diff --git a/web/src/maplibre/custom3DLayer.ts b/web/src/maplibre/custom3DLayer.ts
index 3ae0ee7..5fdb559 100644
--- a/web/src/maplibre/custom3DLayer.ts
+++ b/web/src/maplibre/custom3DLayer.ts
@@ -71,6 +71,7 @@ export function createCustom3DLayer(id: string, map: Map): Layer3DHandle {
   let renderer: THREE.WebGLRenderer | null = null
 
   let origin: MercatorCoordinate | null = null
+  let originLngLat: [number, number] | null = null
   let dirty          = true
   let currentLines:  Line3D[]  = []
   let currentPoints: Point3D[] = []
@@ -96,6 +97,7 @@ export function createCustom3DLayer(id: string, map: Map): Layer3DHandle {
     sphereMeshes.length = 0
     sphereIds.length    = 0
     origin = null
+    originLngLat = null
   }
 
   function rebuildGeometry() {
@@ -109,6 +111,7 @@ export function createCustom3DLayer(id: string, map: Map): Layer3DHandle {
     if (!firstPt) return
 
     const [lon0, lat0, alt0] = firstPt
+    originLngLat = [lon0, lat0]
     // Use exaggerated altitude so geometry aligns with visual terrain.
     origin = MercatorCoordinate.fromLngLat([lon0, lat0], alt0 * TERRAIN_EXAGGERATION)
     const mpu = origin.meterInMercatorCoordinateUnits()
@@ -198,9 +201,14 @@ export function createCustom3DLayer(id: string, map: Map): Layer3DHandle {
       // modelViewProjectionMatrix (used by terrain tiles) but absent from
       // mercatorMatrix (passed to custom layers).  Subtracting it here makes
       // our exaggerated-altitude objects align with the visual terrain surface.
-      const mpu              = origin.meterInMercatorCoordinateUnits()
-      const centerElevM      = (map as unknown as { transform: { elevation: number } }).transform?.elevation ?? 0
-      const centerElevMerc   = centerElevM * mpu
+      //
+      // Use MercatorCoordinate.fromLngLat (not origin.meterInMercatorCoordinateUnits)
+      // because altitude z uses 1/(circumference*cos(lat)) while mpu gives
+      // cos(lat)/circumference — they differ by cos²(lat), causing ~2× error at lat 50°.
+      const centerElevM    = (map as unknown as { transform: { elevation: number } }).transform?.elevation ?? 0
+      const centerElevMerc = originLngLat
+        ? MercatorCoordinate.fromLngLat(originLngLat, centerElevM).z
+        : centerElevM * origin.meterInMercatorCoordinateUnits()
 
       const m = new THREE.Matrix4().fromArray(matrix)
       const t = new THREE.Matrix4().makeTranslation(