounds at about 7000UUs above the playspace
16) Save the map
17) Test that the overhead lines up to the actual play space. If it does, rock on. If it doesn't return to step 1 and do it right instead.