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## Re: [TowerTalk] terrain profile algorithm

 To: towertalk@contesting.com Re: [TowerTalk] terrain profile algorithm Ian White GM3SEK Ian White GM3SEK Sat, 16 Jun 2012 18:23:39 +0100 mailto:towertalk@contesting.com>
 ```Jim Lux wrote: >On 6/16/12 9:55 AM, Ian White GM3SEK wrote: >> Jim Lux wrote: >>> I'm looking for an algorithm for my "generate all profiles for HFTA" app.. >>> I've taken a couple trivial shots at this, but I'm pretty sure someone >>> out there knows of a "better way". >>> >>> here's what I want.. >>> >>> I have an array of heights vs distance at fairly close post spacings (on >>> the order of 10 meters) maybe 1000 points or so, representing the >>> profile along a particular radial. What I want to do is "compress" that >>> into about 100 or so segments that are connected (e.g. HFTA input). >>> That is, I want to convert that to a series of x,y coordinates connected >>> by straight lines that "adequately" represents the terrain. >>> >>> I started with a simple "if the adjacent point is within X meters in >>> altitude, then collapse the two into one", but that produces profiles >>> that somehow don't look right (and I can't say much about their HFTA >>> fidelity). If the underlying terrain were a ziggurat or mesas with >>> valleys, it would probably work, but it turns a gentle slope into a >>> staircase. >>> >>> This is a classic problem in cartographic generalization, and, as it >>> happens, this kind of profile is well represented by a fractal, but not >>> that this helps me much. I was hoping that someone out there knew of a >>> clever way to do it. >>> >>> Preferably in Python, since that's what I've written everything else in, >>> but I can convert anything.. so if you have one written in COBOL, that's >>> fine. >>> _______________________________________________ >> >> If may be easier than you think, because HFTA already makes a linear >> extrapolation between data points, regardless of horizontal separation. >> >> Faced with 360 sets of computer-generated radial data at 100m intervals >> going out to 200km, my manual algorithm was: >> >> 1. Cut off the data somewhat beyond the visual horizon. You can do this >> the old-fashioned way by looking at a map, by writing your own routine >> or by plotting an approximate horizon profile using www.heywhatsthat.com >> >> 2. Wherever there are>=3 points at the same height, remove all but the >> first and last. Large areas of water are obvious targets [insert other >> flatlander jokes here]. >> >> 3. If the number of points is still too large for HFTA, consider >> removing redundant data anywhere that the slope can be approximated by a >> straight line between the first and last points. The further away from >> the antenna, the less any errors will matter, so work inward from the >> greatest distance. >> >> 4. Repeat 2 and 3 until the data set is small enough for HFTA to handle. >> >> 5. And then - most important - go outside and LOOK AT WHAT'S REALLY >> THERE in the immediate foreground of the antenna. >> >> Don't hesitate to modify the satellite data wherever you know better. >> You are much closer to the reality on the ground! For example, dense >> areas of trees would have given a good radar reflection from the >> tree-tops, but at HF the true ground level will be a better >> approximation. > > >Yes.. that's the sort of algorithm, but I'm looking at something automated.. > >Step 2 is basically what I have now.. > >It's the step 3 part that is tricky to automate.. You want straight line >segments that approximate a section, but that are continuous with the >segments on either side. Before I spent significant time designing and >coding such an algorithm, I thought I'd ask. > One thing I forgot: falling ground on the far (shadowed) side of distant ridges can almost certainly be simplified in quite a brutal way. This might be a good first target in step 3. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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