PostGIS 1.5.8 Manual - Fedora Project Packages GIT repositories

PostGIS 1.5.8 Manual31 / 310A second [theoretical] example may be that of a GIS analyst trying to locate all wharfs or docks that intersect a lake’sboundary on a line and where only one end of the wharf is up on shore. In other words, where a wharf is within, but notcompletely within a lake, intersecting the boundary of a lake on a line, and where the wharf’s endpoints are bothcompletely within and on the boundary of the lake. The analyst may need to use a combination of spatial predicates toisolate the sought after features:• ST_Contains(lake, wharf) = TRUE• ST_ContainsProperly(lake, wharf) = FALSE• ST_GeometryType(ST_Intersection(wharf, lake)) = ’LINESTRING’• ST_NumGeometries(ST_Multi(ST_Intersection(ST_Boundary(wharf), ST_Boundary(lake)))) = 1... (needless to say, this could get quite complicated)So enters the Dimensionally Extended 9 Intersection Model, or DE-9IM for short.4.3.6.1 TheoryAccording to the OpenGIS Simple Features Implementation Specification for SQL, "the basic approach to comparing two geometriesis to make pair-wise tests of the intersections between the Interiors, Boundaries and Exteriors of the two geometries andto classify the relationship between the two geometries based on the entries in the resulting ’intersection’ matrix."BoundaryInteriorExteriorThe boundary of a geometry is the set of geometries of the next lower dimension. For POINTs, which have a dimension of0, the boundary is the empty set. The boundary of a LINESTRING are the two endpoints. For POLYGONs, the boundaryis the linework that make up the exterior and interior rings.The interior of a geometry are those points of a geometry that are left when the boundary is removed. For POINTs,the interior is the POINT itself. The interior of a LINESTRING are the set of real points between the endpoints. ForPOLYGONs, the interior is the areal surface inside the polygon.The exterior of a geometry is the universe, an areal surface, not on the interior or boundary of the geometry.