[GRASS-dev] r.terraflow performance tweak

[Markus Neteler]

Here the answer/solution from Laura.
I have backported her fix to 6.4.svn.

Markus

On Tue, Aug 26, 2008 at 5:47 PM, Laura Toma <ltoma at bowdoin.edu> wrote:
> Hi Markus,
>
> They are right,  the ratio of flow to contour can be smaller than 1, so
> there is no need for tci to be positive.
>
> I commented out the warning in the grass_trunk.  I don't have a working
> version of grass6_devel, so if it needs to be changed there as well, maybe
> somebody can copy it over.
>
> -Laura
>
>
> On Aug 26, 2008, at 5:07 AM, Markus Neteler wrote:
>
>> Hi Laura,
>>
>> do you have any suggestion here?
>>
>> Thanks
>> Markus
>>
>>
>> ---------- Forwarded message ----------
>> From: roderikk <roderikk at gmail.com>
>> Date: Mon, Aug 25, 2008 at 10:51 AM
>> Subject: Re: [GRASS-dev] r.terraflow performance tweak
>> To: grass-dev at lists.osgeo.org
>>
>>
>>
>> Hi all,
>>
>> I am trying to use r.terraflow now as well to analyse a very large
>> dataset.
>> However, I run into the same issue as the OP. I get millions of warning
>> messages that are not really relevant, because a negative tci is in my
>> eyes
>> just valid, however, adding an hour to my initial test run on a smaller
>> sample.
>>
>> I think the check on line 143 of sweep.cc should see if the flow or the
>> slope are negative (giving rise to the log of a negative number resulting
>> in
>> an error). I am not sure yet what a negative tci means, but a small flow
>> with a large slope will result in a ratio smaller than 1 giving a negative
>> value for the logarithm taken.
>>
>> Has any work been done on this?
>>
>> Sincerely,
>>
>> Roderik
>>
>>
>>
>>
>> Ludwig M Brinckmann wrote:
>>>
>>> Looking through the code of r.terraflow I discovered that there is one
>>> compile time only option that significantly reduces/improves the
>>> performance
>>> of r.terraflow. At the moment builds produce the less performant version
>>> by default, which for such a long running program is not good. Only those
>>> who read and modify the source code can get the faster version.
>>>
>>> options.h, l. 47, #defines OUTPUT_TCI by default, causing additional
>>> files
>>> generated and in my case a significant (I think millions) number of error
>>> messages from this TCI option from sweep.cc line 143 (tci negative).
>>>
>>> Not being a GIS person I have no idea what the error means, the output
>>> from
>>> r.terraflow seemed to correlate with reality despite the message, and
>>> even
>>> the author of this bit seemed not sure what the error signified: see line
>>> 141: is this true?. Printing the error a zillion times added a night to
>>> my program run.