Since X-Plane 8.50, airports have had a border polygon that defines the boundary of the airport surface area. The airport boundary polygon is probably the least understood aspect of airports, particularly because it takes effect when we cut DSFs, not when you load X-Plane.
The airport boundary does a few things:
- If sloped runways are not enabled, it defines (roughly) the area that X-Plane will flatten to the airport elevation. If you have ever entered a silly field elevation in WED and then turned off sloped runways, you’ll see that the flattening process is not exact.
- When we create base mesh DSFs, the area of the boundary polygon gets a grass surface texture.
- When we create base mesh DSFs, the area of the boundary polygon has no autogen buildings. (These two points are actually one in the same – the zoning of ‘airport’ in the DSF creation tool adds grass and removes autogen at the same time.)
- When we create base mesh DSFs, the elevation under the boundary polygon has large bumps and spikes removed.
No More Bezier Curves
If you’ve used WED 1.2, you may have noticed that it refuses to validate airports with bezier curve boundaries. This is because we are trying to stop the use of bezier curves for airport boundaries.
The problem with bezier curves is that they can very easily be self-intersecting, but WED has no good test for this. The result is (apparently) valid WED airport layouts that later crash the DSF generator. Every time we cut DSFs, we find a handful of boundary polygons that need to be hand-fixed due to bad bezier outlines. There were 3 in this last batch.
Furthermore, the DSF generator cannot actually use bezier curves – it has to convert them to polygons. So the author will not get a surface area like the one they expected – there is a conversion that must take place.
Faced with a bezier curve technology that produces crashes and isn’t actually used, I decided to switch to straight polygonal airport boundaries. So: when you work on new airports, please remove bezier curves from your boundary polygons.