20 mile radius

jphilapy

Howdy,

Can anyone give me an idea of how to do the search that allows you locate this or that within a mile radius. For example on friendster.com their search allows you to search for other people that are within a certain distance from you. So how does the database keep tracking of these distances?

Thanks,
Jeff

leatherback

It probably uses a database where a postal code is linked to their LatLon position. After that it is basic mathematics

Jack_McSlay

Yep. This question has more mathematics to it than it does proggramming. The biggest problem you might face is about how to compensate the longitude, because at the equator , 10° of longitude means approximately 1,787.77 miles, and as you go north or south the value gradually decreases to zero.

Wikipedia has some useful info:
http://en.wikipedia.org/wiki/Longitude

One thing tho, I suggest you use a range wider than 20 miles. In several cases, that would be the equivalent of list everyone who lives in the same city

jphilapy

leatherback wrote:
It probably uses a database where a postal code is linked to their LatLon position. After that it is basic mathematics

hi leatherback, thanks for your reply.

I am thinking that too. I am just wondering if if this is something that I can load on my own server, or is this some service that is provided by say the postal service or something similar.

I believe that part of that basic math you speak of, will include the calculation of the arch of earth. I read that in passing. I will learn more as I read I suppose.

Jeff

leatherback

jphilapy wrote:
hi leatherback, thanks for your reply.

I am thinking that too. I am just wondering if if this is something that I can load on my own server, or is this some service that is provided by say the postal service or something similar.

I believe that part of that basic math you speak of, will include the calculation of the arch of earth. I read that in passing. I will learn more as I read I suppose.

Jeff

Hey Jeff,

There probably are databases that provide postal code / latlon facilities online (And if you cannot find any, let me know, and I might just start one 😃). However, these services typically will charge yu for each postal code requested, which could get costly if you do not do your programming effectively (e.g., store the location of areas you know).

As for the calcs: See jack comment. You need to be somewhat carefull in just assuming that the world is 42000 km circumference, in 360 degrees, and therefor 1 degree represents * meters. It changes with latitude. As in these applications it doesn't come too close, you can probably just get away with applying a cosine of latitude as multiplication factor. Ideally you find a converter using a projection in meters.

Sxooter

This is a pretty good choice for a GIS (Geo Info Systems) setup, like PostGIS. Pretty sure MySQL has some GIS addons as well, don't know how good they are. These systems are built to map things in a 3D, spherical realm.

leatherback

Sxooter wrote:
This is a pretty good choice for a GIS (Geo Info Systems) setup, like PostGIS. Pretty sure MySQL has some GIS addons as well, don't know how good they are. These systems are built to map things in a 3D, spherical realm.

Yeah, However to have someone learn about Geographic Information Systems for something simple like this may be a bit of an overkill.. For the distances mentioned here one can assume the globe to be flat, and distortions due to geoidial differences can be ignored.