diff --git a/B05_Profile/B05_Profile_Api_TerrainGrid.ts b/B05_Profile/B05_Profile_Api_TerrainGrid.ts
index 647f5dcad..14d5387c6 100644
--- a/B05_Profile/B05_Profile_Api_TerrainGrid.ts
+++ b/B05_Profile/B05_Profile_Api_TerrainGrid.ts
@@ -73,7 +73,7 @@ export function designTerrainGridNow(projectId: string): DesignTerrainGrid | nul
 /** 한 번에 물을 수 있는 점 수 — 서버 `MAX_POINTS` 와 같은 값. */
 const ELEVATION_CHUNK = 4000;
 
-/** 계산 묶음 ground 단계에 넣을 격자 · 서버 길(PLAN 346-1). 서버 길 값은 mm 자리(응답 그대로). */
+/** 계산 묶음 ground 단계에 넣을 격자 · 서버 길(PLAN 346-1). 서버 길도 소수 6자리 — 격자 길과 같은 값. */
 export function groundStageDeps(projectId: string): Pick<GroundStageDeps, "grid" | "remote"> {
   return {
     grid: () => designTerrainGridNow(projectId),
@@ -88,7 +88,7 @@ export function groundStageDeps(projectId: string): Pick<GroundStageDeps, "grid"
             method: "POST",
             credentials: "include",
             headers: { "Content-Type": "application/json" },
-            body: JSON.stringify({ points: chunk, terrain }),
+            body: JSON.stringify({ points: chunk, terrain, decimals: 6 }),
             signal,
           });
           if (!response.ok) throw new Error(`지반고 요청 실패: HTTP ${response.status}`);
diff --git a/B05_Profile/B05_Profile_Router_Terrain.py b/B05_Profile/B05_Profile_Router_Terrain.py
index 7af958978..379907567 100644
--- a/B05_Profile/B05_Profile_Router_Terrain.py
+++ b/B05_Profile/B05_Profile_Router_Terrain.py
@@ -62,6 +62,8 @@ class ElevationRequest(BaseModel):
 
     points: list[tuple[float, float]] = Field(..., min_length=1, max_length=MAX_POINTS)
     terrain: AnalysisTerrain | None = None
+    #: 표고 자릿수 — 화면 값은 mm(3) · 계산 묶음 서버 길은 6(격자 표본 · 서버 생성과 같은 자리).
+    decimals: int = Field(3, ge=0, le=6)
 
 
 def _sample(project_root: Path, params: dict, points: list[tuple[float, float]]):
@@ -112,7 +114,9 @@ async def read_route_elevations(project_id: UUID, request: ElevationRequest) ->
     return {
         "status": "success",
         "project_id": str(project_id),
-        "z": [None if not ok else round(float(value), 3) for value, ok in zip(z, valid)],
+        "z": [
+            None if not ok else round(float(value), request.decimals) for value, ok in zip(z, valid)
+        ],
         "valid": [bool(ok) for ok in valid],
     }
 
diff --git a/resources/tester/test_344_1_terrain_grid_route.py b/resources/tester/test_344_1_terrain_grid_route.py
index 69476e550..05c363bc3 100644
--- a/resources/tester/test_344_1_terrain_grid_route.py
+++ b/resources/tester/test_344_1_terrain_grid_route.py
@@ -114,3 +114,30 @@ def test_받을_자리_서명은_manifest_구성과_평활(tmp_path):
     )
     assert terrain_router._design_grid({**PARAMS, "method": "tin"}, models, confirmed) is None
     assert terrain_router._design_grid(PARAMS, models, None) is None
+
+
+def test_지반고_자릿수_기본_3_묶음은_6(tmp_path, monkeypatch):
+    root = tmp_path
+    models = root / "B04_PreProcess" / "models"
+    models.mkdir(parents=True)
+    x = np.arange(0.0, 11.0)
+    np.savez(
+        models / "dtm_csf_smooth.npz",
+        x=x,
+        y=x,
+        z=np.add.outer(x * 0.0, x * 0.1234567),
+        valid_mask=np.ones((11, 11), dtype=bool),
+    )
+    _patch(monkeypatch, root, None)
+
+    async def params(_connection, _project_id):
+        return dict(PARAMS)
+
+    monkeypatch.setattr(terrain_router, "get_surface_confirmation_params", params)
+    ask = terrain_router.ElevationRequest
+    plain = asyncio.run(terrain_router.read_route_elevations(PID, ask(points=[(5.5, 5.0)])))
+    fine = asyncio.run(
+        terrain_router.read_route_elevations(PID, ask(points=[(5.5, 5.0)], decimals=6))
+    )
+    assert plain["z"] == [round(5.5 * 0.1234567, 3)]
+    assert fine["z"] == [round(5.5 * 0.1234567, 6)]