WordPress custom table with post locations and query to select posts within distance.

locations.php

<?php

function wkd_sync_location_to_table( $post_id ) {
	// Sync locations
	global $wpdb;

	$location_id = $wpdb->get_var( $wpdb->prepare( "SELECT id FROM $wpdb->wkd_post_locations WHERE post_id = %d;", $post_id ) );

	$format = array(
		'post_id' => '%d',
		'lat'     => '%f',
		'lng'     => '%f',
	);

	$data = array(
		'post_id' => $post_id,
		'lat'     => get_post_meta( $post_id, '_wkd_mortician_lat', true ),
		'lng'     => get_post_meta( $post_id, '_wkd_mortician_lng', true ),
	);

	if ( $location_id ) {
		$result = $wpdb->update( $wpdb->wkd_post_locations, $data, array( 'id' => $location_id ), $format );
	} else {
		$result = $wpdb->insert( $wpdb->wkd_post_locations, $data, $format );

		if ( $result ) {
			$location_id = $wpdb->insert_id;
		}
	}
}

/**
 * Posts clauses
 *
 * http://codex.wordpress.org/WordPress_Query_Vars
 * http://codex.wordpress.org/Custom_Queries
 *
 * @param array $pieces
 * @param WP_Query $query
 * @return string
 */
function wkd_locations_posts_clauses( $pieces, $query ) {
	global $wpdb;

	// Fields
	$fields = ',
		location.id AS location_id,
		location.lat AS location_lat,
		location.lng AS location_lng
	';

	// Join
	$join = "
		LEFT JOIN
			$wpdb->wkd_post_locations AS location
				ON $wpdb->posts.ID = location.post_id
	";

	// Order by
	$orderby = $pieces['orderby'];

	// Location
	$latitude  = $query->get( 'latitude' );
	$longitude = $query->get( 'longitude' );

	if ( ! empty( $latitude ) && ! empty( $longitude ) ) {
		$radius = 6371; // KM

		$latitude1 = $wpdb->prepare( '%f', $latitude );
		$latitude2 = 'location.lat';
		$longitude1 = $wpdb->prepare( '%f', $longitude );
		$longitude2 = 'location.lng';

		$d = "$radius * ACOS( COS( RADIANS( $latitude1 ) ) * COS( RADIANS( $latitude2 ) ) * COS( RADIANS( $longitude1 ) - RADIANS( $longitude2 ) ) + SIN( RADIANS( $latitude1 ) ) * SIN( RADIANS( $latitude2 ) ) )";

		$fields .= ",
			$d AS distance
		";

		// Where
		// @see http://www.scribd.com/doc/2569355/Geo-Distance-Search-with-MySQL
		$distance = $query->get( 'distance' );

		if ( ! empty( $distance ) ) {
			$pieces['groupby'] .= $wpdb->prepare( ' HAVING distance < %f', $distance );
		}

		// Order by
		$order  = $query->get( 'order' );

		switch ( $query->get( 'orderby' ) ) {
			case 'distance':
				$orderby = 'distance ' . $order;

				break;
		}
	}

	$pieces['fields'] .= $fields;
	$pieces['join']   .= $join;

	$pieces['orderby'] = $orderby;

	return $pieces;
}

add_filter( 'posts_clauses', 'wkd_locations_posts_clauses', 20, 2 );