Postgresql earthdistance - earth_box with radius

Question

Please, can you explain me this behaviour of earth_box function ... or what I'm doing wrong?

data used

40.749276, -73.985643 = Empire State Building - is in my table
40.689266, -74.044512 = Statue of Liberty - is my current position in select - 8324m far from Empire State Building

my table

=> select id, latitude, longitude, title from requests;
 id | latitude  | longitude  |         title
----+-----------+------------+-----------------------
  1 | 40.749276 | -73.985643 | Empire State Building

distance from Empire State Building to Statue of Liberty

=> SELECT id, latitude, longitude, title, earth_distance(ll_to_earth(40.689266, -74.044512), ll_to_earth(latitude, longitude)) as distance_from_current_location FROM requests ORDER BY distance_from_current_location ASC;
 id | latitude  | longitude  |         title         | distance_from_current_location
----+-----------+------------+-----------------------+--------------------------------
  1 | 40.749276 | -73.985643 | Empire State Building |               8324.42998846164

My current position is Statue of Libery which is more than 8000m far from Empire State Buildng, but select return row with id 1 even when radius is only 5558m ! Can you explain me this behaviour or what is wrong?

=> SELECT id,latitude,longitude,title FROM requests WHERE earth_box(ll_to_earth(40.689266, -74.044512), 5558) @> ll_to_earth(requests.latitude, requests.longitude);
 id | latitude  | longitude  |         title
----+-----------+------------+-----------------------
  1 | 40.749276 | -73.985643 | Empire State Building

versions of extensions and postgresql

=> \dx
                                     List of installed extensions
      Name      | Version |   Schema   |                         Description
 ---------------+---------+------------+--------------------------------------------------------------  cube          | 1.0     | public     | data type for multidimensional
 cubes  earthdistance | 1.0     | public     | calculate great-circle
 distances on the surface of the Earth  plpgsql       | 1.0     |
 pg_catalog | PL/pgSQL procedural language

 => select version();
                                                                version
 --------------------------------------------------------------------------------------------------------------------------------------  PostgreSQL 9.4beta2 on x86_64-apple-darwin13.3.0, compiled by Apple
 LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn), 64-bit

thank you noe

Answer 1

The problem here is that earth_box gets Statute Miles. 8324.42998846164 meters are near 5.172560986623845 statute miles Unit Converter

The solution: convert the radius into Statute Miles units

earth_box(ll_to_earth(40.689266, -74.044512), 5558/1.609) //doesn't return results

earth_box(ll_to_earth(40.689266, -74.044512), 9000/1.609) //does.

Answer 2

As per the doc, by default, the radius is expressed in meters.

I just tested out and it looks from the documentation that you need both earth_box and earth_distance in your WHERE clause statement.

So, you need to use both earth_box and earth_distance in conjunction to get correct results.

Also in doc in earth_box function description it says:

Some points in this box are further than the specified great circle distance from the location, so a second check using earth_distance should be included in the query.

So the following will return results

SELECT  earth_distance(ll_to_earth(40.749276, -73.985643), ll_to_earth(40.689266,-74.044512)) distance
FROM (SELECT 1) test
WHERE
    (earth_box(ll_to_earth(40.749276, -73.985643), 9000) @> ll_to_earth(40.689266,-74.044512))
              AND earth_distance(ll_to_earth(40.749276, -73.985643), ll_to_earth(40.689266,-74.044512)) <= 9000
     order by distance desc

but this won't as the actual distance is about 8324.429988461638 meters

SELECT  earth_distance(ll_to_earth(40.749276, -73.985643), ll_to_earth(40.689266,-74.044512)) distance
FROM (SELECT 1) test
WHERE
    (earth_box(ll_to_earth(40.749276, -73.985643), 6000) @> ll_to_earth(40.689266,-74.044512))
              AND earth_distance(ll_to_earth(40.749276, -73.985643), ll_to_earth(40.689266,-74.044512)) <= 6000
     order by distance desc