Print to Scale

<!--
  示例：按比例尺出图
  要点：由目标比例尺反算 resolution，setResolution 后打印范围可确定
  依赖：OpenLayers 10.3.1 · jsDelivr CDN（底图 OSM，无 Key）
-->
<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta name="robots" content="noindex,follow">
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>按比例尺出图</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/ol@10.3.1/ol.css">
<style>
  html,body{height:100%;margin:0}
  body{display:flex;flex-direction:column;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif;color:#16181d}
  .bar{flex:0 0 auto;padding:8px 12px;background:#f6f7f9;border-bottom:1px solid #e3e6ea;
       display:flex;gap:14px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar b{color:#16181d;font-weight:600}
  .bar select,.bar input{font-size:12px;padding:2px 4px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>纸张
    <select id="paper">
      <option value="a4">A4 横向</option>
      <option value="a3">A3 横向</option>
      <option value="letter">Letter 横向</option>
    </select>
  </label>
  <label>比例尺
    <select id="scale">
      <option value="10000">1 : 10 000</option>
      <option value="25000" selected>1 : 25 000</option>
      <option value="50000">1 : 50 000</option>
    </select>
  </label>
  <span>当前 resolution <b id="res">—</b> m/px</span>
  <span>图幅 <b id="size">—</b></span>
</div>
<div id="map"></div>
<script src="https://cdn.jsdelivr.net/npm/ol@10.3.1/dist/ol.js"></script>
<script>
(function () {
  'use strict';
  if (!window.ol) {
    document.getElementById('map').textContent = 'OpenLayers 加载失败，请检查 cdn.jsdelivr.net 是否可访问';
    return;
  }

  var DPI = 96;
  var MM_PER_INCH = 25.4;
  var PAPERS = {
    a4: [297, 210],
    a3: [420, 297],
    letter: [279.4, 215.9]
  };

  var map = new ol.Map({
    target: 'map',
    layers: [new ol.layer.Tile({ source: new ol.source.XYZ({url: 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}', attributions: 'Tiles © Esri'}) })],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.391, 39.907]),
      zoom: 12
    }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  /* 比例尺 = 地图上一米 / 实地米数，1 像素 = resolution 米，1 像素 = 1/DPI 英寸 */
  function resolutionFor(scale) {
    return scale / DPI * (MM_PER_INCH / 1000);
  }

  function applyPrintView() {
    var paper = PAPERS[document.getElementById('paper').value];
    var scale = Number(document.getElementById('scale').value);
    var res = resolutionFor(scale);
    map.getView().setResolution(res);

    document.getElementById('res').textContent = res.toFixed(2);
    document.getElementById('size').textContent =
      (paper[0] * res / 1000).toFixed(2) + ' × ' +
      (paper[1] * res / 1000).toFixed(2) + ' km';
  }

  document.getElementById('paper').addEventListener('change', applyPrintView);
  document.getElementById('scale').addEventListener('change', applyPrintView);
  applyPrintView();
})();
</script>
</body>
</html>