function  getLongitude(scr_x){ 

   var map_image = getMap();
   var drag_mx1 =  getM( BX2IX(scr_x,map_image),  imageXMin(), imageXMax()); 
   var drag_mx2 =  getM( BX2IX(scr_x,map_image),  imageXMax(), imageXMin());  

   var n1 = drag_mx1 *  mapXMin();  
   var n2 = drag_mx2 *  mapXMax();  

   // this is an override for moveing off the map to the east  
   if(drag_mx1 <= 0){  
     return  mapXMax();  
   }  
  
   // this is an override for moveing off the map to the west  
   if(drag_mx2 <= 0){  
     return  mapXMin();  
   }   
  
   return (n1+n2)/(drag_mx1+drag_mx2);  

 }