[postgis-users] Comparing St_Distance() with Google API

[Morten Sickel]

If you look into the json google produces, you should see that the start-
and endpoints are identical - and they are neither equal to neigher your
start- nor endpoint I would guess there are some rounding going on
somewhere - that the google api just is not able to distinguish between
points so close together.



Morten


James David Smith skrev:
> Hi there everyone,
>
> I've been messing about using the Google Directions API thing
> recently, and came across some odd results. Code to demonstrate is
> below.
>
> Essentially the distance between two points using PostGIS is 18
> metres, yet when I translate the points to Lat/Long and then stick
> them in the Google API the distance comes out as 1 metre.
>
> Does anyone have any thoughts on this?
>
> Thanks
>
> James
>
> --------------------------------------------------------------------
> -- Calculate a distance between two points using SRID 27700 (OSGB36):
>
> SELECT St_Distance(
> St_SetSRID(ST_makePoint(531058, 178463), 27700) ,
> St_SetSRID(ST_makePoint(531041, 178455), 27700)
> )
>
> RESULT = 18.7882942280559  (I presume metres)
> ---------------------------------------------------------------------
> -- Now calculte a distance between two points using SRID 4326 (WGS84):
>
> SELECT St_Distance(
> St_Transform(St_SetSRID(ST_makePoint(531058, 178463), 27700), 4326),
> St_Transform(St_SetSRID(ST_makePoint(531041, 178455), 27700), 4326)
> )
>
> RESULT = 0.000256838311729703 (I presume degrees)
>
> ---------------------------------------------------------------------
> -- Now calculate the distance using the Google Directions API using
> the Lat/Lon values:
>
> http://maps.googleapis.com/maps/api/directions/json?origin=51.4899343224464%20-0.113689479559085&destination=51.4898663658699%20-0.113937164484113&mode=walking&sensor=false&units=metric
>
> RESULT = "1 m"
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