How to update field values of a content type's nodes when when viewing a particular content type page
hook_node_view() will be used in this scenario.

function hook_node_view($node, $view_mode, $langcode) {
  if($node-&gt;type == 'park' &amp;&amp; $view_mode == 'full')
   {
    /* change the value of proximity here after applying your formula. 
      * Here you have complete node array, so you alter the result easily here.
    */

   }
 }