Exporting rasters with the gdal_translate and gdalwarp GDAL commands

In this recipe, you will see a couple of main options for exporting PostGIS rasters to different raster formats. They are both provided as command-line tools, gdal_translate and gdalwarp, by GDAL.

Getting ready

You need the following in place before you can proceed with the steps required for the recipe:

1. You need to have gone through the previous recipe and imported tmax 2012 datasets (12 .bil files) as a single multiband (12 bands) raster in PostGIS.
2. You must have the PostGIS raster format enabled in GDAL. For this purpose, check the output of the following command:

      $ gdalinfo --formats | grep -i postgis

The output of the preceding command is as follows:

      PostGISRaster (rw): PostGIS Raster driver

3. You should have already learned how to use the GDAL PostGIS raster driver in the previous two recipes. You need to use a connection string composed of the following parameters:

      $ gdalinfo PG:"host=localhost port=5432
      dbname='postgis_cookbook' user='me' password='mypassword'
      schema='chp01' table='tmax_2012_multi' mode='2'"

4. Refer to the previous two recipes for more information about the preceding parameters.

How to do it...

The steps you need to follow to complete this recipe are as follows:

1. As an initial test, you will export the first six months of the tmax for 2012 (the first six bands in the tmax_2012_multi PostGIS raster table) using the gdal_translate command:

      $ gdal_translate -b 1 -b 2 -b 3 -b 4 -b 5 -b 6
      PG:"host=localhost port=5432 dbname='postgis_cookbook'
      user='me' password='mypassword' schema='chp01'
      table='tmax_2012_multi' mode='2'" tmax_2012_multi_123456.tif

2. As the second test, you will export all of the bands, but only for the geographic area containing Italy. Use the ST_Extent command to get the geographic extent of that zone:

      postgis_cookbook=# SELECT ST_Extent(the_geom) 
      FROM chp01.countries WHERE name = 'Italy';

The output of the preceding command is as follows:

3. Now use the gdal_translate command with the -projwin option to obtain the desired purpose:

      $ gdal_translate -projwin 6.619 47.095 18.515 36.649
      PG:"host=localhost port=5432 dbname='postgis_cookbook'
      user='me' password='mypassword' schema='chp01'
      table='tmax_2012_multi' mode='2'" tmax_2012_multi.tif

4. There is another GDAL command, gdalwarp, that is still a convert utility with reprojection and advanced warping functionalities. You can use it, for example, to export a PostGIS raster table, reprojecting it to a different spatial reference system. This will convert the PostGIS raster table to GeoTiff and reproject it from EPSG:4326 to EPSG:3857:

      gdalwarp -t_srs EPSG:3857 PG:"host=localhost port=5432 
      dbname='postgis_cookbook' user='me' password='mypassword' 
      schema='chp01' table='tmax_2012_multi' mode='2'" 
      tmax_2012_multi_3857.tif

How it works...

Both gdal_translate and gdalwarp can transform rasters from a PostGIS raster to all GDAL-supported formats. To get a complete list of the supported formats, you can use the --formats option of GDAL's command line as follows:

$ gdalinfo --formats

For both these GDAL commands, the default output format is GeoTiff; if you need a different format, you must use the -of option and assign to it one of the outputs produced by the previous command line.

In this recipe, you have tried some of the most common options for these two commands. As they are complex tools, you may try some more command options as a bonus step.

See also

To get a better understanding, you should check out the excellent documentation on the GDAL website:

• Information about the gdal_translate command is available at http://www.gdal.org/gdal_translate.html
• Information about the gdalwarp command is available at http://www.gdal.org/gdalwarp.html