PostGIS
Synopsis
geometry +`*`+ST_FilterByM*(`geometry `geom, double precision
min, double precision max = null, boolean
returnM = false`)`;
Description
Filters out vertex points based on their M-value. Returns a geometry with only vertex points that have a M-value larger or equal to the min value and smaller or equal to the max value. If max-value argument is left out only min value is considered. If fourth argument is left out the m-value will not be in the resulting geometry. If resulting geometry have too few vertex points left for its geometry type an empty geometry will be returned. In a geometry collection geometries without enough points will just be left out silently.
This function is mainly intended to be used in conjunction with ST_SetEffectiveArea. ST_EffectiveArea sets the effective area of a vertex in its m-value. With ST_FilterByM it then is possible to get a simplified version of the geometry without any calculations, just by filtering
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Availability: 2.5.0
Examples
A linestring is filtered
SELECT ST_AsText(ST_FilterByM(geom,30)) simplified
FROM (SELECT ST_SetEffectiveArea('LINESTRING(5 2, 3 8, 6 20, 7 25, 10 10)'::geometry) geom) As foo;
-result
simplified
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LINESTRING(5 2,7 25,10 10)