function onChangeArea(field) {
   var f = document.forms["calc_landarea"];
   if (field.id=="landarea_initial"  ||  field.id=="landarea_units") {
      InputArea = new Area(f["landarea_initial"].value, f["landarea_initial_units"].value);
   }
   calc_area();
}

function calc_area() {
   f = document.forms["calc_landarea"];

   if (InputArea != null) {
      val = InputArea.getArea(f["landarea_result_units"].value);
      f["landarea_result"].value = DisplayFormat(val);
   }
}