ST_Buffer on Geography — Iteration

In my previous post, Imagico said:

This is nice but still fails in one important aspect – it will create significant inaccuracies once your buffering distance reaches values where buffering in a single local coordinate system is getting problematic which, depending on your precision requirements, will likely happen at distances of a few degrees. A solution to this would require either an iterative split and merge technique of a completely different approach on a very basic level (i.e. attempting truly spherical buffering).

Ok, ok. I kinda forgot about this aspect. The proper way to do this is, as always is to do it at a lower level thank in an SQL function, preferably calculating the buffer on the spheroid, ellipsoid, or geoid. But, ouch. I think we can munge something good enough for now.

We’ll still do the buffer:

WITH transformed AS (
SELECT ST_Transform(g1::geometry, _ST_BestSRID(g1)) AS geom
),
buffered AS (
SELECT ST_Buffer(geom, radius_of_buffer, num_seg_quarter_circle) AS geom
FROM transformed
),
transformed_4326 AS (
SELECT ST_Transform(geom, 4326) AS geom FROM buffered
)
...

But now that we’ve performed the buffer, we need to determine that the buffered geometry is not too large for our local coordinate system. To do that, we’ll compare the best SRID for our buffered geometry to the original best SRID that we calculated based on the original input geometry.

SELECT
CASE WHEN _ST_BestSRID(geom) = _St_BestSRID(g1) THEN geom::geography

If the two SRIDs aren’t equal, then we need to use the new best SRID based on buffered geometry and buffer using that new best SRID.

ELSE ST_Transform(ST_Buffer(ST_Transform(g1::geometry, _ST_BestSRID(geom)), radius_of_buffer, num_seg_quarter_circle), 4326)::geography

Now let’s put it all together:

CREATE OR REPLACE FUNCTION ST_Buffer(g1 geography, radius_of_buffer float,
num_seg_quarter_circle integer)
RETURNS geography AS
$BODY$

WITH transformed AS (
SELECT ST_Transform(g1::geometry, _ST_BestSRID(g1)) AS geom
),
buffered AS (
SELECT ST_Buffer(geom, radius_of_buffer, num_seg_quarter_circle) AS geom
FROM transformed
),
transformed_4326 AS (
SELECT ST_Transform(geom, 4326) AS geom FROM buffered
)
SELECT
 CASE WHEN _ST_BestSRID(geom) = _St_BestSRID(g1) THEN geom::geography
 ELSE ST_Transform(ST_Buffer(ST_Transform(g1::geometry, _ST_BestSRID(geom)), radius_of_buffer, num_seg_quarter_circle), 4326)::geography
 END
 FROM transformed_4326

$BODY$
LANGUAGE sql VOLATILE;


CREATE OR REPLACE FUNCTION ST_Buffer(g1 geography, radius_of_buffer float,
buffer_style_parameters text)
RETURNS geography AS
$BODY$

WITH transformed AS (
SELECT ST_Transform(g1::geometry, _ST_BestSRID(g1)) AS geom
),
buffered AS (
SELECT ST_Buffer(geom, radius_of_buffer, buffer_style_parameters) AS geom
FROM transformed
),
transformed_4326 AS (
SELECT ST_Transform(geom, 4326) AS geom FROM buffered
)
SELECT
 CASE WHEN _ST_BestSRID(geom) = _St_BestSRID(g1) THEN geom::geography
 ELSE ST_Transform(ST_Buffer(ST_Transform(g1::geometry, _ST_BestSRID(geom)), radius_of_buffer, buffer_style_parameters), 4326)::geography
 END
 FROM transformed_4326

$BODY$
LANGUAGE sql VOLATILE;

So, what’s the affect? Pretty significant really.

SELECT ST_Buffer(
	 ST_GeomFromText(
	  'POINT(0 0)'
	 )::geography, 5000000, 50);

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Blue line showing correct buffer, dashed red line showing distorted buffer from using wrong local coordinate system.

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