PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

5.2 Exporting Rasters
User can export existing rasters from PostGIS database as many small raster files named image1.tif,
image2.tif et etc. Each file corresponds to one row of a raster table.
gdal_translate "PG:host=’localhost’ dbname=’’ user=’’
password=’’ table=’table_name’ mode=’1’" c:/temp/image\#.tif
Or user can produce only one big raster file composed by available rasters of a raster table:
gdal_translate "PG:host=’localhost’ dbname=’’ user=’’
password=’’ table=’table_name’ mode=’2’" c:/temp/image.tif
5.3 Get Raster Statistics
• Get summary statistics The ST_SummaryStats(raster, band, exclude_nodata_value) function
returns summary statistics consisting of count, sum, mean, stddev (standard deviation), min
and max for a given raster band of a raster. Band 1 is assumed if no band is specified. By
default, exclude_nodata_value parameter is true.
SELECT rid, ST_SummaryStats(rast) As stats
FROM dummy_rast;
• Get pixel histogram The ST_Histogram(raster, band, categories, width[ ]) function returns a
set of (min, max, count, percent) records summarizing raster data distribution for a number of
categories(the number of resulting records) of a given band. Number of categories are autocomputed
if it is not specified. Band 1 is assumed if no band is specified. The width[] parameter
is an array that indicates the width of each category. If the number of categories is greater than
the number of widths, the widths are repeated.
SELECT rid, band, ST_Histogram(rast) As stats
FROM dummy_rast;
• Computes quantiles The ST_Quantile(raster, quantiles[]) function computes quantiles for a
raster in the context of the population. A pixel value could be examined to be at 25%, 50%,
75% percentile of the raster data that contains all possible pixel values.
SELECT ST_Quantile(rast, 0.75) As value
FROM dummy_rast WHERE rid=1;
• Get number of pixels The ST_ValueCount(raster, values[]) function returns a set of records
containing a pixel band value and the number of pixels in a given band that have a given set
of values. If no band is specified, band number 1 is used by default. By default nodata value
pixels are not counted and pixel band values are rounded to the nearest integer.
SELECT ST_ValueCount(rast) As value
FROM dummy_rast WHERE rid=1;
5.4 Display Rasters
Normally any softwares, that use GDAL to read raster and enable a database connection, are able
to display raster directly from PostGIS database. Examples of such software include:
• QGIS plugin
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