Saturday, November 10, 2007

The problem with KLGA

With X-Plane 820 global scenery, you can fly with "sloped runways" (this option drapes the runways like a piece of cloth over the terrain no matter how strange the terrain is) at most locations, but not La Guardia, where there is a huge hump in the runway.

It turns out the problem with KLGA is that there are vertices in the mesh that are adjacent to both water and the airport. The field height is a few meters above sea level, so we cannot decide what height those vertices should be? (Zero for the water, or a few meters for the airport?) In the 820 global scenery, the water wins, pullling down any part of the airport that is coastal.

For the version 9 re-render of the global scenery I am trying to fix this by creating a buffer zone of a few meters around coastal parts of the airport. This assures that we will have at least TWO vertices at the corners of the airport. The inner one can then be at field level and the outer one at sea level. The triangles in between these two "rings" form a sloped transition area (more sloped if the airport is far above water level), allowing the inner area to remain flat.

We'll have to see how much this helps coastal airports - in hindsight I'm surprised that a lot more airports weren't as unmanageable as KLGA.

(As a side note, it appears that the SRTM raw elevation data is at its least reliable in airports - there's something about them, perhaps the metal or concrete or heat that causes a ton of drop-outs and false reflections. So we have to pre-process the airport areas pretty heavily to use them at all - and sometimes this pre-processing doesn't even work)


Austin G said...

Hi Ben,

EGCC is another airport you might like to take a look at with this problem - I'm not sure it's exactly the same issue, because it's nowhere near the coast, but a stream runs under one of the runways and this causes a big dip halfway along when sloped runways is switched on.

Worth a look, anyway.

Jim said...

You might also want to check KPHX as well. There is a pretty significant ledge on the runway at that airport that disappears with "Allow sloped runways" unchecked.


Cormac said...

The problem at EGCC is completely different - the SRTM data predates the building of the problem runway in real life. That's not to say Ben's pre-processing couldn't perhaps deal with such sudden changes in elevation along the length of a runway perhaps?

I'd say London City EGLC would be a good test candidate, given the runway is built on an old dock.

I wonder too is there a possibility of the beach-generating' process ignoring stretches of coastline with straight sections at right angles so that city docks and wharfs retain their man-made shape properly? Something to chew on...