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

[Hamish]

Hamish wrote:
[...]
> > the ideal solution is to have both curvature and refection
> > corrections implemented as flags in the new & improved
> > r.viewshed. [...]

Benjamin wrote:
> 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.

ok, done for r.viewshed in r46423. Number of visible cells
reduces slightly when the curvature flag is used, and rebounds
ever so slightly when the refraction flag is used.
Please test.


> 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

I could not get at the Yoeli(1985) article as it's behind a
paywall my univ does not subscribe to. Can anyone say what's in
it?

> 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.

I don't mind that / it is not so different from the physics I do
in my day job, and just using a fudge factor of +1/7th leaves me
feeling like the job is poorly done. Passing the coeff off to the
user without further guidance seems like a bit of a cop out. I
suppose there is a gradient in the coeff as you move from the
tropics to high latitudes, daily temperature, Linke factor,
humidity, aerosols, etc ... ?


> But given that most DEMs have an inherent vertical error that
> is greater than the influence of these effects,

can we quantify that? for example what's STRM 95% confidence
accuracy?

> 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).

it would be good for us to do a rough back of the envelope calc
to justify that before fully forgetting about it.

I guess for the worst case scenario we could try the views from
Mt. Everest and/or Olympus Mons and see what difference it makes.


thanks,
Hamish