Union_polygon (dissolve)

geometric-functions union_polygon (dissolve)

syntax

  • union_polygon(polygon_data_item)

description

union_polygon(polygon_data_item) results in a parameter with all polygons of the polygon_data_item, in which the lines between adjacent polygons are removed.

In other GIS software the term dissolve is often used for this operation.

applies to

attribute polygon_data_item with an ipoint or spoint value-type

conditions

  1. The composition type type of the polygon_data_item item needs to be polygon.
  2. The domain-unit of the polygon_data_item item must be of value type uint32.
  3. The order of points in the polygon_data_item needs to be clockwise for exterior bounds and counter clockwise for holes in polygons (right-hand-rule).

This function result in problems for (integer) coordinates larger than 2^25 (after translation where the first point is moved to (0, 0)). If your integer coordinates for instance represent mm, 2^25[mm] = about 33[km]. The reason is that for calculating intersections, products of coordinates are calculated and casted to float64 with a 53 bits mantissa (in the development/test environment of boost::polygon these were float80 values with a 64 bits mantissa). We advise to keep the size of your integer coordinates for polygons limited and for instance do not use a mm precision for country borders (meter or kilometer might be sufficient).

since version

7.042

example 

parameter<ipoint> geometry (polygon) := union_polygon(ipolygon(District/geometry));

see also