Discussion:
Terrain elevation variances
(too old to reply)
Andrew Gillanders
2017-06-23 01:22:12 UTC
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I am trying to figure out an inconsistency I am seeing in terrain elevation. I have two versions of my scenery, one with only the default land class and one with all land classes (about 18). I am using $src/scripts/perl/examples/find_elevations.pl to interrogate FG. What is puzzling is that I am getting significantly different terrain elevation figures in each case.

My scenery build procedure is…
1. gdalchop ./work/SRTM-1 ./data/SRTM-1/*.hgt
2. terrafit -x 20000 ./work/SRTM-1
3. genapts850 --input=./data/airports/apt.dat --work=./work --min-lon=150 --max-lon=154 --min-lat=-30 --max-lat=-26
4. ogr-decode --max-segment 500 --area-type Default ./work/Default ./data/shapefiles/MyCoastline

For the default landlcass only scenery, I then run…
5. tg-construct --work-dir=./work --output-dir=./output/Terrain --min-lon=150 --max-lon=154 --min-lat=-29 --max-lat=-26 AirportArea SRTM-1 AirportObj Default

For the other version, I run ogr-decode for all the land classes, then the same tg-construct command as above, with the addition of the other land classes.
./find_elevations.pl
152.807678900 -27.647306600
152.807678900 -27.647306600 78.30243432
^C

For the scenery with all land classes, I get this…
./find_elevations.pl
152.807678900 -27.647306600
152.807678900 -27.647306600 49.97904767
^C

My question is this: does the addition of more land classes alter the elevation of the terrain? I don’t understand why there would such a difference between the two versions of the scenery, as they are using the same DEM.

Thanks in advance for any enlightenment.
Andrew Gillanders
Thorsten Renk
2017-06-23 04:07:03 UTC
Permalink
Post by Andrew Gillanders
My question is this: does the addition of more land classes alter the
elevation of the terrain?
Conceptually landclasses introduce sub-divisions of the mesh, and that
would presumably alter the interpolation process between the points
defined by the elevation data. So just based on looking at wireframe data
of the terrain, I would have expected this outcome.

* Thorsten
Andrew Gillanders
2017-06-23 07:14:07 UTC
Permalink
Thanks, Thorsten. That makes sense, although the difference in elevations (49.98 m against 78.30 m) I saw seems rather large?

Andrew
Post by Andrew Gillanders
My question is this: does the addition of more land classes alter the elevation of the terrain?
Conceptually landclasses introduce sub-divisions of the mesh, and that would presumably alter the interpolation process between the points defined by the elevation data. So just based on looking at wireframe data of the terrain, I would have expected this outcome.
* Thorsten
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