[gdal-dev] Gdal-Grid lidar.

[Nicolas Cadieux]

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".

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.interpolate.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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