On Thu, 2003-06-12 at 09:29, Dennis Allison wrote:
  

You proably should use a spherical earth model to compute distances.
    

Depending on your accuracy requirements.  

The way I read the original post, the goal was to constrain search
results to within some number of miles from a given zip code.

If that's being used for figuring out how much fuel to put on an
airplane, you're going to want far more accurate figures.  :-)

But if the purpose of this is to get some proxy for driving distance,
there's no compelling reason to be any more accurate than within a mile
or so... distance "as the crow flies" is only a bare approximation of
driving distance anyway.

  

Database search with a "distance from" key is difficult and requires
specialized mechanisms.  
    

Depending, again, on how good the results need to be.

Assuming your *best* search would be defined by point p and some radius
r, you could get results *almost* as good by searching for points
bounded by p +/- r.  A select consisting of two simple range criteria is
pretty easy to set up and virtually indistinguishable from *correct*
results in a decent range of probable applications.

Of course, you *could* use that method as a first pass before performing
more expensive computation on the results.  But just this one pass might
be enough to know what things are *about* X miles from some point.

You won't win any comp sci awards for this process, but it's a pragmatic
solution that's good enough for many cases.  Your mileage may vary. :-)

FWIW,

Dylan