[gdal-dev] GDAL/OGR Python Question

[Brian Walawender]

Frank,

Thanks for tip.  I was able to rasterize my polygon and used that to
calculate my sum.  Very  fast.

bw

On Fri, Dec 3, 2010 at 2:28 PM, Frank Warmerdam <warmerdam at pobox.com> wrote:

> Brian Walawender wrote:
>
>> Hello,
>>
>> I am looking for some assistance in optimizing a section of code for
>> faster performance.   Here is my problem, I have GeoTiff that contains 1 km
>> x 1 km population data over an area roughly the size of the continental US
>> (7020 x 3000).   I am trying to calculate the population within a polygon.
>>  I am doing this by finding the extent of the polygon and reading in that
>> section of the GeoTiff  using the ReadAsArray function.  At this point I can
>> quickly calculate the population within the extent by using the numpy sum.
>>  However, the only way I can figure to sum the points within the polygon is
>> to iterate over the array checking each point using ogr.  If the polygon is
>> very large, this can take an extended period of time.  Is there a faster way
>> to calculate the population within a large polygon?  My code and some sample
>> output is below.
>>
>
> Brian,
>
> I think you would be better off rasterizing your mask polygon and operating
> on the masked raster.  It might be sufficient to set all cells outside the
> region to 0 in your target raster if you are just summing.  Or you might
> generate the mask separately and just check it instead of doing an
> expensive point in polygon test.
>
> The test script for the rasterize function is available at the following
> url and might provide a hint of how to use it.
>
>  http://svn.osgeo.org/gdal/trunk/autotest/alg/rasterize.py
>
> You can also find more about the rasterize api from it's C++ docs at:
>
>  http://www.gdal.org/gdal__alg_8h.html#50caf4bc34703f0bcf515ecbe5061a0a
>
> Good luck,
> --
>
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>
>


-- 

Brian Walawender

Technique Development Meteorologist

Scientific Services Division - Central Region Headquarters

816-268-3114 - Office

816-805-6497 - Cell
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