Yes, I must agree with George: ADE would be best while working on an airport flatten... (you'll also find many other delightfully easy to do things that might then be of interest to you... in addition to flattens).
ADE really is very simple to understand and use (plus it's
free) !
Alternatively you could undertake the risky method of manually editing the active copy of Scenery.cfg file and putting in an old-fashioned flatten statement (which believe it or not... is
still honored by FSX) !
PS: Again, I offer the friendly reminder that if you wanted to convince someone here to knock off a quick flatten BGL for you (it literally takes seconds to do), you must
declare a desired elevation in feet or meters MSL (not AGL) for that flatten's surface !
(Sorry, I'm rebuilding my system right now and can't do it for you)
BTW: FS2002 SCASM / AREA16N flattens are NOT honored by FSX, but FS2004 format "LWM" flattens are; however their sizes and edges may vary with terrain mesh LOD settings, so for best results use an FSX SDK compliant method to generate a CVX Vector type flatten such as ADE and SBuilder allow.
There is also another method with a simple modified "XYZ" ASCII text file (saved with a BLN extension) that could be put together using notepad, then processed via Jim Keirs' BLN2SHP utility, and handed off to SHP2VEC (from the FSX SDK) for final BGL output.
I believe BLN2SHP was (and may still be) a run-time module for SBuilderX to create some types of vector object BGLs.
Come up with a desired flatten elevation in a reply here, and if I get some time tomorrow, I'll see if that alternate route might be further simply explained by an additional post here.
IMPORTANT: You'd need the FSX Deluxe Version's SDK utility "SHP2VEC" installed to do
any of the above procedures yourself.
But I strongly encourage you to take a look at ADE, as it will be very handy for making/editing your current airport or other scenery area of interest.
Not certain of what the ground elevation needs to be (remembering it must work OK with the ARP and RWY elevations) ?
Try using this handy "crosshairs" aircraft placed on the ground in slew mode to get a precise elevation readout via the gauge in the cockpit:
I use the excellent Finney Ground Cross Hairs Plus "user aircraft", with mods for FSX texture visibility and a precision cockpit gauge in both Feet and Meters by Jim Robinson; a "must-have" for FS scenery development IMHO.
FYI: The 3D object and visual model datum point sits exactly at ground level as reported by FSUIPC (after read out by SimConnect in FSX)
Oh, and it's fun to fly (it internally uses the "Jet Pack Man" aircraft from a few years ago) !
http://library.avsim.net/search.php?SearchTerm=ch_pnl.zip&CatID=root&Go=Search
GaryGB