Return a point approximating the center of the given region.
For a point, this is the point itself.
For a circle, it is the center point.
For a box, it is the point whose latitude is half-way between the northern
and southern limits and whose longitude is half-way between the western and
For a linestring, it is the middle point in the line.
For polygons and complex polygons, an approximate centroid
is returned. This approximation is rough, and useful for quick comparisons.