4

We have a satellite image loaded into PostGIS. We've calculated a reflectance raster from that image and stored the results as 32BF pixels. There are 54 1000x1000 tiles in the reflectance raster.

I want to create a GeoTIFF of that reflectance raster. Individual tiles work fine. But using an ST_Union on the reflectance raster column raises an exception when executing the query below from Python:

SELECT ST_AsTIFF(ST_Union(refl_rast), 'GTiff') gtiff
FROM sat_rast_calc

InternalError: invalid memory alloc request size 1344128795

I know I can export the tiles individually and merge them with gdal_merge.py afterwards. I'd prefer to export this GTiff in one step though.

Would any of these be likely to help?

  1. Using a different pixel type
  2. Passing in another ST_AsGTIFF option
  3. Increasing a PostGRES memory parameter
  4. Increasing a Linux kernel parameter
  5. Doing something else
  • 1344128795 bytes are actually 1.3 gigabytes – nickves Dec 10 '15 at 22:26
1

The in-memory limit for one row in PostgreSQL is 1 GB. Try using a less memory hungry pixel type. You can also try to pass a compression parameter to ST_AsTiff() but I think internal work is done without compression. Try also to compress tiles BEFORE ST_Union() them just in case... Rasters in PostGIS are desinged to work well as small raster chunk. Not to manipulate big rasters as a whole.

  • I don't like the solution of raising the memory limit. Ram should be treated as a temporary place and certainly not the best place of putting petabytes of data. – nickves Dec 10 '15 at 22:35

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