return height above ground in feet

def ground_height(self):
        '''return height above ground in feet'''
        lat = self.pkt['I105']['Lat']['val']
        lon = self.pkt['I105']['Lon']['val']
        global ElevationMap
        ret = ElevationMap.GetElevation(lat, lon)
        ret -= gen_settings.wgs84_to_AMSL
        return ret * 3.2807