Distance between 2 POINTs in Postgis in srid 4326 in metres

Question

This is probably a simple question, but I'm not very good at PostGIS and don't fully grok all of this.

Basically I have a table (nodes) with a POINT column (point). I have created an index on this column

create index nodes__points on nodes using gist (point) 

The column was created with

select addgeometrycolumn('nodes', 'points', 'POINT', 4326, 2) 

I am using srid 4326 because I'm adding data that's in the form (latitude, longitude). (i.e. the co-ordinate system where the position of Dublin, Ireland is lat=53.353 lon=-6.264 (which I've added with GeomFromText('POINT(-6.264 53.535)'))).

For each point, I want to find all points that are roughly within a 1km box centred on that point (so selcet a.id, count(*) from nodes as a, nodes as b where SOME_DISTANCE_FUNCTION_HERE(a.point, b.point, 1000) group by a.id;. It doesn't have to be exact, just a rough hueristic figure. a 1km bbox is fine, a 1km circle is fine. It doesn't have to be exactly 1km, just that order of magnitude.

The ST_Distance/ST_DWithin/etc. all use the units of the SRID, which for 4326/WGS64 is degrees (so 1 = 1 degree of lattitude/longitude). But I want to use metres.

I tried ST_distance_sphere and st_dwithin which can use metres, but if I do that, the explain shows that the index isn't being used.

How can I get roughly what I want, and use the geographical index?

UPDATE: This is on PostgreSQL 9.1 and PostGIS 2.0 svn build.

Answer 1

You could use ST_Transform to use meters, also remeber that not all functions are available with geography types but if you really need speed use ST_DWithin, is the fastest way. Here's an aproximation of conversions between degrees and meters:

| places | degrees    | distance | | ------ | ---------- | -------- | | 0      | 1.0        | 111 km   | | 1      | 0.1        | 11.1 km  | | 2      | 0.01       | 1.11 km  | | 3      | 0.001      | 111 m    | | 4      | 0.0001     | 11.1 m   | | 5      | 0.00001    | 1.11 m   | | 6      | 0.000001   | 0.111 m  | | 7      | 0.0000001  | 1.11 cm  | | 8      | 0.00000001 | 1.11 mm  |