Saturday, November 01, 2008

What Does the Airport Boundary Do

The apt.dat 850 file format defines a polygonal "airport boundary". But what exactly does it do? It does different things when creating DSFs and when rendering them.

Inside X-Plane it has relatively little effect:
  • It is one of many elements that counts toward the land area that will be flattened. (Runways and taxiways are also used.)
  • We do not actually render any special terrain or fences.
The airport boundary has more of an effect during scenery creation.
  • If an airport has an airport boundary, we do not calculate the airport's boundary ourselves - instead f we use the specified boundary.
  • All land intersecting the airport boundary is turned to airport terrain.
  • The DEM is flattened within the airport boundary to reduce the slope of high frequency bumps.
There is one aspect of airport creation that the airport boundary is not involved in: filling in water to make land under runways. When we create an airport, we actually calculate three boundary polygons:
  • The inner ring is closest to runways and taxiways (very close) with very little simplification. It is filled in with land if it is wet.
  • A second ring slightly farther from runways and taxiways is also filled in with land if it is wet.
  • The outermost ring is a lot farther out, but does not fill in Water. This is the ring that the airport boundary can replace.
Why do we need two inner rings? Well, if an airport is next to the water but not at sea level, we need to induce two sets of mesh points, the outer ones which drop down to sea level and the inner ones which are at airport level. You can see the importance of this at KLGA, where one of the runways dropped to sea level at its midpoint in version 8, but not version 9.

Since the airport boundary polygon provides only one ring, it cannot be used for this purpose. At some point in the future, we might use the airport boundary for rings 2 and 3, or use a smaller version of the airport polygon for rings 1 and 2.

For now, my recommendation is: the airport boundary should trace out the entire airport premesis, not including water.


Dan said...

Because we can't cut holes in the airport boundary (as far as I know), are we allowed to use more than one boundary that are joined sharing polygon coordintes?

There are special cases where this is desirable. It could be a large area between two taxiways far apart that should not be affected by any flattening, airport ground textures, the lack of forrests etc.

Murmur said...

Automatically sdding fences to the boundary of every airport would be great for the immersion factor, without appreciable performance cost. Do you think it will be possible in the near future?

nickw said...

Many small farm and bush airports are unfenced. A rule like "only fence if paved and lighted" could be used - but I bet someone would find exceptions even to that.