Postgresql earthdistance - earth_box with radius - postgresql

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