fix(infrastructure): sample visible ground below the ellipsoid

docs/CURRENT-STATE.md

@@ -2048,8 +2048,8 @@ Unchanged overlay entries do not request another frame; moved positions,
 changed membership and re-enabling still publish. Ground sampling waits for
 visible globe tiles to settle, and skips markers whose ground the camera
 cannot see (unseen ground reads through the Earth and would retry every 2 s).
-Ground is tested at loaded globe terrain, or across 0-9,000 m when the globe
-is hidden, so photoreal views skip only ground well out of view. Until it is
+Ground is tested at loaded globe terrain, or across -9,000 to 9,000 m when the
+globe is hidden, so photoreal views skip only ground well out of view. Until it is
 seen, a stem keeps its unsampled height, which can show for a stem whose base
 is just off screen. Sampling uses valid loaded terrain as a floor while
 retaining rooftop and below-sea-level elevations. A hidden globe does not

src/data/localGeojson.test.mjs

@@ -797,7 +797,7 @@ test('an unseen base is not sampled and arms no retry until it is in view', asyn
  * `cameraM` metres above the ellipsoid, looking at the base's ground at
  * `groundM` (or directly away from it).
  */
-function aimRealCamera(env, { groundM, cameraM, southM, away = false }) {
+function aimRealCamera(env, { groundM, cameraM, southM, away = false, fov = Math.PI / 3 }) {
   const entity = env.dataSources[0].entities.values[0];
   const base = Cesium.Cartographic.fromCartesian(entity.__localBaseCartesian);
   const ground = Cesium.Cartesian3.fromRadians(base.longitude, base.latitude, groundM);
@@ -822,7 +822,7 @@ function aimRealCamera(env, { groundM, cameraM, southM, away = false }) {
   );
   const up = Cesium.Cartesian3.cross(right, direction, new Cesium.Cartesian3());
   const frustum = new Cesium.PerspectiveFrustum({
-    fov: Math.PI / 3,
+    fov,
     aspectRatio: 800 / 600,
     near: 1,
     far: 1e8,
@@ -841,7 +841,10 @@ function aimRealCamera(env, { groundM, cameraM, southM, away = false }) {
 const loadedTerrain = (height) => ({ show: true, tilesLoaded: true, getHeight: () => height });
 const hiddenGlobe = { show: false, tilesLoaded: true, getHeight: () => undefined };
 
-for (const { label, globe, groundM, cameraM, southM, baseOffScreen } of [
+for (const { label, globe, groundM, cameraM, southM, fov, baseOffScreen } of [
+  // The Dead Sea (about -430 m) under a hidden globe, seen steeply down
+  // through a narrow view: both 0 m and 9,000 m above it are off screen.
+  { label: 'hidden-globe depression', globe: hiddenGlobe, groundM: -430, cameraM: 20, southM: 180, fov: Math.PI / 12, baseOffScreen: true },
   // 2,500 m terrain from 3 km away at a shallow angle: the ellipsoid-height
   // base is far below the bottom of the screen.
   { label: 'high ground with loaded terrain', globe: loadedTerrain(2_500), groundM: 2_500, cameraM: 3_500, southM: 3_000, baseOffScreen: true },
@@ -857,7 +860,7 @@ for (const { label, globe, groundM, cameraM, southM, baseOffScreen } of [
     const clock = installFakeClock(t);
     t.after(() => { env.layer.destroy(env.viewer); env.cleanup(); });
     env.viewer.scene.globe = globe;
-    const baseVisibility = aimRealCamera(env, { groundM, cameraM, southM });
+    const baseVisibility = aimRealCamera(env, { groundM, cameraM, southM, fov });
     if (baseOffScreen)
       assert.equal(baseVisibility, Cesium.Intersect.OUTSIDE, 'precondition: the ellipsoid-height base is off screen');
 

src/data/localGeojsonCore.js

@@ -1159,9 +1159,10 @@ function groundSamplePoint(record, height, result) {
  * retry, each one a full offscreen render; 110 unseen data centres there held
  * rest frames at 58-589 ms. Unsampled bases sit on the ellipsoid, so the test
  * uses loaded globe terrain when there is some. Otherwise (a hidden globe) it
- * tests the vertical from 0 to GROUND_SAMPLE_MAX_ABS_HEIGHT_M, every height a
- * sample can accept, so high ground in view is never mistaken for an unseen
- * base. Unseen ground is sampled on the first walk that sees it.
+ * tests the vertical across +/-GROUND_SAMPLE_MAX_ABS_HEIGHT_M, every height a
+ * sample can accept, so high or below-ellipsoid ground in view is never
+ * mistaken for an unseen base. Unseen ground is sampled on the first walk that
+ * sees it.
  */
 function groundSampleInView(viewer, record) {
   const camera = viewer.camera;
@@ -1170,7 +1171,11 @@ function groundSampleInView(viewer, record) {
   const known =
     Number.isFinite(terrain) &&
     Math.abs(terrain) <= GROUND_SAMPLE_MAX_ABS_HEIGHT_M;
-  const low = groundSamplePoint(record, known ? terrain : 0, groundSampleLow);
+  const low = groundSamplePoint(
+    record,
+    known ? terrain : -GROUND_SAMPLE_MAX_ABS_HEIGHT_M,
+    groundSampleLow,
+  );
   const high = known
     ? low
     : groundSamplePoint(