On 11/07/18 05:46, Huidae Cho wrote:
Hello Rich,
g.region creates a rectangular computational region while r.mask can
create an irregular mask. If you use r.mask, any cells outside the mask
will be ignored, so you have to make sure that your mask is big enough
to cover your basin. In terms of computational time, r.mask can be
faster depending on what modules you need. For example, when you run
flow accumulation, you may save some time using r.mask because you don't
have to calculate flow accumulation outside your area of interest. If
you use g.region that covers your basin, you calculate flow accumulation
outside the basin within the rectangular region. So in general, I would
say r.mask is preferred if and only if the mask is correct.
See also the general raster info:
https://grass.osgeo.org/grass74/manuals/rasterintro.html which explains
that:
1. Raster output maps have their bounds and resolution equal to those
of the current computational region.
2. Raster input maps are automatically cropped/padded and rescaled
(using nearest-neighbour resampling) to match the current region.
3. Raster input maps are automatically masked if a raster map named
MASK exists. The MASK is only applied when reading maps from the disk.
In other words: the MASK only applies for reading. For a given region
and MASK, interpolation will happen for the entire region, but only
using raster input data which falls within the MASK (vector input data
is read from the entire region generally). If you erase the MASK, you
will see that the result of interpolation (and other raster commands)
covers the entire region.
Moritz
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