id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
304	Geography Best SRID or ?	robe	pramsey	"I'm a bit puzzled maybe its because I haven't thought about how the curvature of the earth affects area much, but I would expect that if I buffer a point, any point on the globe by say 10 meters, that each polygon would be the same area.  It more or less is if I convert to UTM zone, buffer, convert back to geography --> take the area.  But not when I use ST_Buffer geography.  What gives?

(If I buffer by 10 meters -- I expect the answer to be pi()*10^2 (314 +- 2).  Really 312 because of the inaccuracy of the default buffer (using 8 segs per quarter circle).

and it is for the most part except when I go to Helsinki, Bergen.  My plot of land is around 288 sq meters.  I guess I shouldn't be visiting those places.


{{{
SELECT city, ST_Area(ST_Buffer(geography(the_geom), 10),true) As geog_area, 
	ST_Area(geography(ST_Transform(ST_Buffer(ST_Transform(the_geom,utm_srid), 10), 4326))) As utm_geog_area
FROM (VALUES ('Helsinki', ST_GeomFromText('POINT(24.9767 60.1964)',4326), 32635 ),
	('Bergen', ST_GeomFromText('POINT(5.4907 60.35)',4326), 32631 )
	) f(city, the_geom,utm_srid);
}}}

the above query yields

{{{
  city   |    geog_area     |  utm_geog_area
---------+------------------+------------------
Helsinki | 287.711725821719 | 312.298024061475
Bergen   | 288.120740812147 | 312.249761770185
}}}






"	defect	closed	medium	PostGIS 1.5.0	postgis	trunk	fixed