Bg_buffer_multi_polygon

geometric-functions bg_buffer_multi_polygon

syntax

  • bg_buffer_multi_polygon(polygon_data_item, size, nr_angles)

definition

bg_buffer_multi_polygon(polygon_data_item, size, nr_angles) results in a polygon data-item with buffer polygons around each polygon in the polygon_data_item. The resulting data item has the same domain-unit as the polygon_data_item.

The size arguments indicates the buffer size, which must be a single Float64 value or a Float64 attribute with the same domain as the polygon_data_item.

The nr_angles arguments indicates how many angles are used to make the buffer angles, which must be a single UInt8 value or a UInt8 attribute with the same domain as the polygon_data_item. More angles results in smoother circles but also in more data.

The bg_ prefix of the function name indicates that the implementation of operator uses boost geometry library, more specifically, the buffer function.

A variant bg_buffer_single_polygon with the same signature can be used to buffer single polygons.

description

The bg_buffer_multi_polygon function can be used as alternative for the polygon-inflated and polygon-deflated functions. The bg_buffer_multi_polygon is much faster, but for complex polygons it might be less accurate.

  • Use a positive size argument to increase polygons (outer rings including islands are increased, inner rings (lakes) are decreased).
  • Use a negative size argument to decrease polygons (outer rings including islands are decreased, inner rings (lakes) are increased).

If only the buffer polygon without the orginal polygon is needed, use the sub-(difference) function the cut out the original polygon from the result of the bg_buffer_multi_polygon function.

The bg_buffer_multi_polygon does not always work correctly with holes and lakes if polygon geometries are not valid. If self-interactions occur in polygons we noticed unexpected results.

We advice to use e.g. QGis to check the validity of your polygons before applying the bg_buffer_multi_polygon function. Within the GeoDMS you can use the mul-(polygon-intersection) function (first and second argument are the same) to clean your polygon topology.

If that does not solve the issue, we advice to first use the split_polygon function to make a set with single polygons first and perform the bg_buffer_multi_polygon function on the single polygons.

applies to

since version

8.031

example 

attribute<fpoint> polygon_buffer (polygon, district) := bg_buffer_multi_polygon(polyset/geometry, 10.0, 16b);

more examples of buffer functions can be found here: Buffer processing

see also