使用控制比例尺线保存地图

// export png
document.getElementById('export-png').addEventListener('click', function() {
  map.once('postcompose', function(event) {
    var canvasElement = event.context.canvas;
    var MIME_TYPE = "image/png";
    var imgURL = canvasElement.toDataURL(MIME_TYPE);
    var dlLink = document.createElement('a');
    dlLink.download = "carte"; //fileName;
    dlLink.href = imgURL;
    dlLink.dataset.downloadurl = [MIME_TYPE, dlLink.download, dlLink.href].join(':');
    document.body.appendChild(dlLink);
    dlLink.click();
    document.body.removeChild(dlLink);
  });
  map.renderSync();
});