Contents
Which is the outer boundary of a polygon?
A polygon is a representation of an area. The outer boundary of the polygon is represented by a ring. This ring is a linestring that is both closed and simple as defined above. Holes within the polygon are also represented by rings. Polygons are used to represent objects whose size and shape are important.
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 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.
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”.
When to use a geometry constructor in PostGIS?
Fortunately, PostGIS supports emitting and consuming geometries in a large number of formats: The most common use of a constructor is to turn a text representation of a geometry into an internal representation:
What kind of representations are included in PostGIS?
PostGIS has extended that to include 3- and 4-dimensional representations; more recently the SQL-Multimedia Part 3 ( SQL/MM) specification has officially defined their own representation. Our example table contains a mixture of different geometry types.
How are the geometries defined in PostGIS in SQL?
In conformance with the Simple Features for SQL ( SFSQL) specification, PostGIS provides two tables to track and report on the geometry types available in a given database. The first table, spatial_ref_sys, defines all the spatial reference systems known to the database and will be described in greater detail later.