Flatten terrain based on polygon area

CesiumJS
1

Hi, can anyone see from the code below why my terrain is not being flattened if it falls within the red polygon?

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="utf-8">
  <!-- Include the CesiumJS JavaScript and CSS files -->
  <script src="https://cesium.com/downloads/cesiumjs/releases/1.119/Build/Cesium/Cesium.js"></script>
  <script src="https://cdn.jsdelivr.net/npm/@turf/turf@7.0.0"></script>
  <link href="https://cesium.com/downloads/cesiumjs/releases/1.119/Build/Cesium/Widgets/widgets.css" rel="stylesheet">
</head>

<body>
  <div id="cesiumContainer"></div>
  <script type="text/javascript">
    // Your access token can be found at: https://ion.cesium.com/tokens.
    // This is the default access token from your ion account
    Cesium.Ion.defaultAccessToken = 'REMOVED';

    const viewer = new Cesium.Viewer('cesiumContainer', {
      terrain: Cesium.Terrain.fromWorldTerrain(),
    });

    viewer.camera.flyTo({
      destination: Cesium.Cartesian3.fromDegrees(-122.4175, 37.655, 400),
      orientation: {
        heading: Cesium.Math.toRadians(0.0),
        pitch: Cesium.Math.toRadians(-15.0),
      }
    });

    const mountainPolygonEntity = viewer.entities.add({
      polygon: {
        hierarchy: new Cesium.PolygonHierarchy(Cesium.Cartesian3.fromDegreesArray([
          -122.5, 37.6,
          -122.4, 37.6,
          -122.4, 37.7,
          -122.5, 37.7
        ])),
        material: Cesium.Color.RED.withAlpha(0.5),
        outline: true,
        outlineColor: Cesium.Color.BLACK
      }
    });

    const positions = mountainPolygonEntity.polygon.hierarchy.getValue().positions;
    const edge1 = Cesium.Cartesian3.subtract(positions[1], positions[0], new Cesium.Cartesian3());
    const edge2 = Cesium.Cartesian3.subtract(positions[2], positions[1], new Cesium.Cartesian3());
    const normal = Cesium.Cartesian3.normalize(Cesium.Cartesian3.cross(edge1, edge2, new Cesium.Cartesian3()), new Cesium.Cartesian3());

    const clippingPlanes = new Cesium.ClippingPlaneCollection({
      planes: [new Cesium.Plane(normal, 0)]
    });

    const terrainProvider = new Cesium.CesiumTerrainProvider({
      terrain: Cesium.Terrain.fromWorldTerrain(),
      requestVertexNormals: true,
      requestWaterMask: true,
      hasWaterMask: true,
      clippingPlanes: clippingPlanes
    });

    Promise.resolve(viewer.terrainProvider.readyPromise).then(function () {
      const updateHeightPromises = mountainPolygonEntity.polygon.hierarchy.getValue().positions.map(position => {
        const cartographic = Cesium.Cartographic.fromCartesian(position);
        const longitude = cartographic.longitude;
        const latitude = cartographic.latitude;

        // Set the height to 0 to flatten the terrain
        const flattenedHeight = 0;

        return viewer.terrainProvider.requestTileGeometry(new Cesium.Cartographic(longitude, latitude, 0))
          .then(function (updatedGeometry) {
            console.log(updatedGeometry);
            if (updatedGeometry && updatedGeometry.terrainData && updatedGeometry.terrainData.length > 0) {
              const terrainHeightSample = updatedGeometry.terrainData[0]; // You might need to determine the index based on the provided geometry
              // Update the terrain height in the red polygon area to the flattened height
              terrainHeightSample.height = flattenedHeight;
              return Cesium.Cartographic.fromRadians(longitude, latitude, flattenedHeight);
            }
          });
      });

      Promise.all(updateHeightPromises).then(updatedPositions => {
        mountainPolygonEntity.polygon.hierarchy = new Cesium.PolygonHierarchy(updatedPositions);
      });
    });


  </script>
</body>

</html>