bp_overlay_polygon

geometric-functions bp_overlay_polygon

syntax

  • bp_overlay_polygon (first_polygon_data_item, second_polygon_data_item) (formerly known as overlay_polygon

description

bp_overlay_polygon(first_polygon_data_item, second_polygon_data_item) results in a new uint32 domain-unit with an entry for each intersecting part of the first_polygon_data_item and the second_polygon_data_item.

The function generates three subitems for the new domain unit:

  • geometry: the geometry of the resulting polygons (in the figure the yellow polygons). This attribute has the same values-unit as the first_polygon_data_item and second_polygon_data_item attributes.
  • first_rel: a relation for the new domain unit towards the domain of the first_polygon_data_item.
  • second_rel: a relation for the new domain unit towards the domain of the second_polygon_data_item.

In QGIS, this operation is known as intersect.

It is similar to:

unit<uint32> CartesianProduct := combine(DomainUnit(first_polygon_data_item), DomainUnit(second_polygon_data_item))
{
    attribute<ValuesUnit(first_polygon_data_item)> Geometry(poly) := first_polygon_data_item[first_rel] * second_polygon_data_item[second_rel];
    attribute<bool>   IsIntersecting := area(Geometry, Float64) > 0.0;
}
unit<uint32> result := select(CartesianProduct/IsIntersecting)
{
    attribute<DomainUnit(first_polygon_data_item)> first_rel := collect_by_cond(., CartesianProduct/first_rel);
    attribute<DomainUnit(second_polygon_data_item)> second_rel := collect_by_cond(., CartesianProduct/second_rel);
    attribute<ValuesUnit(first_polygon_data_item)> Geometry(poly) := collect_by_cond(., CartesianProduct/Geometry);
}

conditions

  1. The composition type of the first_polygon_data_item and second_polygon_data arguments needs to be polygon with an ipoint or spoint value-type..
  2. The values-unit of the first_polygon_data_item and second_polygon_data arguments items must match.
  3. The order of the points in the first_polygon_data_item and second_polygon_data needs to be clockwise for exterior bounds and counterclockwise for holes in polygons (right-hand-rule).

This function results 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 cast to float64 with a 53-bit mantissa (in the development/test environment of boost::polygon, these were float80 values with a 64-bit mantissa). We advise you to keep the size of your integer coordinates for polygons limited and, for instance, do not use an mm precision for country borders (a meter or kilometre might be sufficient).

since version

15.6.0

example 

unit<uint32> intersect := bp_overlay_polygon(Building/geometry, District/geometry);