[GRASS-user] r.damflood and Courant-Friedrich-Lewy stability condition

Anna Petrášová kratochanna at gmail.com
Wed Dec 30 16:16:51 PST 2015


On Wed, Dec 30, 2015 at 6:40 PM, Panagiotis Mavrogiorgos <pmav99 at gmail.com>

> Hi all,
> In r.damflood if you try to run a calculation using invalid input you get
> a warning WRT Courant-Friedrich-Lewy stability condition
> <https://en.wikipedia.org/wiki/Courant%E2%80%93Friedrichs%E2%80%93Lewy_condition>.
> If the wikipedia article is accurate and it is indeed a necessary condition
> for convergence wouldn't it make sense to raise a fatal error (or whatever
> is more appropriate) as soon as the condition isn't respected? I mean I
> don't see much point in running a potentially lengthy simulation that will
> give meaningless results. Am I wrong in this?

I was getting this warning several times in the beginning of the
simulation, but the simulation still produced reasonable results. Lowering
the time step would probably eliminate it. I think the warning is
appropriate and shouldn't be changed to fatal error in my opinion, unless
the original authors or anyone else who fully understands it say otherwise.



> If my suggestion has indeed merit, could anyone suggest the most
> appropriate way to do this?I don't mind submitting a patch but I am afraid
> that I am not really familiar with GRASS C APIs.
> These are the relevant lines:
> https://trac.osgeo.org/grass/browser/grass-addons/grass7/raster/r.damflood/SWE.c#L504
> https://trac.osgeo.org/grass/browser/grass-addons/grass7/raster/r.damflood/SWE.c#L708
> thank you,
> Panos
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