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I'm planning a custom WordPress theme where the Custom Post Type (CPT) will have latitude and longitude coordinate as it's meta value. The latitude and longitude will be displayed as a Marker in a Google Map.

So far I don't have any problem in showing the Google Map and the CPT as it's Marker. That is if I query the CPT using the default order.

The problem occurs when I need n CPTs that is closest to a coordinate whether it is current user position or a location clicked on the map. Another situation is to query all CPTs that is in x km radius of a coordinate.

The question is : How do I query posts based on latitude-longitude value that is saved on the post meta?

I'm not really sure how to do it, but I think the latitude and longitude value should be saved in a separate custom field.

Thank you in advance.

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up vote 4 down vote accepted

This is a simple mathimatical problem. You will indeed need access to both your longitude and latitude, so save it in a metafield.

than you will have to query your posts like this as a sql query. Haven't got a chance to test it. and or pour it into wordpress. Don't have access to my test env now. But I guess you could do it yourself :) if not I'll do it later on when I can.

set @latitude = xxx; — center latitude
set @longitude = xxx; — center longitude
set @distance = xx; — search distance

select p.ID, p.post_name, ((ACOS(SIN(@latitude * PI() / 180) * SIN(`latitude.meta_value` * PI() / 180) + COS(@latitude * PI() / 180) * COS(`latitude.meta_value` * PI() / 180) * COS((@longitude – `longitude.meta_value`) * PI() / 180)) * 180 / PI()) * 60 * 1.1515) AS distance
from wp_posts p
left join wp_postmeta latitude on latitude.post_id = p.ID and latitude.meta_key = ‘_latitude’
left join wp_postmeta longitude on longitude.post_id = p.ID and longitude.meta_key = ‘_longitude’
having distance < @distance;
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This is a great start. Thank you very much. I'm not that good on sql queries, I'll respond if i've figured it out. – ifdion Aug 14 '12 at 10:26
+1, looks promising. – amit Aug 14 '12 at 13:27
I was wondering if you managed to get it to work? – woony Aug 17 '12 at 18:16

For anyone looking for this answer in the future, I took woony's code the extra mile and got everything working within Wordpress' post_meta table structure. This assumes you have two separate custom fields, one for Latitude (city_latitude) and one for longitude (city_longitude). Just pass the latitude, longitude, and distance parameters into a WP function and you should be set.

Here's the WP function. Drop this in your functions.php file:

function get_nearby_cities($lat, $long, $distance){
    global $wpdb;
    $nearbyCities = $wpdb->get_results( 
        city_latitude.meta_value as cityLat,
        city_longitude.meta_value as cityLong,
        ((ACOS(SIN($lat * PI() / 180) * SIN(city_latitude.meta_value * PI() / 180) + COS($lat * PI() / 180) * COS(city_latitude.meta_value * PI() / 180) * COS(($long - city_longitude.meta_value) * PI() / 180)) * 180 / PI()) * 60 * 1.1515) AS distance,
        wp_postmeta AS city_latitude
        LEFT JOIN wp_postmeta as city_longitude ON city_latitude.post_id = city_longitude.post_id
        INNER JOIN wp_posts ON wp_posts.ID = city_latitude.post_id
    WHERE city_latitude.meta_key = 'city_latitude' AND city_longitude.meta_key = 'city_longitude'
    HAVING distance < $distance
    ORDER BY distance ASC;"

        return $nearbyCities;

Return the value in your template file:

$nearbyCities = get_nearby_cities(get_post_meta($post->ID, 'city_latitude', true), get_post_meta($post->ID, 'city_longitude', true), 25);
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Have a look at the Geo Data Store plugin. It creates an optimized table for your lat/long coordinates and has two template tags to use.

More info and code example in this post

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