Tissot

<!--
  示例：OpenLayers 等距圆与投影变形
  要点：同一组圆在 EPSG:4326 与 EPSG:3857 视图下的形状差异，用两颗地图并排对比
  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>等距圆与投影变形</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;flex-wrap:wrap;font-size:12px;color:#58606e}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  .bar select,.bar input[type=range]{font-size:12px}
  .bar .val{font-family:ui-monospace,Consolas,monospace;color:#1f5fd0;min-width:44px}
  .bar .sep{width:1px;height:16px;background:#d8dce2}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
  .maps{flex:1 1 auto;min-height:0;display:grid;grid-template-columns:1fr 1fr;gap:2px;background:#ccd3db}
  .map{min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <span>左：EPSG:4326 视图</span>
  <span class="sep"></span>
  <span>右：EPSG:3857 视图</span>
  <span class="sep"></span>
  <span>圆半径约 600 km</span>
</div>
<div class="maps">
  <div id="map4326" class="map"></div>
  <div id="map3857" class="map"></div>
</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').textContent = 'OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net';
    return;
  }


  var circleStyle = new ol.style.Style({
    fill: new ol.style.Fill({color: 'rgba(255, 145, 0, 0.14)'}),
    stroke: new ol.style.Stroke({color: 'rgba(224, 106, 0, 0.9)', width: 1})
  });

  var features4326 = [];
  var features3857 = [];
  var radius = 600000 / 111320;

  for (var lon = -180; lon < 180; lon += 30) {
    for (var lat = -60; lat <= 60; lat += 30) {
      features4326.push(new ol.Feature({
        geometry: ol.geom.Polygon.circular([lon, lat], radius, 64)
      }));
      features3857.push(new ol.Feature({
        geometry: ol.geom.Polygon.circular([lon, lat], radius, 64)
          .transform('EPSG:4326', 'EPSG:3857')
      }));
    }
  }

  function makeMap(target, projection, layers, center, zoom) {
    return new ol.Map({
      target: target,
      layers: layers,
      view: new ol.View({
        projection: projection,
        center: center,
        zoom: zoom
      })
    });
  }

  var map4326 = makeMap(
    'map4326',
    'EPSG:4326',
    [
      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'})}),
      new ol.layer.Vector({
        source: new ol.source.Vector({features: features4326, wrapX: false}),
        style: circleStyle
      })
    ],
    ol.proj.fromLonLat([105, 30], 'EPSG:4326'),
    3
  );

  var map3857 = makeMap(
    'map3857',
    'EPSG:3857',
    [
      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'})}),
      new ol.layer.Vector({
        source: new ol.source.Vector({features: features3857, wrapX: false}),
        style: circleStyle
      })
    ],
    ol.proj.fromLonLat([105, 30]),
    2
  );

  map4326.getView().setMinZoom(1);
  map3857.getView().setMinZoom(1);

})();
</script>
</body>
</html>