Re: Latitude / Longitude

Tim Perdue wrote:
> Hi Folks -
>
> I'm working on a global weather database for a client and have hit an issue
> which I'm sure has been solved before. Unfortunately, the site search doesn't
> pull up anything.
>
> Essentially, I've got two tables, one with city/county/lat/long and another
> with lat/long/weather data.
>
> None of the lat/longs in the two tables match up directly, so I can't do a
> simple join of the two tables. I need to join on closest proximity on the
> lat/long fields.
>
> Any suggestions? It seems to me this will be pretty expensive on CPU resources
> unless there's a really elegant trick uncovered.

I see you've gotten some other help, but in case you're interested, I'll give
you another alternative. Here's a plpgsql function I wrote a while ago based
on the Haversine formula:

CREATE FUNCTION "zipdist" (float8,float8,float8,float8 ) RETURNS float8 AS '
DECLARE
lat1 ALIAS FOR $1;
lon1 ALIAS FOR $2;
lat2 ALIAS FOR $3;
lon2 ALIAS FOR $4;
dist float8;
BEGIN
dist := 0.621 * 6371.2 * 2 *
atan2( sqrt(abs(0 + pow(sin(radians(lat2)/2 -
radians(lat1)/2),2) + cos(radians(lat1)) * cos(radians(lat2)) *
pow(sin(radians(lon2)/2 - radians(lon1)/2),2))),sqrt(abs(1 -
pow(sin(radians(lat2)/2 - radians(lat1)/2),2) + cos(radians(lat1)) *
cos(radians(lat2)) * pow(sin(radians(lon2)/2 - radians(lon1)/2),2))));
return dist;
END;
' LANGUAGE 'plpgsql';

I used the following PHP code to start looking for a match in a small circle,
and then expand it if no matches were found:

$dist = INIT_DIST;
$cnt = 0;
$cntr = 0;
do {
if ((! $zip == "") && (! $dist <= 0)) {
$sql = get_zip_sql($lon1d,$lat1d,$dist,$numtoshow);
$rs = connexec($conn,$sql);
$rsf = rsfetchrs($rs);
$dist *= 2;
$cntr++;
} else {
$cntr = 10;
}
} while (count($rsf) < $numadvisorstoshow && $cntr < 10);

Hopefully you get the idea.

As was suggested, you can narrow the results using a box to make the query
perform better, and then sort by distance to get the closest alternative.
Here's the related part of get_zip_sql():

function get_zip_sql($lon1d,$lat1d,$dist,$numtoshow)
{
$sql = "
SELECT DISTINCT <fields>
FROM tbl_a AS a
,tbl_d AS d
,tbl_a_zipcodes AS az
,tbl_zipcodes as z
WHERE
abs(z.lat - $lat1d) * 60 * 1.15078 <= $dist
and abs(z.long - $lon1d) * 60 * 1.15078 <= $dist
and zipdist($lat1d,$lon1d,lat,long) <= $dist
and z.zip = az.zipcode
<other criteria>
ORDER BY
LIMIT $numtoshow;
";

return $sql;
}

The "X * 60 * 1.15078" converts differences in degrees lat/long into rough
distances in miles. The zipdist function returns a more-or-less exact distance
using the Haversine formula.

Hope this helps. Let me know if you want/need more explanation of any of this.

Joe