I think stackoverflow is better for this specific question.

Having said that most databases (on the backend) have spatial extension where you can run queries like 'where inside(point,geometry)'. So you create a bounding box or circle around the user, then run a query. That's faster than calculating distances between all users. The databases have special indices for geographic lookup. Both point and geometry probably need to be specified in WKT format. http://www.gaia-gis.it/spatialite-2.1/SpatiaLite-manual.html http://docs.mongodb.org/manual/applications/geospatial-index...

Assuming you have the coordinates for the each online user, iterate that collection, create a CLLocation object from the coordinate data, and check that object against CLLocation method distanceFromLocation with a radius value.

https://developer.apple.com/library/ios/documentation/CoreLo...:

Yes. When the user opens the app I will get their current location and add it to a column in the database 'currentLocation' Then I will query the database like so: select * from users where userOnlineStatus = 'online' and currentLocation = ""; <- That is what I would want to do. Its just calculating the distance so that not all random users are showing up.

Also as @mtmail mentions, the better path for this is probably not on the device itself. It would get expensive where N number of users is really high. Perhaps at the service layer, send the device location to the service, and have the service return the nearby users.