I'm in the process of moving a GIS app from manual distance
calculations (using the haversine formula) to PostGIS using
ST_Distance_Spheroid (with WGS84).
The geometry column is just storing longitude/latitude points (in
radians).
The distances I get with ST_Distance_spheroid (and ST_Distance_Sphere)
are just about exactly 28 times as large as the haversine distance,
and I can't figure out where that ratio is coming from.
I initially entered the data with no SRID, but that doesn't make a
difference: I get the same ratio if I enter the data points as WGS84.
It's probably a unit conversion issue, but the ratio isn't a familiar
one.
When I spot-check known locations, the haversine distances are very
close to the real-world values, and the maximum values there match the
range I specify in ST_DWithin.
Any ideas/pointers?
(Mac OS X 10.5.8, PostgreSQL 8.2.4, PostGIS 1.4, GEOS 3.1.1, Proj 4.6.1)
Ta,
-Stephen
-----------------------------------------
Stephen Sample, NPPA IT Manager/Web Developer
+919/383-7246 x11, <[email protected]>
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