[GRASS-user] GRASS & earth curvature correction (viewsheds, LOS)

[Benjamin Ducke]

Dear list --

I just recently had the opportunity to revisit
the issue of earth curvature correction in r.los
(and, by extension, r.cva). The curvature of the
Earth's surface means that visibility will be
over-estimated if the line-of-sight algorithm
does not correct for its effect. I initially
implemented a tentative correction in r.cva and
it was with some anxiety that I saw that (untested)
method spread into the source code of r.los.

However, after some testing and comparison with
the output of ArcGIS' Viewshed module, I can now
confirm that the correction method works just fine.

So that's the good news. The remaining itch is that
atmospheric refraction is not corrected for in any
GRASS line-of-sight based module. That effect is
smaller, but it works against the effect of earth
curvature. This means that a module which corrects
for curvature, but not refraction, will tend to
over-compensate (by about 6/7).

I have not tested r.viewshed's curvature correction.
But if it uses the same method as r.los, then the
same observations apply.

As an alternative to all this mess, I have attached
a little script which allows the user to pre-process
the digital elevation model, in effect producing
a "fake" planar model that "fools" LOS algorithms into
operating in a plane instead of on a curved surface.
This little trick works as long as the region is
not more than a few hundred kilometers across. Kudos
to Bill Huber of Quantitative Decisions, who made the 
details of the computations available via the user 
forums of a well-known GIS producer. More details
can be found in the HTML man page included in the
attached ZIP file.

Pre-processing the elevation data and then turning
OFF any built-in correction methods of any LOS
module used will make the computations transparent
and more comparable.

Btw.: When comparing the output of ArcGIS' Viewshed
with that of r.los (or any other LOS modules I have
tried, including some from SEXTANTE), you will notice
that the ArcGIS viewsheds are always more contiguous
and less "noisy". That software seems to use some
internal tolerance or smoothing to achieve this effect.
I am not sure it's a desirable thing to emulate, though.
Better smooth the result manually and in a controlled
manner using e.g. a median filter, if desired.



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