[GRASS-dev] [GRASS GIS] #3554: r.slope.aspect: add flag/functionality to not shrink

[Steven Pawley]

Handling of nodata values at raster borders for modules which involve focal
functions would be handy to have for several GRASS GIS modules, not just
for r.slope.aspect. IMHO r.mapcalc, r.texture, and r.resamp.interp (if
using bilinear or cubic resampling) would also benefit from this option
(perhaps other tools as well). Although some users may be concerned that
filling nodata values at raster borders can create edge effects, in most
cases the common approaches (such as mirroring, reflecting or wrapping)
leave little in the way of an obvious edge effect, and also satisfy what I
think are most user's expectations that the output raster will have the
same dimensions as the input.

Although a similar effect can be achieved by growing the input raster
first, this can be inconvenient when used as part of a complex set of focal
function operations. An example is that I have just completed working on an
updated r.valley.bottom module, which required multiple r.grow operations
as the algorithm repeatedly generalizes the input DEM and calculate
flatness and lowness using r.slope.aspect and r.mapcalc focal functions. It
also appears that most other tools, including SAGA-GIS, scipy (image
filters), and gdaldem  provide a method to handle raster borders.


On Sat, Apr 28, 2018 at 2:55 PM GRASS GIS <trac at osgeo.org> wrote:

> #3554: r.slope.aspect: add flag/functionality to not shrink
> -------------------------+-------------------------
>  Reporter:  hellik       |      Owner:  grass-dev@…
>      Type:  enhancement  |     Status:  new
>  Priority:  normal       |  Milestone:  8.0.0
> Component:  Raster       |    Version:  svn-trunk
>  Keywords:               |        CPU:  All
>  Platform:  All          |
> -------------------------+-------------------------
>  at the moment, the result maps of r.slope.aspect, e.g. slope etc, are
>  shrinked by 1 pixel.
>
>  adding a flag/functionality to not shrink the result maps e.g. by adding
>  the same value as the -1 off pixel, some interpolations or similar as an
>  enhancement.
>
> --
> Ticket URL: <https://trac.osgeo.org/grass/ticket/3554>
> GRASS GIS <https://grass.osgeo.org>
>
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