[GRASS-user] Hardware config/limits

[Markus Metz]

On Tue, May 31, 2011 at 11:15 PM, Glynn Clements
<glynn at gclements.plus.com> wrote:
>
> nunosousa84 wrote:
>
>> I've noticed that i have serious problems dealing with large raster maps and
>> some GRASS commands in Grass plugin, like r.surf.contour, shaded relief,
>> some raster calcs, etc... Im using 1 by 1 resolution, maybe it is a bit
>> exagerated, but even at 5 by 5 the calculation process is very slow. Im
>> using a Intel Core 2 Duo E8400 and 4Gb 8500mhz DDR2 Memory. Im about to do
>> an upgrade, should i opt for 8Gb memory, or i better opt for a good CPU
>> (maybe quad core)? Whats your experience dictates to improve performance?
>
> Multiple cores won't help; GRASS modules are single-threaded.
>
>> The strange thing is that Ubuntu System Monitor says only one core is at
>> 100% full capacity but the other is only 20%.
>
> That's to be expected.
>
>> Ans memory usage is only 700Mb, so whats causing all this lag and
>> slow calc, the bar almost dont moves?
>
>> I just run the r.surf.contour command and it takes infinite
>> time
>
> r.surf.contour is the problem. The documentation says:
>
>        The running of r.surf.contour is very sensitive to the
>        resolution of rasterized vector map. If multiple contour lines
>        go through the same raster, slight anomalies may occur. The
>        speed of r.surf.contour is dependent on how far "apart" the
>        contour lines are from each other (as measured in raster
>        cells). Since a flood fill algorithm is used, the program's
>        running time will grow exponentially with the distance between
>        contour lines.
>
> Maybe there's a faster approach along the lines of r.grow.distance?

Interpolating contour lines to get a surface should be a means of last
resort if nothing else is available. For elevation, there are various
datasets available, most importantly SRTM which easily beats contour
lines in terms of accuracy.

Markus M