How to create Union of two geometries in PostGIS?

How to create Union of two geometries in PostGIS?

We will use the geomunion OpenGIS function to generate another MULTIPOLYGON PostGIS layer, named polygon1_union, composed of the polygons of polygon1 layer merged for the common value of CODE field. We will get the polygon1_union layer, as in the picture: To create polygon1 layer, just execute in your PostGIS environment the following SLQ code:

What does a union do in PostgreSQL geometry?

Unions the input geometries, merging geometry to produce a result geometry with no overlaps. The output may be a single geometry, a MultiGeometry, or a Geometry Collection. Comes in several variants:

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.

Which is the aggregate variant of the St Union function?

Array variant: returns a geometry that is the union of an array of geometries. Aggregate variant: returns a geometry that is the union of a rowset of geometries. The ST_Union () function is an “aggregate” function in the terminology of PostgreSQL.

How to merge polygons in a GIS task?

I was asked this week to perform a fairly simple GIS task: buffer provincially significant wetlands by 120m, buffer other evaluated wetlands by 30m, and merge the results with an existing wetland layer. This would obviously involve a dissolve process since the buffered wetlands would likely create significant overlap between polygons.

How many multipolygons are there in PostGIS 1?

The original PostGIS layer, polygon1, is composed of 7 multipolygons, as you can realize querying your database: In these 7 multipolygons there are 3 ones with a code value of 2, 3 ones with a code value of 3, and only one with a code value of 1.

How to simplify PostGIS geometries but keep them contiguous?

In both case, for simplifying, you can choose the ST_Simplify and ST_SimplifyPreserveTopology functions based on Douglas-Peucker algorithm or the ST_SimplifyVW function based on the Visvalingam-Whyatt algorithm if you use PostGIS 2.2+. I have posted another solution on my blog at www.spheraware.com/blog/post/late-tessellate.

Which is the Union of two input geometries?

Two-input variant: returns a geometry that is the union of two input geometries. If either input is NULL, then NULL is returned. Array variant: returns a geometry that is the union of an array of geometries. Aggregate variant: returns a geometry that is the union of a rowset of geometries.

What does the St _ Union of the geometry column of?

In your query you are using the simple one that is creating the union of two single geometries. What you are doing is cross joining your two tables, and make union of every combination of the geometries of your two tables, resulting in n*m multipolygons, where n is the row count of your first table and m the count of the second one.