diff --git a/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py b/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py
index e9c13960..a5f57920 100644
--- a/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py
+++ b/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py
@@ -144,17 +144,9 @@ def build_sheet_surface_model(
     stem = f"dtm_{SHEET_SOURCE_FILTER}"
     model_path = models_dir / f"{stem}.npz"
     preview_path = models_dir / f"{stem}_preview.glb"
-    atomic_npz(
-        model_path,
-        x=x_coords,
-        y=y_coords,
-        z=z_grid,
-        valid_mask=valid_grid,
-        resolution=np.array([SHEET_SURFACE_GRID_M], np.float32),
-    )
-
-    vertices, faces = _preview_mesh(x_coords, y_coords, z_grid, valid_grid)
     finite_z = z_grid[valid_grid]
+    # bounds를 npz에 같이 넣는다 — 등고선 API가 이 값을 화면 원점으로 쓴다. 없으면
+    # LAS structured.npz로 폴백해 메시(glb) 원점과 어긋난다(LAS 없는 설계는 아예 실패).
     bounds = np.array(
         [
             [x_coords[0], x_coords[-1]],
@@ -162,6 +154,17 @@ def build_sheet_surface_model(
             [float(finite_z.min()), float(finite_z.max())],
         ]
     )
+    atomic_npz(
+        model_path,
+        x=x_coords,
+        y=y_coords,
+        z=z_grid,
+        valid_mask=valid_grid,
+        bounds=bounds,
+        resolution=np.array([SHEET_SURFACE_GRID_M], np.float32),
+    )
+
+    vertices, faces = _preview_mesh(x_coords, y_coords, z_grid, valid_grid)
     write_glb(preview_path, vertices, faces, bounds)
 
     logger.info(