[gdal-dev] Gdal-Grid lidar.

[Even Rouault]

Le vendredi 12 juin 2015 23:18:26, Nicolas Cadieux a écrit :
> Hi Even,
> Thank you very much.  I increased the buffer to 1.5 GB
> (512+512+514)*1024*1024 and I can now work much much faster with
> gdal_grid.
> 
> Does gdal_fillnodata have the same type of variable that could speed things
> up?  It is also slow and does not seem speeded up by the "All_CPUS".

No, no tunable for fillnodata, and it doesn't support multi-threaded 
parallelization.

> 
> I will use warp and the raster calculator in the future so you have more
> magic up your sleeve, I would appreciate it.
> 
> Cheers and thanks again.
> 
> Nicolas Cadieux M.Sc.
> Les Entreprises Archéotec inc. 
> 8548, rue Saint-Denis Montréal H2P 2H2
> Téléphone: 514.381.5112  Fax: 514.381.4995
> www.archeotec.ca
> 
> On Jun 5, 2015 19:13, Even Rouault <even.rouault at spatialys.com> wrote:
> > Le samedi 06 juin 2015 00:18:45, Nicolas Cadieux a écrit :
> > > Hi,
> > > I have been using gdal_grid to interpolate LiDAR data for some time
> > > now. This would be a typical command:
> > > 
> > > gdal_grid -l P0344_sol2004_XYZI_MTM8_CGVD28_150529 -txe 348733.0
> > > 401135.0 -tye 5568587.0 5773796.0 -a
> > > nearest:radius1=3.0:radius2=3.0:angle=0.0:nodata=0.0 -outsize 52402.0
> > > 205209.0 -of GTiff
> > > H:/Nicolas/pytemp/OutPut\P0344_sol2004_XYZI_MTM8_CGVD28_150529.vrt
> > > H:/Nicolas/pytemp/OutPut\P0344_sol2004_XYZI_MTM8_CGVD28_150529.tif
> > > --confi g GDAL_NUM_THREADS ALL_CPUS
> > > 
> > > As you can see, this is a huge raster and the csv. file (Pointed with a
> > > .vrt) contains over a billion points.  I am trying to do this in one
> > > shot to avoid boundary effects.  (I know I can split this .csv file
> > > into smaller bits and make this more efficient).   The computer I am
> > > using has 12 cores, 64BG of memory (smaller than the csv file) and a
> > > 1TB pcie solid state drive.  It should manage the task but it has been
> > > running for 5 days with less than 10% done).  I have access to a super
> > > computer with gdal if all else fails.
> > > 
> > > This is my question.  The "--config GDAL_NUM_THREADS ALL_CPUS" or
> > > "--config GDAL_NUM_THREADS 12" does not seem to work with the nearest
> > > neighbor algorithm. (It seem to work with the -a count command)  If I
> > > use this on a smaller data set, I see no difference in speed or cpu
> > > usage between using the "ALL_CPUS" or not using this switch.  Is this
> > > a bug or simply a limitation of the nearest neighbor algorithm?
> > 
> > Nicolas,
> > 
> > The threading mechanism is generic for all algorithms . Add "--debug on"
> > and you should see traces like "GDAL_GRID: Using 12 threads". Now, a
> > reason for it not being efficient is that the work buffer used by
> > gdalgrid (16 MB) is too small w.r.t to nearest speed. Hum, actually
> > looking more closely at the code, I see that the quad tree to index the
> > points will be rebuilt for each work buffer. This is likely the main
> > cause of the inefficiency and why you don't see measurable differences
> > between non threaded or threaded computations (since the building of the
> > quadtree is not multithreaded). If you compiled from source, you could
> > try changing the "const GUInt32 nDesiredBufferSize = 16*1024*1024;" line
> > in apps/gdal_grid.cpp to a much higher value. A more proper solution
> > would be to avoid rebuilding the quad tree for each work buffer. This is
> > mainly code restructuring.
> > 
> > An alternative to gdal_grid would be to use gdal_rasterize +
> > gdal_fillnodata. This is usually much much faster, so you could try to
> > check if it gives the results you expect.
> > 
> > You could also try scipy :
> > http://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.interpol
> > ate.griddata.html#scipy.interpolate.griddata Not sure how it behaves with
> > that number of points.
> > 
> > Even
> > 
> > > I don't want to move this
> > > to a super computer if it will not multi-thread properly. 
> > > 
> > > Thanks,
> > > Nicolas Cadieux
> > > 
> > > (Gdal1.11.2 downloaded with OSGEO4W 64bit install with QGIS 2.8 on
> > > windows 7_64)  I use GDAL from the command line not from QGIS.
> > > 
> > > 
> > > Nicolas Cadieux M.Sc.
> > > Les Entreprises Archéotec inc.
> > > 8548, rue Saint-Denis Montréal H2P 2H2
> > > Téléphone: 514.381.5112  Fax: 514.381.4995
> > > www.archeotec.ca
> > > _______________________________________________
> > > gdal-dev mailing list
> > > gdal-dev at lists.osgeo.org
> > > http://lists.osgeo.org/mailman/listinfo/gdal-dev

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