Run FSX and go to KADW, switch to slew mode and top-down view. Run TCalcX. This will tell you the folder and file containing the default flatten, in this case Microsoft Flight Simulator X\Scenery\0302\scenery\cvx2718.bgl:
Open TmfViewer and load the above file and press the "+" key on the keypad to zoom in:
TCalcX also shows the current lat/long, so in TmfViewer, select Jump/lat long and copy/paste the lat/long values from TCalcX:
Andrews is contained in two Qmid11 rectangles so you need to create an exclusion rectangle to the west of the boundary, and covering part of the next Qmid11 rectangle:
Looking at the status bar, you can see the lat/long of the cursor. Put the cursor at the SW corner and make a note of the coordinates, something like 38.799938, -76.887691 now move to the NE corner and again note the coordinates, say, 38.808249,-76.869548.
Now run ADE and place two position markers somewhere near the noted coordinates and lock them:
Select Options/Units and change Coord to "Decimal"
Edit the lat/long of each marker to the values you noted:
Now select polygon and draw a rectangle roughly through your position markers:
Set the tag to be Exclude Specific/Airport Backgrounds Flatten:
Now create another polygon, this time surrounding your new airport layout and tag this as Airport Background/
Flatten Mask Class Map ExcludeAutogen:
Close FSX, Compile the airport and copy both bgl files to an active scenery folder then restart FSX:
George