Reprojection By Code

<!--
  示例：代码注册投影
  要点：ol.proj.addProjection 加 addCoordinateTransforms，自定义投影即可参与转换
  依赖：OpenLayers 10.3.1 · jsDelivr CDN（底图 OSM，无 Key）
  替代：官方用 proj4 定义 EPSG:21781，这里用球面 Lambert 方位等积公式自行实现
-->
<!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>代码注册投影</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}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
  table.mono td{font-family:ui-monospace,Consolas,monospace;font-size:11px}
</style>
</head>
<body>
<div class="bar">
  <span>自定义投影 <b>LOCAL:LAEA</b></span>
  <span>坐标转换 <b id="cv">—</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 R = 6371008.8;
  var D2R = Math.PI / 180;

  /* 球面 Lambert 方位等积投影，原点取东经 116°、北纬 39° */
  var LON0 = 116 * D2R;
  var LAT0 = 39 * D2R;

  function forward(coord) {
    var lon = coord[0] * D2R;
    var lat = coord[1] * D2R;
    var k = Math.sqrt(2 / (1 + Math.sin(LAT0) * Math.sin(lat) +
      Math.cos(LAT0) * Math.cos(lat) * Math.cos(lon - LON0)));
    return [
      R * k * Math.cos(lat) * Math.sin(lon - LON0),
      R * k * (Math.cos(LAT0) * Math.sin(lat) -
        Math.sin(LAT0) * Math.cos(lat) * Math.cos(lon - LON0))
    ];
  }

  var custom = new ol.proj.Projection({
    code: 'LOCAL:LAEA',
    units: 'm',
    extent: [-4e6, -4e6, 4e6, 4e6],
    global: false
  });

  ol.proj.addProjection(custom);
  ol.proj.addCoordinateTransforms('EPSG:4326', custom, forward, null);

  var toCustom = ol.proj.getTransform('EPSG:4326', 'LOCAL:LAEA');
  var backToMap = ol.proj.getTransform('LOCAL:LAEA', 'EPSG:3857');

  var PTS = [
    ['北京', 116.391, 39.907],
    ['天津', 117.201, 39.084],
    ['石家庄', 114.515, 38.043],
    ['济南', 117.120, 36.651],
    ['太原', 112.549, 37.871]
  ];

  var features = PTS.map(function (p) {
    /* 先在自定义投影里算出坐标，再转回地图投影落图 */
    var local = toCustom([p[1], p[2]]);
    var xy = backToMap(local);
    var f = new ol.Feature({
      geometry: new ol.geom.Point(xy),
      name: p[0],
      local: local
    });
    f.setStyle(new ol.style.Style({
      image: new ol.style.Circle({
        radius: 6,
        fill: new ol.style.Fill({ color: '#2f8f5b' }),
        stroke: new ol.style.Stroke({ color: '#fff', width: 2 })
      }),
      text: new ol.style.Text({
        text: p[0],
        offsetY: -14,
        font: '500 12px sans-serif',
        fill: new ol.style.Fill({ color: '#16181d' }),
        stroke: new ol.style.Stroke({ color: '#fff', width: 3 })
      })
    }));
    return f;
  });

  var layer = new ol.layer.Vector({ source: new ol.source.Vector({ features: features }) });

  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'}) }), layer],
    view: new ol.View({ center: ol.proj.fromLonLat([116.2, 38.6]), zoom: 6 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  var first = features[0].get('local');
  document.getElementById('cv').textContent =
    '北京 → ' + first[0].toFixed(0) + ', ' + first[1].toFixed(0) + ' m';

  map.on('singleclick', function (evt) {
    var ll = ol.proj.toLonLat(evt.coordinate);
    var local = toCustom(ll);
    document.getElementById('cv').textContent =
      ll[0].toFixed(3) + ',' + ll[1].toFixed(3) + ' → ' +
      local[0].toFixed(0) + ', ' + local[1].toFixed(0) + ' m';
  });
})();
</script>
</body>
</html>