Geo Distance vs ST_DistanceSphere

[Barrett Clark]

I'm trying to compare the distance calculation that RGeo performs to ST_DistanceSphere. I thought they should give the same result given a spherical factory, but they don't. I'm hoping someone can help me understand why.

Here is what I'm doing:

Using RGeo:

factory = RGeo::Geographic.spherical_factory(:srid => 4326)

point_sfo = factory.point(-122.3748433, 37.61900194)

point_lga = factory.point(-73.87260917, 40.77724306)

distance_meters = point_sfo.distance(point_lga)

distance_miles = distance_meters * 0.000621371

=> 2575.9719632552024

Using PostGIS:

SELECT ST_DistanceSphere(

  ST_GeomFromText('POINT(-122.3748433 37.61900194)',4326), -- SFO

  ST_GeomFromText('POINT(-73.87260917 40.77724306)',4326)  -- LGA

)::NUMERIC * 0.000621371 AS dist_miles

2573.09304783170447076

Thank you,

Barrett

[Court Simas]

I, too, would love to know what's going on here.