Keep vehicles off rooftops and bridge decks: probe the mesh where the vehicle is

On the photoreal stack the rendered-mesh floor took one probe at each coarse
cell's centre (0.001 degrees) and kept it for the session. A centre on a roof
passed the +80 m DEM window and lifted every vehicle in the cell to roof height
(a Haymarket bus drew 23.5 m above the street, on the garage roof); a centre in
the river put Red Line trains on the Longfellow Bridge 6-9 m under the deck.

The sampler now probes at the caller's point. A cell with a reading is probed
again only at spots about 17 m apart from earlier ones, eight at most, and a
spot refused only because the cell's DEM prior was cold is tried again later.
The floor keeps each reading that passes the gate at the point it was taken
(two within about 5 m keep the lower; sixteen at most per cell) and answers a
read from the reading nearest the asked point, so a viaduct and the street
beside it in one cell keep their own heights.

src/services/groundFloor.js

@@ -418,13 +418,26 @@ export function createGroundFloor({ terrain, signal }) {
   // is the photogrammetric MESH, which sits above it (measured ~17 m at the
   // Austin airport apron). DEM-flooring therefore still buried sprites/trails
   // inside the mesh. These cells hold the RENDERED surface height, sampled
-  // one-shot per coarse cell near the viewer (scene.sampleHeight — the same
+  // at the callers' points, a few per coarse cell near the viewer, each kept
+  // where it was taken and read back from the nearest (scene.sampleHeight — the same
   // source groundSnap uses, which is why 3D models always looked right) and
   // reported here by meshFloorSampler.js. Design principle (owner): NEVER
   // below the visible surface; slightly above is always fine. The DEM stays
   // the instant global prior/fallback and the sanity gate for samples.
-  /** @type {Map<string, number>} coarse cell key -> rendered-surface height (m, ellipsoidal). */
+  /** @type {Map<string, Array<{lat: number, lon: number, h: number}>>}
+   *  coarse cell key -> rendered-surface readings (m, ellipsoidal), each kept
+   *  at the point it was probed. A read answers from the reading nearest the
+   *  asked point, so a viaduct and the street beside it in one cell keep their
+   *  own heights. */
   const _meshCells = new Map();
+  /** @constant {number} Readings closer than this (degrees, about 5 m) are
+   *  one spot: the lower one is kept there. */
+  const MESH_READING_SAME_SPOT_DEG = 0.00005;
+  /** @constant {number} Readings kept per cell. The sampler probes a cell at
+   *  most eight times, but ground snaps report at each aircraft's position;
+   *  past this many, a reading away from the others is not kept, so a busy
+   *  apron cell stays a short list to scan. */
+  const MESH_READINGS_PER_CELL = 16;
   /** @type {boolean} Whether mesh cells apply — true only in the google-3d
    *  regime (photoreal stack). On globe stacks the rendered surface IS the
    *  terrain provider (Re:Earth on keyless), so the DEM is already correct
@@ -444,7 +457,8 @@ export function createGroundFloor({ terrain, signal }) {
 
   /**
    * Records a sampled rendered-surface height for the cell containing the
-   * coordinate (one-shot latch — first accepted sample wins).
+   * coordinate, unchecked, when the cell has no reading yet. The sampler goes
+   * through `reportValidatedMeshFloorCell`, which keeps a reading per point.
    * @param {number} lat @param {number} lon @param {number} heightM
    */
   function reportMeshFloorCell(lat, lon, heightM) {
@@ -456,7 +470,7 @@ export function createGroundFloor({ terrain, signal }) {
       return;
     const c = coarseFloorCoord(lat, lon);
     const key = `${c.lat},${c.lon}`;
-    if (!_meshCells.has(key)) _meshCells.set(key, heightM);
+    if (!_meshCells.has(key)) _meshCells.set(key, [{ lat, lon, h: heightM }]);
   }
 
   /** Return whether a rendered-mesh sample is plausible against a real DEM prior. */
@@ -471,9 +485,11 @@ export function createGroundFloor({ terrain, signal }) {
 
   /**
    * Validate and record a rendered-mesh floor through the single shared gate.
-   * Callers must not bypass this with raw `reportMeshFloorCell`: a coarse-LOD,
-   * rooftop, or aircraft hit would otherwise win the session-long first-write
-   * latch and poison every floor consumer in the cell.
+   * Callers must not bypass this with raw `reportMeshFloorCell`: a coarse-LOD
+   * or aircraft hit would otherwise poison every floor consumer in the cell.
+   * The reading is kept at the point it was taken; a second reading at the
+   * same spot (within about 5 m) keeps the lower of the two. A cell keeps at
+   * most MESH_READINGS_PER_CELL readings.
    * @returns {boolean} Whether the sample passed and was reported.
    */
   function reportValidatedMeshFloorCell(lat, lon, heightM) {
@@ -481,13 +497,41 @@ export function createGroundFloor({ terrain, signal }) {
     const cell = coarseFloorCoord(lat, lon);
     const prior = cachedRealEllipsoidalGround(cell.lat, cell.lon);
     if (!meshFloorSampleWithinPrior(heightM, prior)) return false;
-    reportMeshFloorCell(cell.lat, cell.lon, heightM);
+    const key = `${cell.lat},${cell.lon}`;
+    const readings = _meshCells.get(key);
+    if (!readings) {
+      _meshCells.set(key, [{ lat, lon, h: heightM }]);
+      return true;
+    }
+    const same = readings.find(
+      (r) =>
+        Math.abs(r.lat - lat) < MESH_READING_SAME_SPOT_DEG &&
+        Math.abs(r.lon - lon) < MESH_READING_SAME_SPOT_DEG,
+    );
+    if (same) {
+      if (heightM < same.h) same.h = heightM;
+    } else if (readings.length < MESH_READINGS_PER_CELL) {
+      readings.push({ lat, lon, h: heightM });
+    }
     return true;
   }
 
   /**
-   * Synchronous mesh-cell read — null when the cell is unsampled OR the
-   * google-3d regime is inactive (globe stacks must floor on the DEM).
+   * Whether the cell holding the point has the real DEM prior that
+   * `reportValidatedMeshFloorCell` judges a reading against.
+   * @param {number} lat @param {number} lon
+   * @returns {boolean}
+   */
+  function meshFloorPriorReady(lat, lon) {
+    if (!Number.isFinite(lat) || !Number.isFinite(lon)) return false;
+    const c = coarseFloorCoord(lat, lon);
+    return cachedRealEllipsoidalGround(c.lat, c.lon) != null;
+  }
+
+  /**
+   * Synchronous mesh-cell read — the cell's reading nearest the point, or
+   * null when the cell is unsampled OR the google-3d regime is inactive (globe
+   * stacks must floor on the DEM).
    * @param {number} lat @param {number} lon
    * @returns {number|null}
    */
@@ -495,8 +539,20 @@ export function createGroundFloor({ terrain, signal }) {
     if (!_meshPreferred) return null;
     if (!Number.isFinite(lat) || !Number.isFinite(lon)) return null;
     const c = coarseFloorCoord(lat, lon);
-    const h = _meshCells.get(`${c.lat},${c.lon}`);
-    return h != null ? h : null;
+    const readings = _meshCells.get(`${c.lat},${c.lon}`);
+    if (!readings) return null;
+    if (readings.length === 1) return readings[0].h;
+    const k = Math.cos((lat * Math.PI) / 180) ** 2;
+    let best = readings[0],
+      bestD = Infinity;
+    for (const r of readings) {
+      const d = (r.lat - lat) ** 2 + k * (r.lon - lon) ** 2;
+      if (d < bestD) {
+        bestD = d;
+        best = r;
+      }
+    }
+    return best.h;
   }
 
   /** Test hook: clears mesh cells (never called at runtime — cells are valid all session). */
@@ -662,6 +718,7 @@ export function createGroundFloor({ terrain, signal }) {
     reportMeshFloorCell,
     meshFloorSampleWithinPrior,
     reportValidatedMeshFloorCell,
+    meshFloorPriorReady,
     cachedMeshFloor,
     _clearMeshFloorCellsForTest,
     cachedGroundFloor,