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

[Hamish]

Benjamin wrote:
> 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.

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.

> 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.

I'm not sure, but I suspect it might. MarkusM?

> 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.

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? *)

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..?


[*] @MarkusM: re. changing out-of-view to NULL in r.viewshed, I
now notice in the help page that NULL was previously reserved for
NULLs in the original DEM. But in that case I suspect that
everything in the shadow of the DEM's NULLs should also be NULL,
as the visibility will be unknowable. I'm mildly in favour of
keeping the code as-is, and changing the help page to match.


> 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.

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?


> 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.

absolutely. If it's a common task maybe offer a flag to run
that automatically, but certainly don't mess with the results
by default.


thanks,
Hamish