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

[Benjamin Ducke]

[snip]
> >
> > However, after some testing and comparison with
> > the output of ArcGIS' Viewshed module, I can now
> > confirm that the correction method works just fine.
> 
> It gives confidence sure, but there's no way of knowing if their
> black box is actually correct or not, maybe we make the same
> mistakes? :) All the better to follow/cite published articles.
> 

Of course, that will always be a problem. But I have
compared three independent methods: r.los with built-in
correction, ArcGIS with built-in correction, and r.ecurv.comp.
In general, the outputs are very similar, even down to
small detail. The remaining differences must be in 
implementation details and the fact that r.los does
not implement atmospheric refraction.

I have also run some of the LOS algorithms in SEXANTE
(on a DEM that was processed with r.ecurv.comp, since
they lack any built-in correction). Again, results
were very much the same as r.los and ArcGIS.

Only ArcGIS outputs those contiguous viewsheds, though.
So they must have done something to tune the output.

> certainly the ideal solution is to have both curvature and
> refection corrections implemented as flags in the new & improved
> r.viewshed. (say, is that now ready for moving into grass7? *)

That should be really easy to do. All that's needed is to
amend the existing correction but taking away 1/7 to account
for the adverse effect of refraction.

> 
> Does refraction only work in the vertical or would you be able
> to slightly see around some distant volcano horizontally?
> ..perhaps the phenomenon is related to the atm pressure
> gradient, in which case purely a vertical-only effect..?
> 

Well, that refraction correction is really a rough
simplification of reality. Essentially, it uses the same
amount of correction as ArcGIS. There is some justification
for this. You can find links to articles here:

http://mapaspects.org/content/effects-curvature-earth-refraction-light-air-and-fuzzy-viewsheds-arcgis-92

(@Markus N: maybe that answers your question, as well?)

But in summary, accounting for realistic refraction conditions
would be much more complex, as it would also have to take into
plus different refraction at different elevations, etc.

But given that most DEMs have an inherent vertical error that
is greater than the influence of these effects, I am not sure 
it's worth spending too much time on (it might be for very
long distance visibility -- I just don't know).
 
> Do you think that noise happens because of a translation of the
> coarse horizontal grid cell size when it is translated into the
> vertical plane?

That's a really interesting thought. 
It could very well be a quantization effect of that kind.
That might explain why the noisy patches seem to show 
structural features of the DEM.

Cheers,

Ben


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