I posted previously in my blog that we use airport boundaries to flatten airports when we make the DSFs, as well as place airport grass. Let me clarify this a bit: there are two stages of flattening that we do.
- The flattening in the DSF is a “soft” flattening – we do not completely flatten the area, but rather we try to remove high frequency spikes in the elevation data, so that the airport maintains its overall shape without being unusable. This flattening process is very precise in that it only happens inside the airport area. Because we decide the mesh when making the DSF, we can shape the mesh to flatten only the airport area.
- If flattening in the sim is enabled, we then do a “hard” flatten of an area including the airport and some surrounding area. This flatten is truly flat and destroys any topography for the airport and a lot of surrounding terrain. Because the mesh is already defined, X-Plane is limited in its choices of where it can choose to flatten or not flatten.
Now that first soft flattening is what we use the airport boundary for. In X-Plane 800 the US DSF flattening worked very poorly. For the 820 flattening it worked well for most cases, but had some very specific bugs. (Typically the confluence of water, airport, and/or a DSF tile in close proximity would cause the three different processors for water, airport, and boundaries to come into conflict.)
I am going to try to further improve the flattening algorithm for the v9 render, with the hope of fixing some of the buggy cases from the 820 render. I am sure there will still be some broken cases – with 14,000+ DSF tiles, one of them is bound to be weird…but my hope is that with each re-render we can get closer to a render where users can run with sloped runways enabled and enjoy realistic non-flat runways.