Caution: only tested with
MSFS 2002 and 2004

Flatten Flight Simulator terrain

Flatten bgl files can be used to flatten terrain mesh to a fixed elevation in most locations. For example:

Airport scenery includes several components, not all of which are always aligned correctly with the airport elevation. This can cause problems with disappearing taxiways and parking areas, bleeding of grass through the pavement ... This problem occurs with both default and enhanced mesh. The solution offered here involves the creation of small "Flatten" bgl files for the airports. These files will adjust the airport elevation to that indicated in the Map View data for the airport. This often resolves the problem.

Lakes and ponds at elevations very different from the surrounding terrain. These are excellent candidates for this type of fix. When care is taken in selecting key points around the perimeter, the improvement can be dramatic. Use the Ground Elevation value to determine the best elevation for the Area.

Flatten switches can also be used, but are limited to rectangular shapes, and a maximum of 10 per scenery layer.

The application uses FSUIPC (already registered) to track aircraft position and ground elevation. Two approaches are supported:

  • Area16N: Use to define a new "feature" and specify its elevation. This approach is easiest, works anywhere, and should be sufficient for most purposes. Simply Slew the aircraft around the perimeter of the area to be flattened, recording key polygon vertex locations with a mouse click as you go. Once the area has been defined, a single mouse click creates the bgl file immediately using these stored pairs of vertex coordinates. (Named for the SCASM function which creates the bgl file.)
  • LWM: This approach involves editing (mostly automated) the information FS uses to define existing "features" in the sim (LWM polygon data). While elegant in concept, it is more complex, and requires at least a casual understanding of the Scenery SDK, more computer literacy, and a bit of patience as well. For lakes, it also requires use of the hyp*.bgl files provided with FS2002.
    This approach works well when using a grid overlay in the sim to help identify LOD13 quadrants. Flatten-Grid Image

You can download my system here: (6 December, 2003)

Three simple steps (lakes, using the Area16N method):

1. Collect the data

  • Open Flight Simulator and go to the feature to be patched.
  • Enter an Area name and Elevation for the flatten bgl. Get the correct runway elevation from Map View, other elevations can be estimated from the Ground Elevation reading provided by the application.
  • Slew your aircraft around the border of the feature and record the coordinates at key locations. The starting point does not matter, but you must enter the data in a counter-clockwise sequence.

    The Blue Vertices indicate positons already recorded for the current Area, the Blue Links define the area that will be included in the flatten.bgl for that Area. The Black Vertices and Links represent a previously defined Area. The Red Point indicates the current position of the aircraft.

2. Create the bgl patch file

  • Click the <Create bgl(s)> button

3. Install the patch in Flight Simulator

  • move it to ADDON SCENERY\scenery (FS will use it by default if Addon Scenery is checked in your Scenery Library)
  • or, (recommended)
    • create another folder, with a \scenery subdirectory;
      e.g. ADDON SCENERY\airport patches\scenery\
    • move the bgl file there
    • "Add" it to FS using the Scenery Library menu