PostGIS 1.5.8 Manual - Fedora Project Packages GIT repositories

PostGIS 1.5.8 Manual226 / 310NoteGeography: For geography this is really a thin wrapper around the geometry implementation. It first determines thebest SRID that fits the bounding box of the 2 geography objects (if geography objects are within one half zone UTM butnot same UTM will pick one of those) (favoring UTM or Lambert Azimuthal Equal Area (LAEA) north/south pole, andfalling back on mercator in worst case scenario) and then intersection in that best fit planar spatial ref and retransformsback to WGS84 geography.ImportantDo not call with a GEOMETRYCOLLECTION as an argumentPerformed by the GEOS moduleAvailability: 1.5 support for geography data type was introduced.This method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1. s2.1.1.3This method implements the SQL/MM specification. SQL-MM 3: 5.1.18ExamplesSELECT ST_AsText(ST_Intersection(’POINT(0 0)’::geometry, ’LINESTRING ( 2 0, 0 2 )’:: ←↪geometry));st_astext---------------GEOMETRYCOLLECTION EMPTY(1 row)SELECT ST_AsText(ST_Intersection(’POINT(0 0)’::geometry, ’LINESTRING ( 0 0, 0 2 )’:: ←↪geometry));st_astext---------------POINT(0 0)(1 row)---Clip all lines (trails) by country (here we assume country geom are POLYGON or ←↪MULTIPOLYGONS)-- NOTE: we are only keeping intersections that result in a LINESTRING or MULTILINESTRING ←↪because we don’t-- care about trails that just share a point-- the dump is needed to expand a geometry collection into individual single MULT* parts-- the below is fairly generic and will work for polys, etc. by just changing the where ←↪clauseSELECT clipped.gid, clipped.f_name, clipped_geomFROM (SELECT trails.gid, trails.f_name, (ST_Dump(ST_Intersection(country.the_geom, trails. ←↪the_geom))).geom As clipped_geomFROM countryINNER JOIN trailsON ST_Intersects(country.the_geom, trails.the_geom)) As clippedWHERE ST_Dimension(clipped.clipped_geom) = 1 ;--For polys e.g. polygon landmarks, you can also use the sometimes faster hack thatbuffering anything by 0.0-- except a polygon results in an empty geometry collection--(so a geometry collection containing polys, lines and points)-- buffered by 0.0 would only leave the polygons and dissolve the collection shell←↪