----- 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. _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
