[Octave-patch-tracker] [patch #9974] [octave forge](mapping) gc2sc scxsc gcxsc

[Philip Nienhuis]

Follow-up Comment #14, patch #9974 (project octave):

Well, maybe not so fast ;-)

While fixing the scxsc.m tests I now hit the issue that gc2sc.m is a bit
inaccurate.
As touched in passing in comment #11, in test #2 the great circles from test
#4 in gcxgc.m are morphed into small circle notation, but along the way enough
precision is lost to screw up the test in scxsc.m.
Maybe I could use your assistance in improving gc2sc.m's precision.

About precision and haversines, indeed for small radii scxsc.m gets
inaccurate, but trying with haversines only gave me more inaccuracy. I want to
figure that out more thoroughly.
You can try for yourself with tangent circles, e.g.:

[la, lo] = scxsc (0, 0, 1, 0, 2, 1)

and/or maybe smaller radii and thus smaller distances between circle centers.
Or even a degenerate case with zero radius:

[la, lo] = scxsc (0, 0, 0, 0, 2, 2)

For large circles (r > 30 degrees) scxsc.m turns out to be quite accurate.

In the wiki <https://wiki.octave.org/Mapping_package> there's a long list of
functions needing implementation.

Would you have any experience and/or interest in projections?
I'd very much like to have projection functions in the mapping package based
on the proj package. I've already tried a few things but very basic. Once
projections are implemented a next step would be implementing geoshow etc.
One difficult step is to parse projection strings into something more
manageable.

So you see there s always work to be done ;-)


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