double offsetY = WORLD_ENVELOPE.getMinimum(1);
try { //take available tiles from database
leftTile = file.minColumn(zoomLevel);
rightTile = file.maxColumn(zoomLevel);
bottomTile = file.minRow(zoomLevel);
topTile = file.maxRow(zoomLevel);
} catch (SQLException e) {
throw new IOException(e);
}