﻿ 5. Detailed description of the Actions > 5.10. TA - GIS - Geographical Information System > 5.10.14. ConcaveHull GIS

# 5.10.14. ConcaveHull GIS

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Function: ConcaveHull

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Short description:

Create POLYGON’s from GEOMETRIES

Long Description:

To contruct some POLYGONs from POINTs inside Anatella, you can either use the tesselateGIS action or the ConcaveHullGIS Action.

There are, basically, 2 use cases to use this Action:

Use the “Union” mode to merge many small POLYGONs into bigger MULTIPOLYGONs: See the section 5.10.9.2. for an example where we merge tother 6048 geometries into only 95 geometries:

Use the Convex/Concave mode to recreate a POLYGON from a set of points. If the input geometries are not points, Anatella will dissolve the input geometries into many points before recreating POLYGONs from the points.

 A polygon is convex when there are NO "dents" or indentations in it (no internal angle is greater than 180°)   The opposite idea is called "concave".

Given in input a set of points that “looks like” a “S” shape, the ConcaveHull GIS action produces as output this geometry (in yellow):

To create a ConcaveHull, the basic approach is that it first creates a convexhull of the geometry and then uses the ST_ClosestPoint function to cave in the hull to transform it into a concave hull.

The typical use case is to create a POLYGON from a set of coordinates that represents different given classes of Customers (one class per partition). The ConcaveHullGIS Action will then (re-)create a POLYGON that encompass all the Customers inside the same class.

This Action assumes that the geometries are defined in a standard PLANAR coordinate system (i.e. not inside the SRID 4326), so for best results, you shouldn’t use it inside the SRID 4326 4326 (i.e. use the ReprojectGIS action to do all computations in a better SRID).