What I'm trying to do is basically plot a rectangle on a runway that goes parallel with the runway.

When I start out everything works as expected:

enter image description here

But what I notice is that as I go further down the runway it varies significantly from the centerline as we see in the end of my rectangle:

enter image description here

This is not what I would expect, as I would expect the centerline to continue to match the runway centerline, or at least be closer.

My calculation for the points to plot is rather simplistic, as it is just the runway heading +/- the magnetic variation and it leaves me with this result. So my question is, is there anything else in my calculation I am missing in order to get this to be more precise?

  • 4
    I'm not sure this has anything to do with aviation, it looks like a topography question. Have you considered the historical variation in the local magnetic anomaly? Maybe the runway was aligned when built, but isn't now. – AEhere Oct 14 '19 at 22:07
  • I'm not sure what is driving you to use MagVar for this task, but you would be way more precise if you simply lined it up visually. – Michael Hall Oct 15 '19 at 17:51
  • 1
    Where are you getting the runway magnetic heading? Runway headings are never whole numbers. Runway 09 might by 094.256°. That extra .256 looks to be the error in your calculations. What country are you referring to? – wbeard52 Oct 16 '19 at 2:20

Without knowing more specific about your calculation, it's impossible to know what you might have left out. But probably the biggest would be, where exactly are you getting the "runway heading" information from? As far as I know, these are documented only to the nearest degree, which leaves plenty of room for error.

If you're getting the information from some aeronautical facilities database, then you should already have access to the true heading of the runway. This is likely to be a lot closer to the actual heading for a couple of reasons: a) magnetic variation changes over time, sometimes even over a short time, and b) the runway was likely constructed according to the true desired heading of the runway.

Another piece of information commonly available would be the lat/lon of the runway end. For the purposes of overlaying some visual element on the runway, I would think that this data would be more useful. Presumably you've got some sort of geolocation implementation that registers the displayed photographic image with other elements (the display, if nothing else). This same geolocation can be used to find the midpoints of the short ends of your rectangle, and then of course from there it's a simple matter to draw the actual rectangle with whatever width you like.

  • Actually you bring up a good point that I hadn't thought about, I do have the runway end coordinates for both ends. So perhaps a better approach would be to find the heading between the 2 runway end points and use that as the effective "runway heading" and so then for visualization purposes it might accomplish what I'm looking for. – ez4nick Oct 14 '19 at 23:38
  • If all you're really trying to do is draw the rectangle, I doubt you need to even calculate the heading per se. Just map the runway end coordinates from lat/lon to whatever drawing coordinate system you're using, and calculate the rectangle in that coordinate system. How exactly to do this depends on the "visualization system" you're using, but it's been my experience that trying to compute complex shapes in an abstract system just to produce those same shapes in the concrete rendering environment is harder than moving to the rendering coordinate system as early in the process as possible. – Peter Duniho Oct 14 '19 at 23:56