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

[Markus Metz]

On Sun, Apr 29, 2018 at 7:36 AM, Steven Pawley <dr.stevenpawley at gmail.com>
wrote:
>
> 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).

r.texture already has the -n flag: Allow NULL cells in a moving window
r.slope.aspect has a new -e flag: Compute output at edges and near NULL
values (same like gdaldem -compute_edges)
r.mapcalc has the isnull() function and the special operators "&&&" and
"|||" handling NULL values
r.resamp.interp needs to be done for bilinear, bicubic, lanczos,
implementation could be relatively easy in the raster library functions

Markus M

> 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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>
>
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