What do you call an overlay in PostGIS?
One question that comes up often during our PostGIS training is “how do I do an overlay?” The terminology can vary: sometimes they call the operation a “union” sometimes an “intersect”.
How to calculate the overlapping parts of a pair of shapes in PostGIS?
So an overlapping set of three circles becomes a non-overlapping set of 7 polygons. Calculating the overlapping parts of a pair of shapes is easy, using the ST_Intersection () function in PostGIS, but that only works for pairs, and doesn’t capture the areas that have no overlaps at all.
Which is the best spatial join test for PostGIS?
My go-to performance test for PostGIS is the point-in-polygon spatial join: given a collection of polygons of variables sizes and a collection of points, count up how many points are within each polygon. It’s a nice way of testing indexing, point-in-polygon calculations and general overhead.
Can you turn a collection of overlapping polygons into a non-overlapping set?
What they mean is, “can you turn a collection of overlapping polygons into a collection of non-overlapping polygons that retain information about the overlapping polygons that formed them?” So an overlapping set of three circles becomes a non-overlapping set of 7 polygons.
How does the St Union array in PostGIS 14 work?
The ST_Union array and set variants use the fast Cascaded Union algorithm described in http://blog.cleverelephant.ca/2009/01/must-faster-unions-in-postgis-14.html A gridSize can be specified to work in fixed-precision space. The inputs are snapped to a grid of the given size, and the result vertices are computed on that same grid.
What does the St Union function do in PostgreSQL?
The ST_Union() function is an “aggregate” function in the terminology of PostgreSQL. That means that it operates on rows of data, in the same way the SUM() and AVG() functions do and like most aggregates, it also ignores NULL geometries.