PostGIS
Spatial and Geographic objects for PostgreSQL
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Tips
Getting distinct pixel values and pixel value counts of a raster
PostGIS raster has so so many functions and probably at least 10 ways of doing something some much much slower than others. Suppose
you have a raster, or you have a raster area of interest — say elevation raster for example, and you want to know the distinct pixel values in the area.
The temptation is to reach for ST_Value function in raster, but there is a much much more efficient function to use, and that is the ST_ValueCount function.
ST_ValueCount function is one of many statistical raster functions available with PostGIS 2.0+. It is a set returning function that returns 2 values for each row: a pixel value (value), and a count of pixels (count) in the raster that have that value. It also has variants that allow you to filter for certain pixel values.
This tip was prompted by the question on stackexchange How can I extract all distinct values from a PostGIS Raster?
Case Studies
Institut Géographique National
The Institut Géographique National (IGN) is the national mapping agency for France, with 1800 employees and a mandate to collect, integrate, manage and distribute reference geographical information for the whole country.
InfoTerra
Infoterra has been in the geospatial business for over 25 years, since its founding in 1980 as the UK National Remote Sensing Centre. In that time, NRSC grew, formed international partnerships with space agencies, expanded into aerial data collection, and was eventually privatized and re-named “Infoterra” in the late 1990s.