public final Geometry polygon() throws GeometryException {
    if (!canBePolygon()) {
      throw new GeometryException("feature_not_polygon", "cannot be polygon", true);
    }
    return polygonGeometry != null ? polygonGeometry : (polygonGeometry = computePolygon());
  }
Returns this feature as a {@link Polygon} or {@link MultiPolygon} in world web mercator coordinates.

@throws GeometryException if an error occurs constructing the geometry, or of this feature should not be
                          interpreted as a line
planetiler-core/src/main/java/com/onthegomap/planetiler/geo/WithGeometry.java