fix(street-level): ignore implausible ground samples when framing a photo

Opening a photo frames it from 140 m behind using scene.sampleHeight for
the ground. Before the Google 3D tiles under the photo have loaded, that
sample can be kilometres wrong: an Austin 360 photo sampled -14,886 m and
the camera flew to -14,811 m. A sample now counts only when it is a
plausible surface height and, once the bare-earth height is known, lies
within -15/+80 m of it; otherwise the bare-earth height is used. FOLLOW's
eye height goes through the same check.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/streetLevel/cameraFollow.js

@@ -1,29 +1,53 @@
 import * as Cesium from 'cesium';
 import { FOLLOW_EYE_HEIGHT_M } from './policy.js';
 
+/** Ellipsoidal heights a land surface can have (Dead Sea shore to Everest, with geoid slack). */
+const SURFACE_MIN_M = -500;
+const SURFACE_MAX_M = 9000;
+/** Rendered mesh window around the bare earth: roofs and bridges, not whole towers. */
+const MESH_BELOW_DEM_M = 15;
+const MESH_ABOVE_DEM_M = 80;
+
 /**
  * Drive the globe camera from the street-level pose: either continuously
  * ("camera follows view") or once, framing the image from behind.
  */
-export function createCameraFollow({ state }) {
+export function createCameraFollow({ state, parts }) {
   const { render } = state.services;
 
   function requestRender() {
     render?.governorRequestRender?.('street-level-follow');
   }
 
+  /**
+   * Ground under a photo for framing and following. The rendered surface is
+   * sampled first, but `sampleHeight` can return nonsense before the tiles
+   * under the point have loaded (kilometres below the ellipsoid), so a sample
+   * counts only when it is a plausible surface height and, once the bare-earth
+   * height is known, lies within the mesh window around it.
+   */
   function groundHeightAt(lon, lat, fallback) {
     const scene = state.viewer?.scene;
     const carto = Cesium.Cartographic.fromDegrees(lon, lat);
+    const caster = parts?.groundCaster;
+    const dem = caster?.groundAt(lon, lat) ?? null;
+    if (dem === null) caster?.prepare([[lon, lat]]);
+    const plausible = (height) =>
+      Number.isFinite(height) &&
+      height >= SURFACE_MIN_M &&
+      height <= SURFACE_MAX_M &&
+      (dem === null ||
+        (height >= dem - MESH_BELOW_DEM_M && height <= dem + MESH_ABOVE_DEM_M));
     let height = null;
     try {
       if (scene?.sampleHeightSupported) height = scene.sampleHeight(carto);
     } catch {
       /* not sampleable yet */
     }
-    if (!Number.isFinite(height)) height = scene?.globe?.getHeight?.(carto);
-    if (!Number.isFinite(height)) height = fallback;
-    return Number.isFinite(height) ? height : 0;
+    if (!plausible(height)) height = dem;
+    if (!plausible(height)) height = scene?.globe?.getHeight?.(carto);
+    if (!plausible(height)) height = fallback;
+    return plausible(height) ? height : 0;
   }
 
   /** Put the Cesium camera where the street-level camera is. */