Hi I am quite a newbie in Ruby. I want to sort a list of locations by relative distance (shortest to farthest relative to my location) on the fly. For example, let say I am at x and y coordinate, I have other points in the map around me. I want to be able to list the other points in terms of the shortest distance from me to the farthest. I store longitude and latitude information on the database. However, I cannot store the relative distance permanently, as (i) number of points change in real time; and (ii) I don't have relationship with the other points, which means the many-to-many tables are still empty. Any ideas?

on 2009-04-17 07:21

on 2009-04-17 07:29

On Apr 16, 2009, at 10:21 PM, Milton Wong wrote: > change > in real time; and (ii) I don't have relationship with the other > points, > which means the many-to-many tables are still empty. > > Any ideas? Google for "earth distance sql"... or trust that the below is right :) This assumes that some_table has columns for latitude and longitude and that you've set variables for latitude, longitude and radius (in miles) to search on. SELECT * FROM some_table WHERE (3960 * 2 * ASIN( SQRT( (1 - COS(RADIANS(latitude - #{latitude}))) / 2 + COS(RADIANS(#{latitude})) * COS(RADIANS(latitude)) * (1 - COS(RADIANS(longitude - #{longitude}))) / 2))) <= #{radius}

on 2009-04-17 18:30

Michael Wong wrote: > Hi I am quite a newbie in Ruby. I want to sort a list of locations by > relative distance (shortest to farthest relative to my location) on the > fly. [...] No need to implement this yourself. Use PostgreSQL, PostGIS, and (if necessary) GeoRuby. This will make the task almost trivial. Best, -- Marnen Laibow-Koser http://www.marnen.org marnen@marnen.org

on 2009-04-18 21:02

Philip Hallstrom wrote: > On Apr 16, 2009, at 10:21 PM, Milton Wong wrote: > SELECT * FROM some_table WHERE > (3960 * 2 * ASIN( SQRT( (1 - COS(RADIANS(latitude - #{latitude}))) > / 2 + COS(RADIANS(#{latitude})) > * COS(RADIANS(latitude)) > * (1 - COS(RADIANS(longitude - #{longitude}))) / 2))) <= #{radius} Thanks Phillip, Michael. I tried the formula and it works!