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copied!<p><strong>EDIT</strong> This location-finder comes up often enough that I've written an article on it.</p> <p><a href="http://www.plumislandmedia.net/mysql/haversine-mysql-nearest-loc/" rel="nofollow">http://www.plumislandmedia.net/mysql/haversine-mysql-nearest-loc/</a></p> <p><strong>Original Post</strong></p> <p>Let's start by dealing with the haversine formula once for all, by putting it into a stored function so we can forget about its gnarly details. NOTE: This whole solution is in statute miles. </p> <pre><code>DELIMITER $$ CREATE FUNCTION distance(lat1 FLOAT, long1 FLOAT, lat2 FLOAT, long2 FLOAT) RETURNS FLOAT DETERMINISTIC NO SQL BEGIN RETURN (3959 * ACOS(COS(RADIANS(lat1)) * COS(RADIANS(lat2)) * COS(RADIANS(long1) - RADIANS(long2)) + SIN(RADIANS(lat1)) * SIN(RADIANS(lat2)) )); END$$ DELIMITER ; </code></pre> <p>Now let's put together a query that searches on the bounding box, and then refines the search with our distance function and orders by distance</p> <p>Based on the PHP code in the your question:</p> <p>Assume <code>$radius</code> is your radius, <code>$center_lat</code>, <code>$center_lng</code> is your reference point.</p> <pre><code>$sqlsquareradius = " SELECT post_id, lat, lng FROM ( SELECT post_id, lat, lng, distance(lat, lng, " . $center_lat . "," . $center_lng . ") AS distance FROM wp_geodatastore WHERE lat >= " . $center_lat . " -(" . $radius . "/69) AND lat <= " . $center_lat . " +(" . $radius . "/69) AND lng >= " . $center_lng . " -(" . $radius . "/69) AND lng <= " . $center_lng . " +(" . $radius . "/69) )a WHERE distance <= " . $radius . " ORDER BY distance "; </code></pre> <p>Notice a few things about this.</p> <p>First, it does the bounding box computation in SQL rather than in PHP. There's no good reason for that, except keeping all the computation in one environment. <code>(radius / 69)</code> is the number of degrees in <code>radius</code> statute miles. </p> <p>Second, it doesn't fiddle with the size of the longitudinal bounding box based on latitude. Instead it uses a simpler, but slightly too large, bounding box. This bounding box catches a few extra records, but the distance measurement gets rid of them. For your typical postcode / store finder app the performance difference is negligible. If you were searching many more records (e.g. a database of all utility poles) it might not be so trivial.</p> <p>Third, it uses a nested query to do the distance elimination, to avoid having to run the distance function more than once for each item.</p> <p>Fourth, it orders by distance ASCENDING. This means your zero-distance results should show up first in the result set. It usually makes sense to list nearest things first.</p> <p>Fifth, it uses <code>FLOAT</code> rather than <code>DOUBLE</code> throughout. There's a good reason for that. The haversine distance formula is not perfect, because it makes the approximation that the earth is a perfect sphere. That approximation happens to break down at roughly the same level of accuracy as the epsilon for <code>FLOAT</code> numbers. So <code>DOUBLE</code> is deceptive numerical overkill for this problem. (Don't use this haversine formula to do civil engineering work like parking lot drainage, or you will get big puddles a couple of epsilon, a few inches, deep, I promise.) It's fine for store-finder applications.</p> <p>Sixth, you are definitely going to want to create an index for your <code>lat</code> column. If your table of locations doesn't change very often, it will help to create an index for your <code>lng</code> column as well. But your <code>lat</code> index will give you most of your query performance gain.</p> <p>Lastly, I tested the stored procedure and the SQL, but not the PHP.</p> <p>Reference: <a href="http://www.scribd.com/doc/2569355/Geo-Distance-Search-with-MySQL" rel="nofollow">http://www.scribd.com/doc/2569355/Geo-Distance-Search-with-MySQL</a> Also my experience with a bunch of proximity finders for health care facilities.</p> <p>--------------- EDIT --------------------</p> <p>If you don't have a user interface that lets you define a stored procedure, that's a nuisance. At any rate, PHP lets you use numbered parameters in the sprintf call, so you can generate the whole nested statement like this. NOTE: You might need %$1f etc. You'll need to experiment with this. </p> <pre><code>$sql_stmt = sprintf (" SELECT post_id, lat, lng FROM ( SELECT post_id, lat, lng, (3959 * ACOS(COS(RADIANS(lat)) * COS(RADIANS(%$1s)) * COS(RADIANS(lng) - RADIANS(%$2s)) + SIN(RADIANS(lat)) * SIN(RADIANS(%$1s)) )) AS distance FROM wp_geodatastore WHERE lat >= %$1s -(%$3s/69) AND lat <= %$1s +(%$3s/69) AND lng >= %$2s -(%$3s/69) AND lng <= %$2s +(%$3s/69) )a WHERE distance <= %$3s ORDER BY distance ",$center_lat,$center_lng, $radius); </code></pre>

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