Spatial analysis: distance, profile and visibility

• Use EllipsoidGeodesic.surfaceDistance for ground distance and Cartesian3.distance for sight lines.
• Await sampleTerrainMostDetailed on an array of Cartographic points before reading height.
• Compare interpolated sight height against sampled terrain, allowing a small epsilon.

var start = Cesium.Cartographic.fromDegrees(116.30, 39.90);
var end   = Cesium.Cartographic.fromDegrees(116.46, 39.92);

var geodesic = new Cesium.EllipsoidGeodesic(start, end);
console.log('地表距离', geodesic.surfaceDistance.toFixed(1), '米');
console.log('方位角', Cesium.Math.toDegrees(geodesic.startHeading).toFixed(1), '度');

var a = Cesium.Cartesian3.fromDegrees(116.30, 39.90, 40);
var b = Cesium.Cartesian3.fromDegrees(116.46, 39.92, 120);
console.log('直线距离', Cesium.Cartesian3.distance(a, b).toFixed(1), '米');

function sampleLine(start, end, count) {
  var out = [];
  for (var i = 0; i <= count; i++) {
    var t = i / count;
    out.push(new Cesium.Cartographic(
      start.longitude + (end.longitude - start.longitude) * t,
      start.latitude + (end.latitude - start.latitude) * t
    ));
  }
  return out;
}

async function buildProfile(start, end, count) {
  var samples = sampleLine(start, end, count || 120);
  await Cesium.sampleTerrainMostDetailed(viewer.terrainProvider, samples);

  return samples.map(function (c) {
    return {
      lng: Cesium.Math.toDegrees(c.longitude),
      lat: Cesium.Math.toDegrees(c.latitude),
      height: c.height
    };
  });
}

var profile = await buildProfile(
  Cesium.Cartographic.fromDegrees(116.30, 39.90),
  Cesium.Cartographic.fromDegrees(116.46, 39.92),
  120
);
var maxHeight = Math.max.apply(null, profile.map(function (p) { return p.height; }));

• With EllipsoidTerrainProvider every sampled height is 0, producing a flat profile.
• Sampling is asynchronous and expensive, so show a loading state and never reuse stale results.