POSTGIS ST_DISTANCE 函数和结果的问题

Question

Hi I have a bit of issues with postgis and calculations.

Using ST_DISTANCE causes a bit of confusion:

SELECT st_distance(
    ST_SetSRID(ST_MakePoint(16.0420,45.8250), 4326),
    ST_SetSRID(ST_MakePoint(16.1675,45.8344), 4326));

returns 0.12585153952177025 as an result.

The result seems a bit odd as on a simple visible checking those points are quite far away and result should be in meters.

For instance :

select st_distance(
  'POINT(15.651955 73.712769 )'::geography,
  'POINT(14.806993 74.131451 )'::geography) AS d;

returns 53536.74349675 which seems to work properly.

Can you tell me what I'm doing wrong?

Answer 1

st_distance returns a distance in the unit of the used coordinate system. For 4326, the unit is degree , so the distance is in degrees (which is meaningless).

As you have done, you can cast to geography first, which returns a result in meters. You can also use ST_DistanceSpheroid to get a distance in meters, or you can use a suitable coordinate system whose unit is in meters.