= Buffer geodetic geometry using meters =

Constructs a buffer of a geodetic (lat/long) geometry using a distance in meters.
Uses a transformation to a suitable UTM zone, so only works for geometries with extent less than a few degrees of longitude. 

Note: not sure if this is better than simply casting to `geography`?

Sources: 
* https://blog.datapolitan.com/2013/11/22/st_buffer_meters/
* [https://gist.github.com/datapolitan/9ef9489eac5686ca3b34 st_buffer_meters.sql]
* [https://gist.github.com/datapolitan/ffdeb086b3c4531d9f4e utmzone.sql]

{{{
/* Function: ST_Buffer_Meters(geometry, double precision)
DROP FUNCTION ST_Buffer_Meters(geometry, double precision);
Usage: SELECT ST_Buffer_Meters(the_geom, num_meters) FROM sometable; */
 
 
CREATE OR REPLACE FUNCTION ST_Buffer_Meters(geometry, double precision)
RETURNS geometry AS
$BODY$
DECLARE
orig_srid int;
utm_srid int;
 
BEGIN
orig_srid:= ST_SRID($1);
utm_srid:= utmzone(ST_Centroid($1));
 
RETURN ST_transform(ST_Buffer(ST_transform($1, utm_srid), $2), orig_srid);
END;
$BODY$ LANGUAGE 'plpgsql' IMMUTABLE
COST 100;

/* Function: utmzone(geometry)
DROP FUNCTION utmzone(geometry);
Usage: SELECT ST_Transform(the_geom, utmzone(ST_Centroid(the_geom))) FROM sometable; */
 
CREATE OR REPLACE FUNCTION utmzone(geometry)
RETURNS integer AS
$BODY$
DECLARE
geomgeog geometry;
zone int;
pref int;
 
BEGIN
geomgeog:= ST_Transform($1,4326);
 
IF (ST_Y(geomgeog))>0 THEN
pref:=32600;
ELSE
pref:=32700;
END IF;
 
zone:=floor((ST_X(geomgeog)+180)/6)+1;
 
RETURN zone+pref;
END;
$BODY$ LANGUAGE 'plpgsql' IMMUTABLE
COST 100;
}}}