Draw and Modify Geodesic

<!--
  说明：绘制与修改大地圆
  核心逻辑：Draw 生成 Circle 后在 drawend 按球面公式转成 72 点等距多边形，再交给 Modify
  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>绘制与修改大地圆</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:10px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar button{font:inherit;padding:4px 10px;border:1px solid #c9d0da;border-radius:5px;
              background:#fff;color:#1f2a37;cursor:pointer}
  .bar button.on{background:#1f5fd0;border-color:#1f5fd0;color:#fff}
  #info{margin-left:auto}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <button id="btnDraw">绘制圆</button>
  <button id="btnModify">修改外环</button>
  <button id="btnClear">清空</button>
  <span id="info">—</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 fromLonLat = ol.proj.fromLonLat;

  /* 球面等距圆：按方位角算出每一点的经纬度，再转回 EPSG:3857 */
  function geodesicRing(center3857, radiusMeters, steps) {
    var c = ol.proj.toLonLat(center3857);
    var lat1 = c[1] * Math.PI / 180;
    var lon1 = c[0] * Math.PI / 180;
    var d = radiusMeters / 6378137;
    var pts = [];

    for (var i = 0; i <= steps; i++) {
      var brng = i / steps * Math.PI * 2;
      var lat2 = Math.asin(Math.sin(lat1) * Math.cos(d) +
        Math.cos(lat1) * Math.sin(d) * Math.cos(brng));
      var lon2 = lon1 + Math.atan2(
        Math.sin(brng) * Math.sin(d) * Math.cos(lat1),
        Math.cos(d) - Math.sin(lat1) * Math.sin(lat2));
      pts.push(fromLonLat([lon2 * 180 / Math.PI, lat2 * 180 / Math.PI]));
    }
    return new ol.geom.Polygon([pts]);
  }

  var source = new ol.source.Vector();

  var layer = new ol.layer.Vector({
    source: source,
    style: new ol.style.Style({
      fill: new ol.style.Fill({ color: 'rgba(31,95,208,0.14)' }),
      stroke: new ol.style.Stroke({ color: '#1f5fd0', width: 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'}) }), layer],
    view: new ol.View({ center: fromLonLat([116.391, 39.907]), zoom: 10, minZoom: 2, maxZoom: 19 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  var draw = new ol.interaction.Draw({
    source: source,
    type: 'Circle',
    style: new ol.style.Style({
      fill: new ol.style.Fill({ color: 'rgba(226,72,60,0.10)' }),
      stroke: new ol.style.Stroke({ color: '#e2483c', width: 2, lineDash: [6, 6] })
    })
  });

  var modify = new ol.interaction.Modify({ source: source });

  draw.setActive(false);
  modify.setActive(false);
  map.addInteraction(draw);
  map.addInteraction(modify);

  draw.on('drawend', function (evt) {
    var circle = evt.feature.getGeometry();
    var ring = geodesicRing(circle.getCenter(), circle.getRadius(), 72);
    evt.feature.setGeometry(ring);
    info.textContent = '已转换：外环 ' + (ring.getCoordinates()[0].length - 1) +
      ' 个点，半径约 ' + Math.round(circle.getRadius() / 1000) + ' km';
  });

  var info = document.getElementById('info');
  var btnDraw = document.getElementById('btnDraw');
  var btnModify = document.getElementById('btnModify');

  function activate(which) {
    draw.setActive(which === 'draw');
    modify.setActive(which === 'modify');
    btnDraw.classList.toggle('on', which === 'draw');
    btnModify.classList.toggle('on', which === 'modify');
    if (which === 'draw') info.textContent = '按住并拖动确定半径';
    if (which === 'modify') info.textContent = '拖动外环折点微调形状';
  }

  btnDraw.addEventListener('click', function () {
    activate(draw.getActive() ? '' : 'draw');
  });
  btnModify.addEventListener('click', function () {
    activate(modify.getActive() ? '' : 'modify');
  });
  document.getElementById('btnClear').addEventListener('click', function () {
    source.clear();
    activate('');
    info.textContent = '已清空';
  });
})();
</script>
</body>
</html>