[GRASSLIST:4860] curvature of the earth: man pages don't warn

[Dan Jacobson]

I bet most grass commands don't take into account the curvature of the
earth.  Nothing wrong with that, as it would complicate things usually
unnecessarily.  However, I wish each man page would mention if it does
or doesn't...

Or is the way programs deal with it is to set limits, e.g. r.los at
100km.

Anyway, e.g. using d.3d to simulate a telephoto lens looking across to
a certain mountain 80 km away... when indeed that mountain would have
been hidden by a different mountain... but the man page didn't warn.

I'll try Glen's DTM bending [[GRASSLIST:4844] 100KM raduis viewshed]

As I'll be bending all my DTMs (like convex prescription glasses,
specifically for patient=my_location) perhaps there is a need for a
standard grass command to do this.  Or maybe I'm the first one
bothered by this.  No critical contracts at stake, just trying to find
which mountain that is behind father in the family photo album.

Hmmm, I wonder how the other leading brands of GIS deal with earth
curvature.
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