[GRASS-user] Re: QGIS GRASS plugin shadedrelief issue

Hamish hamish_b at yahoo.com
Sat Sep 25 01:20:42 EDT 2010

Micha wrote:
> We're used to perceiving shadows below and highlights above
> what is being lit. The same holds for a hill-shaded terrain.
> When we see shadows on a slope, it seems to be south facing,
> and highlights appear north facing. So if you "place" the
> sun at 130° (SE) then you are lighting the south facing
> slope, and making it appear north facing, thus causing the
> valleys to appear to be hills, and the peaks appear as
> valleys. We have a set of ortho-photos that were shot early
> in the morning (sun in the east) and a colleague always
> turns the maps "upside down" (N pointing down) so that the
> hills and valleys look right.
> The most "natural" looking hillshading is with azimuth
> between 270 - 315° (W to NW).

note that in most places GRASS uses the cartesian convention of
theta as degrees counter-clockwise from the positive x-axis, not
navigational convention of degrees being measured clockwise from
north.  (convert between the two with '90-angle')

> > By the way GRASS as any map composer like QGIS to be able to
> > finish the work (put scale bar, north, legend, title,...)?
> Yes, QGIS is a good choice. GRASS does have a full set of
> map composer tools, but some graphic elements require a bit
> more fiddling to get them right. Mind you, the results can
> be quite rewarding.

(see the help pages for the d.* modules and ps.map, and the
cartography section of the grass web site)



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