Reduces the number of vertices in a line or polygon, preserving topology

Works with the following datatypes:

This function works with 2.1 endpoint(s). What does this mean? »

Similar to simplify(...), the simplify_preserve_topology(...) function reduces the number of vertices in a line or polygon using the Ramer–Douglas–Peucker algorithm. This is useful for approximation or for reducing the total size of a geospatial dataset.

The DPR algorithm in action

Unlike simplify(...), this function will not over-simplify geometries.

simplify_preserve_topology(...) takes two arguments:

  • The geometry to be simplified
  • The tolerance, in meters, used to simplify the segments in your geometry.

For example, to get a simplified version of the polygon for the City of Seattle from the Tiger LINE “Places” dataset:$select=name, simplify_preserve_topology(the_geom, 0.001) AS the_geom&name=Seattle

For more information on how simplification works, you should refer to the ST_Simplify PostGIS function.

Example Use Case: Generating WKT Polygons for within_polygon(...)

One handy use of simplify_preserve_topology(...) is to generate simplified polygons that work with our within_polygon(...) function. To use within_polygon(...), you need to generate a string in Well Known Text, a text markup language for representing geometries, and that string also needs to be fit within the limit of a URL string, which can be as short as 2000 characters for some HTTP client libraries.

Fortunately, our CSV output type already encodes polygons in WKT, and you can use simplify_preserve_topology(...) with our CSV output. So, if you wanted to fetch a simplified version of the Seattle city boundary to feed into another SoQL query, you could use a call similar to the one above, but change the output type to get you CSV and WKT:$select=simplify_preserve_topology(the_geom, 0.001) AS the_geom&name=Seattle