Coordinate Convert

<!--
  示例：坐标系转换
  要点：fromLonLat / toLonLat / transform / transformExtent 四种转换对照输出
  依赖：OpenLayers 10.3.1 · jsDelivr CDN
-->
<!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>坐标系转换</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}
  .bar{flex:0 0 auto;padding:8px 12px;background:#f6f7f9;border-bottom:1px solid #e3e6ea;
       display:flex;gap:16px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar code{font-family:ui-monospace,Consolas,monospace;color:#1f5fd0}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <span>点击地图任意位置</span>
  <span>经纬度 <code id="lonlat">—</code></span>
  <span>3857 <code id="merc">—</code></span>
  <span>4326 <code id="geo">—</code></span>
  <span>视野范围 <code id="ext">—</code></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 (typeof ol === 'undefined') {
    document.getElementById('map').innerHTML =
      '<div style="padding:20px;color:#b42318">OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net</div>';
    return;
  }

  var view = new ol.View({
    center: ol.proj.fromLonLat([116.3974, 39.9093]),
    zoom: 13
  });

  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 })
  });

  var lonlatEl = document.getElementById('lonlat');
  var mercEl = document.getElementById('merc');
  var geoEl = document.getElementById('geo');
  var extEl = document.getElementById('ext');

  function fmt(arr, digits) {
    return arr.map(function (v) { return v.toFixed(digits); }).join(', ');
  }

  function reportExtent() {
    var size = map.getSize();
    if (!size) {
      return;
    }
    var extent = view.calculateExtent(size);
    var geoExtent = ol.proj.transformExtent(extent, 'EPSG:3857', 'EPSG:4326');
    extEl.textContent = fmt(geoExtent, 3);
  }

  map.on('singleclick', function (evt) {
    /* 3857 转经纬度 */
    var lonlat = ol.proj.toLonLat(evt.coordinate);
    lonlatEl.textContent = fmt(lonlat, 6);

    /* 经纬度转回 3857 */
    var merc = ol.proj.fromLonLat(lonlat);
    mercEl.textContent = fmt(merc, 1);

    /* 用 transform 做同样的 3857 → 4326，源投影显式写出 */
    var geo = ol.proj.transform(evt.coordinate, 'EPSG:3857', 'EPSG:4326');
    geoEl.textContent = fmt(geo, 6);
  });

  view.on('change:resolution', reportExtent);
  map.once('postrender', reportExtent);
})();
</script>
</body>
</html>