I use a lot of UK data projected in British National Grid. The data are already in that format, and they are elevation data at 50m postings. So they fit real nice with the GB National Grid which of course is measured in metres.
I do a lot of grid operations like line of sight. Sometimes I'm dealing with a line of sight of 100miles. Say from one mountaintop to the other.
Now, if we ignore earth curvature and refraction for a second (and their horizontal component shouldn't matter too much anyway as they ought to apply only in the vertical dimension...) I think I'm right in saying that light travels in straight lines. So let's imagine a nice straight line from mountain A to mountain B. 100 miles apart. Sorry, let's say 200km apart, keep it all metric. A large distance.
Now, if you were to plot that straight line on the OS BNG map, would it also be a straight line? Or would it be slightly curved? This is what I can't work out. Because this has implications for line of sight algorithms.
How far does the straight line on the map grid deviate from the horizontal position of the real line of sight line on the real world? How much are these two lines different?
I need to know, but I can't figure it out. If anyone can explain to what extent the British National Grid distorts directions, I'd find it really helpful. I know that azimuthal equidistant map projections preserve direction (great circles etc) but before using that I need to confirm what kind of horizontal deviation I'd expect over a 200 km line of sight on UK data. Any ideas? Thanks a lot
Edited by mikerouge, 29 May 2010 - 03:28 AM.