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diff --git a/viewer/js/overlay_geom.js b/viewer/js/overlay_geom.js
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+++ b/viewer/js/overlay_geom.js
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+// overlay_geom.js — vector overlays → line-segment vertex arrays for the globe and flat views
+import { R_KM, toVec, interpolate, project, EE_XMAX, PC_XMAX } from './geo.js';
+
+const onSeam = (a, b) => Math.abs(a[0]) === 180 && Math.abs(b[0]) === 180;
+
+export function linesFromGeoJSON(fc, keep = () => true) {
+  const out = [];
+  const add = coords => {                         // split at segments that run along the antimeridian cut
+    let cur = [];
+    coords.forEach((c, i) => {
+      if (i > 0 && onSeam(coords[i - 1], c)) {
+        if (cur.length > 1) out.push(cur);
+        cur = [];
+      }
+      cur.push({ lat: c[1], lon: c[0] });
+    });
+    if (cur.length > 1) out.push(cur);
+  };
+  for (const f of fc.features || []) {
+    const g = f.geometry;
+    if (!g || !keep(f.properties || {})) continue;
+    if (g.type === 'LineString') add(g.coordinates);
+    else if (g.type === 'MultiLineString' || g.type === 'Polygon') g.coordinates.forEach(add);
+    else if (g.type === 'MultiPolygon') g.coordinates.forEach(poly => poly.forEach(add));
+  }
+  return out;
+}
+
+export function densify(lines, stepDeg = 1) {
+  return lines.map(line => {
+    if (line.length < 2) return [...line];
+    const out = [];
+    for (let i = 0; i + 1 < line.length; i++) out.push(...interpolate(line[i], line[i + 1], stepDeg).slice(i === 0 ? 0 : 1));
+    return out;
+  });
+}
+
+export function globeSegments(lines, radius = 1.002, stepDeg = 1) {
+  const pos = [];
+  for (const line of lines) {
+    for (let i = 0; i + 1 < line.length; i++) {
+      const pts = interpolate(line[i], line[i + 1], stepDeg);
+      for (let k = 0; k + 1 < pts.length; k++) {
+        const a = toVec(pts[k].lat, pts[k].lon), b = toVec(pts[k + 1].lat, pts[k + 1].lon);
+        pos.push(a[0] * radius, a[1] * radius, a[2] * radius, b[0] * radius, b[1] * radius, b[2] * radius);
+      }
+    }
+  }
+  return new Float32Array(pos);
+}
+
+// Lines laid on the 3D ground near the camera. Segments with an end inside the cap (cosCap around n, the eye's
+// direction) are cut into pieces ≈ 1/100 of their distance from the eye; each vertex sits on heightM (the drawn
+// ground, m), then is pulled 1% of the way toward the eye: the same pixel, a nearer depth, so the line wins over
+// the ground it lies on but hills in front still hide it. Positions are relative to origin.
+const PULL = 0.01;
+export function drapedSegments(lines, { eye, n, cosCap, origin, heightM }) {
+  const pos = [], kmPerDeg = (R_KM * Math.PI) / 180;
+  const inCap = p => { const v = toVec(p.lat, p.lon); return v[0] * n[0] + v[1] * n[1] + v[2] * n[2] >= cosCap; };
+  const distKm = p => { const v = toVec(p.lat, p.lon); return Math.hypot(v[0] - eye[0], v[1] - eye[1], v[2] - eye[2]) * R_KM; };
+  const put = p => {
+    const v = toVec(p.lat, p.lon), r = 1 + (Math.max(0, heightM(p.lat, p.lon)) + 1) / (R_KM * 1000);
+    for (let i = 0; i < 3; i++) pos.push(v[i] * r + (eye[i] - v[i] * r) * PULL - origin[i]);
+  };
+  for (const line of lines) {
+    for (let i = 0; i + 1 < line.length; i++) {
+      const a = line[i], b = line[i + 1];
+      if (!inCap(a) && !inCap(b)) continue;
+      const pts = interpolate(a, b, Math.max(0.01, Math.min(distKm(a), distKm(b)) / 100) / kmPerDeg);
+      for (let k = 0; k + 1 < pts.length; k++) {
+        put(pts[k]);
+        put(pts[k + 1]);
+      }
+    }
+  }
+  return new Float32Array(pos);
+}
+
+export function flatSegments(lines, view, z = 0.001) {
+  const xmax = view === 'equal_earth' ? EE_XMAX : PC_XMAX;
+  const pos = [];
+  for (const line of lines) {
+    let prev = null;
+    for (const p of line) {
+      const q = project(view, p.lat, p.lon);
+      if (prev && Math.abs(q.x - prev.x) < xmax) pos.push(prev.x, prev.y, z, q.x, q.y, z);
+      prev = q;
+    }
+  }
+  return new Float32Array(pos);
+}
+
+export function graticuleLines(step = 30) {
+  const lines = [];
+  for (let lon = -180; lon <= 180; lon += step) {
+    const l = [];
+    for (let lat = -90; lat <= 90; lat += 2) l.push({ lat, lon });
+    lines.push(l);
+  }
+  for (let lat = -90 + step; lat < 90; lat += step) {
+    const l = [];
+    for (let lon = -180; lon <= 180; lon += 2) l.push({ lat, lon });
+    lines.push(l);
+  }
+  return lines;
+}