[GRASS-user] extract by attribute from a dem

[Hamish]

Charles Ehlschlaeger wrote:
> It is also possible that the region was not set to the geographic
> coordinates of the dem or slope map when performing the r.mapcalc.
> r.slope.aspect is one of the few programs to default to extent and
> resolution of the original map, not the current region.

i.e. set the region bounds to match the input map before running region obeying
modules like r.mapcalc:

g.region rast=elevation.dem

> $ r.slope.aspect ele=elevation.dem slope=slope
> $ r.mapcalc 'new=if( slope <= 20, slope, null())'



I am surprised to see a slope of 89deg for SRTM data, that's a big cliff at
SRTM resolution! Even if there is a huge 90deg cliff, from the plan-view cell's
perspective that is only a small sliver in the cell, and to maintain that slope
over the horizonal resolution of the cell means a *huge* vertical distance
change. I work in very steep fjords with 1000m drops, and the most I see at 10m
res is somewhere around 76deg.


Hamish



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