Reprojection Wgs84

<!--
  示例：WGS84 视图
  要点：view 用 EPSG:4326 时中心直接是经纬度，OSM 瓦片由 OL 自动重投影
  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>WGS84 视图</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;font-variant-numeric:tabular-nums}
  .bar select{font-size:12px;padding:2px 4px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>视图投影
    <select id="pj">
      <option value="EPSG:4326" selected>EPSG:4326（经纬度）</option>
      <option value="EPSG:3857">EPSG:3857（墨卡托）</option>
    </select>
  </label>
  <span>中心 <b id="ctr">—</b></span>
  <span>1 像素 <b id="res">—</b> m</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;
  }

  /* EPSG:4326 下 fromLonLat 原样返回经纬度，3857 下则换成米 */
  var center4326 = ol.proj.fromLonLat([116.391, 39.907], 'EPSG:4326');

  var view = new ol.View({
    projection: 'EPSG:4326',
    center: center4326,
    zoom: 8,
    minZoom: 2
  });

  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: view,
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  function refresh() {
    var c = view.getCenter();
    var p = view.getProjection();
    var isDeg = p.getCode() === 'EPSG:4326';
    document.getElementById('ctr').textContent = isDeg
      ? c[0].toFixed(4) + ', ' + c[1].toFixed(4) + ' °'
      : (c[0] / 1000).toFixed(1) + ', ' + (c[1] / 1000).toFixed(1) + ' km';
    var res = view.getResolution() * (p.getMetersPerUnit() || 1);
    document.getElementById('res').textContent = res < 1
      ? res.toFixed(2)
      : res.toFixed(0);
  }

  view.on('change:resolution', refresh);
  refresh();

  /* View 不支持原地换投影，用 map.setView 替换视图对象 */
  document.getElementById('pj').addEventListener('change', function () {
    var code = this.value;
    var ll = ol.proj.toLonLat(view.getCenter(), view.getProjection());
    var next = new ol.View({
      projection: code,
      center: ol.proj.fromLonLat(ll, code),
      zoom: view.getZoom(),
      minZoom: 2
    });
    map.setView(next);
    view = next;
    view.on('change:resolution', refresh);
    refresh();
  });
})();
</script>
</body>
</html>