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

[James David Smith]

Hi Morton,

Thanks for the email and for spotting the issue. Clearly Google is
rounding the coordinates.

James

On 24 July 2013 12:26, Morten Sickel <morten at sickel.net> wrote:
> 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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>
>
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