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

[Benjamin Ducke]

----- Original Message -----
> Hamish wrote:

[..]
> 
> 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.
> 

Cool, that's what one would expect. Reassuring.

> 
> > 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 ... ?

I am sure there is. But I lack the background to judge this
correctly.
> 
> > 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 think this needs a probabilistic approach, see my other
reply to this thread.]

> > 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.

-- that would rock :)

Ben

> 
> 
> thanks,
> Hamish


------
Files attached to this email may be in ISO 26300 format (OASIS Open Document Format). If you have difficulty opening them, please visit http://iso26300.info for more information.