Re: Radius of a zip code

"Andy Lewis" <jumboc(at)comcast(dot)net> writes:
> I was trying to find all zip codes within a given zip code or radius.

I think there are canned solutions for this available in PostGIS ---
have you looked at that?

> I've tried the following with no luck:

> -- 20 Miles
> --select 20 * 360.0 / (7900.0 * atan2(1,1) * 4.0);
> select * from zip_code where map_loc @
> circle(map_point('dallas','tx','75201'), .290105212724467 ) order by
> city

I'm guessing that the big problem is that you didn't measure longitude
and latitude in identical units in your table, so your "circle" isn't
real circular, and the smaller problem is that "miles" converts to
"degrees of arc" differently at different latitudes.

regards, tom lane