Sort Locations Listing by City in Ascending & Decending Order

gistfile1.txt

// Sort by city. 

add_filter('wpgmp_sorting','wpgmp_sorting_location_field',2,2);
function wpgmp_sorting_location_field($sorting,$map) {

$sorting['city__asc'] ="City A-Z";
$sorting['city__desc'] ="City Z-A";

return $sorting;
}