[asheshouse]

Sounds like a bit of ground levelling is required.
For airfield plates to sit flat the ground needs to be perfectly level.

If you want to try to fix it take a look at map_h.tga in a graphics editor.

First write down the coordinates for the airfield. You can see them in FMB.
1 pixel = 200m so set your graphics software to show a pixel counter and you can find the airfield location.

You should have exactly the same shade of grey across the entire airfield.
Probably 1 pixel is different to the others.
Make them all the same.

Ensure you back up the original file first.

Ashe