diff --git a/src/AcDream.Core/Physics/TerrainSurface.cs b/src/AcDream.Core/Physics/TerrainSurface.cs
index 8337dbe..a3c5f78 100644
--- a/src/AcDream.Core/Physics/TerrainSurface.cs
+++ b/src/AcDream.Core/Physics/TerrainSurface.cs
@@ -51,12 +51,15 @@ public sealed class TerrainSurface
     /// interpolation within that triangle. This matches the visual terrain
     /// mesh exactly.
     ///
-    /// A small slope-proportional upward bias is added to compensate for
-    /// the geometric fact that a point-sampled Z on a tilted triangle
-    /// places the character's center at the surface, but the character's
-    /// feet (which extend forward/backward) clip into the rising terrain.
-    /// The bias is proportional to the max height difference across the
-    /// cell — steeper slope = more lift. On flat ground it's zero.
+    /// Triangle layout (from LandblockMesh.cs index buffer):
+    ///   SWtoNE: tri1 = {BL,TL,BR}, tri2 = {BR,TL,TR}  — shared edge TL→BR (x+y=1 boundary)
+    ///   SEtoNW: tri1 = {BL,TR,BR}, tri2 = {BL,TL,TR}  — shared edge BL→TR (y=x boundary)
+    ///
+    /// NOTE: The SWtoNE "cut" exposes the SW(BL) and NE(TR) corners as isolated
+    /// vertices — the hypotenuse runs NW(TL)→SE(BR), so the dividing test is
+    /// x+y=1 (not y=x). Confusing naming aside, the formula below matches
+    /// TerrainGeometryGenerator.GetHeight (ACME WorldBuilder-ACME-Edition) which
+    /// was verified against the mesh index buffer.
     /// </summary>
     public float SampleZ(float localX, float localY)
     {
@@ -83,17 +86,21 @@ public sealed class TerrainSurface
 
         if (splitSWtoNE)
         {
-            if (ty <= tx)
-                return hBL + (hBR - hBL) * tx + (hTR - hBR) * ty;
+            // Mesh: {BL,TL,BR} and {BR,TL,TR}. Shared hypotenuse = TL(0,1)→BR(1,0).
+            // Dividing line: tx + ty = 1.
+            if (tx + ty <= 1f)
+                return hBL + (hBR - hBL) * tx + (hTL - hBL) * ty;  // BL+BR+TL triangle
             else
-                return hBL + (hTR - hTL) * tx + (hTL - hBL) * ty;
+                return hTR + (hTL - hTR) * (1f - tx) + (hBR - hTR) * (1f - ty);  // TR+TL+BR triangle
         }
         else
         {
-            if (ty <= 1f - tx)
-                return hBL + (hBR - hBL) * tx + (hTL - hBL) * ty;
+            // Mesh: {BL,TR,BR} and {BL,TL,TR}. Shared hypotenuse = BL(0,0)→TR(1,1).
+            // Dividing line: ty = tx.
+            if (ty <= tx)
+                return hBL + (hBR - hBL) * tx + (hTR - hBR) * ty;  // BL+BR+TR triangle
             else
-                return hTR + (hTL - hTR) * (1f - tx) + (hBR - hTR) * (1f - ty);
+                return hBL + (hTR - hTL) * tx + (hTL - hBL) * ty;  // BL+TR+TL triangle
         }
     }