protected function calculateLimitTiles()
  {
    $so = $this->imageGeoZone->getSudOuestPoint();
    $this->soTile = self::$gpm->LatLonToTile($so->getLatitude(), $so->getLongitude(), $this->zoom);
    $ne = $this->imageGeoZone->getNordEstPoint();
    $this->neTile = self::$gpm->LatLonToTile($ne->getLatitude(), $ne->getLongitude(), $this->zoom);

    $this->soTileInfo = self::$gpm->TileLatLonBounds($this->soTile['x'], $this->soTile['y'], $this->zoom);
    $this->soTileInfo['min']['lat'] = $this->soTileInfo['min']['lat']*-1;
    $this->soTileInfo['max']['lat'] = $this->soTileInfo['max']['lat']*-1;
    $this->neTileInfo = self::$gpm->TileLatLonBounds($this->neTile['x'], $this->neTile['y'], $this->zoom);
    $this->neTileInfo['min']['lat'] = $this->neTileInfo['min']['lat']*-1;
    $this->neTileInfo['max']['lat'] = $this->neTileInfo['max']['lat']*-1;
    $this->tilesGeoZone = new GeoZone(
      $this->soTileInfo['max']['lat'], $this->neTileInfo['min']['lat'],
      $this->soTileInfo['min']['lon'], $this->neTileInfo['max']['lon']
    );
  }