Formula for Radial Latitudes and Longitudes

Date: 10/15/2002 at 00:28:44
From: Bill Tait
Subject: Formula for radial latitudes and longitudes

I have a database of places that lists a latitude and longitude for 
each place. I want to make a radius search tool that returns all 
places based on latitude/longitude within a certain radius (say 
5 miles) of a given place. I have already created one that will return 
results of a square or rectangle, but I can't figure out how to do 
this when it's a radius. Obviously calculating the area is easy, but 
to display a list of every lat/lon within that area is the problem.

Any ideas?

Regards,
Bill Tait

Date: 10/15/2002 at 08:31:37
From: Doctor Rick
Subject: Re: Formula for radial latitudes and longitudes

Hi, Bill.

We get lots of questions like this. Here's what you need to do. For 
each point in your database, compute the distance between that point 
and the given point. If it's less than the specified distance, add it 
to your list of places within the circle. I suggest the following 
algorithm for calculating distance:

   Haversine Formula
   http://mathforum.org/library/drmath/view/51879.html 

If you're only considering short distances like 5 miles, you can use 
an approximation for short distances in which the earth can be 
regarded as flat. It's explained here:

   Circle of Radius 1 km around Given Latitude/Longitude
   http://mathforum.org/library/drmath/view/61135.html 

Using this method, you will convert the longitude (a2) and latitude 
(b2) of each point, with a1 and b1 equal to the longitude and 
latitude of the given point, to x and y, then calculate

   r^2 = x^2 + y^2

If this is less than the given distance squared, add the point to the 
list. This approximation requires fewer trig functions, but it still 
requires one. It only needs to be computed once for the given point, 
however, not for each point in the database.

- Doctor Rick, The Math Forum
  http://mathforum.org/dr.math/