public function getUrlPart()
 {
     $s = GoogleStaticMapApi::MAPS_URL;
     $s .= '?';
     $s .= '&sensor=' . ($this->sensor ? 'true' : 'false');
     $s .= '&size=' . $this->size[0] . 'x' . $this->size[1];
     if (!empty($this->zoom)) {
         $s .= '&zoom=' . $this->zoom;
     }
     if (!empty($this->markers)) {
         foreach ($this->markers as $marker) {
             $s .= '&';
             $s .= $marker->getUrlPart();
         }
     }
     if (empty($this->center) && empty($this->markers)) {
         $this->center = new LatLng(50.061822, 19.937353);
         //cracow main square
     }
     if ($this->center instanceof LatLng) {
         $s .= '&center=' . $this->center->getUrlPart();
     }
     return $s;
 }