[GRASS-user] Topo to Raster

[Markus Metz]

On Tue, Mar 4, 2014 at 7:25 PM, Mark Seibel <mseibel at gmail.com> wrote:
>> Hi.
>
>>
>> is it possible in GRASS to perform something similar to "Topo to
>> Raster" as known in ArcGIS [1]?
>>
>> [1]
>> http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z0000007m000000.htm
>
> Are there specific inputs of the "topo to raster" tool that are of
> importance for your application?
>
> Using contour lines as input, I have found the r.surf.nnbathy module to
> perform very well.

Previously, contour lines created by land surveying provided more
detail than available DEMs. Nowadays (since SRTM of 2001), DEMs
provide more detail than contour lines and contour lines are usually
derived from a DEM. Therefore creating a DEM from contour lines which
if in doubt have been created using a DEM is no longer recommended,
rather use any DEM instead.

> For (LiDAR) point data, my preference is v.surf.rst.
> The "topo to raster" tool alters the DEM by filling sinks.

The ArcGIS reference for sink filling is Goodchild and Mark (1987).
ArcGIS thus ignores the literature of the last 27 years. According to
the ArcGIS documentation, "The program assumes that all unidentified
sinks are errors". Identified sinks are those supplied by the user.
Unfortunately for ArcGIS, unidentified sinks are not errors but
usually true terrain elevation, particularly in the year 1987 when
LIDAR was not yet available and DEMs were derived from radar. That
means that the elevation values surrounding sinks are erroneuos rather
than the sinks themselves. Two (of several) methods to deal with sinks
in a more realistic way are the minimum impact approach of Lindsay &
Creed (2005) which alters the DEM (implemented in GRASS as r.hydrodem)
and r.watershed which does not alter the DEM.

In short, you should not use ArcGIS to perform hydrological analysis
or create a DEM for hydrological analysis because the ESRI tools use
methods from the 1980's. Doing something similar to "Topo to Raster"
as known in ArcGIS does not make sense. Rather use
RiverTools/Whitebox/TauDEM/GRASS.

Markus M


> The arcgis
> approach alters the DEM so that their flow routing tool doesnt stop in every
> sink.  My preference is to use the data as close to original source as
> possible, and let the superb GRASS flow and routing algorithms handle
> routing through the sinks automatically.
>
> If one wanted to mimic the arcgis method of filling sinks after
> interpolating, one could run iterations of r.fill.dir to make it
> depression-less.  This isnt necessary with the hydrologic tools in GRASS
> because the r.watershed algorithm is intelligent enough to keep seeking the
> next lowest location the DEM.  Add in the fact that r.watershed has MFD, and
> GRASS quickly surpasses the ESRI hydrologic toolset offerings.
>
> HTH,
> Mark
>
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