[GRASS-dev] r.watershed failing: seg fault and exit code 35584

[Markus Metz]

Chris Carleton wrote:
> Okay thanks. I was using a 'trial-and-error' method for determining the
> threshold, but I think I ought to start large and proceed toward smaller
> values. I haven't come across any analytical method for determining the best
> threshold value for stream extraction.

There are conceptually different approaches to determine a reasonable
threshold. One would be to use the DEM only and estimate what
threshold provides a reasonable stream network. Obviously you can get
vastly different stream networks from a 1m LiDAR DEM and a 90m SRTM
DEM. Such an approach gives an answer to what is possible with the
DEM, which does not necessarily match reality (most of the time I
guess it does not match reality). A different approach would be to
look at rainfall patterns, soil infiltration rates etc, and decide
whether you are interested in ephemeral, intermediate or permanent
streams. For permanent streams, a decision must be made whether you
want to get all, also very small permanent streams, or only major
streams.

Markus M


Some smart person needs to develop a
> statistically sound method for assessing the uncertainty associated with
> geophysical feature extraction so that multiple results can be compared
> meaningfully and we're not left to assess the validity of a result on the
> basis of whether or not it 'looks okay' and preferably that doesn't involve
> going into the field to count streams. I'm going to compile GRASS from
> source with the latest stable release and configure for 64bit support. I
> think the current install was done through the repository so it's behind and
> not optimized. On that note, I'm trying to figure out what the appropriate
> configure flag(s) is(are) for HDF4 and HDF5 support in GDAL so that I can
> open HDF files in GRASS. The only like that I can find that seems to be
> discussing it formally is here:
>
> http://trac.osgeo.org/gdal/wiki/HDF
>
> Does anyone have the answer handy or another link with instructions that are
> more clear? Thanks,
>
> Chris
>
> On 24 August 2010 05:39, Markus Metz <markus.metz.giswork at googlemail.com>
> wrote:
>>
>> Martin Landa:
>> >
>> > Chris Carleton:
>> >> I haven't been able to find something in the mailing lists about this,
>> >> but
>> >> if you know of somewhere that I can find a solution I'd appreciate the
>> >> link.
>> >> I'm running GRASS 6.4 RC5 on Ubuntu LTS 10.04 with a Dell Precision
>> >> T7500
>> >> Workstation. The region settings are as follows;
>> >
>> > RC5 is quite old - please try RC6 or better fresh SVN.
>> >
>> The Ubuntu version and grass version used are both true 64bit?
>> Processing over 130 million cells with r.watershed in memory and 12GB
>> is possible, but only if grass is compiled as a 64bit application.
>>
>> SECTION 4: Watershed determination is pretty much unchanged since RC5,
>> the segfault might also happen in current 6.4.
>>
>> The combination of threshold=10 and over 130 million cells is a bit
>> unusual, producing a very large number of stream segments, but AFAICT
>> not large enough to trigger a segfault.
>>
>> Markus M
>>
>
>