diff --git a/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py b/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py
index dd52ea62..cd6646ad 100644
--- a/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py
+++ b/B04_PreProcess/B04_PreProcess_Engine_SheetSurface.py
@@ -56,7 +56,7 @@ SHEET_SOURCE_FILTER = "sheet"
 
 
 def _load_features_metric(
-    processed_dir: Path, epsg: int, filename: str = _CONTOUR_FILE
+    processed_dir: Path, crs: str, filename: str = _CONTOUR_FILE
 ) -> list[dict[str, Any]]:
     """병합 도엽 레이어(WGS84)를 읽어 사업지 CRS(m)로 재투영한다."""
     path = processed_dir / filename
@@ -72,7 +72,7 @@ def _load_features_metric(
     features = data.get("features")
     if not isinstance(features, list):
         return []
-    transformer = Transformer.from_crs("EPSG:4326", f"EPSG:{epsg}", always_xy=True)
+    transformer = Transformer.from_crs("EPSG:4326", crs, always_xy=True)
 
     def _map(coords: Any) -> Any:
         if not isinstance(coords, list):
@@ -381,7 +381,7 @@ def build_sheet_surface_model(
     processed_dir: Path,
     models_dir: Path,
     route_xy: np.ndarray,
-    epsg: int,
+    crs: str,
     methods: list[str] | None = None,
 ) -> list[dict[str, Any]]:
     """도엽등고선으로 방식별 DTM npz·프리뷰 glb를 만들고 등록용 dict 목록을 돌려준다.
@@ -390,10 +390,10 @@ def build_sheet_surface_model(
     화면에서 바꿔 보며 정해야 한다(2026-08-30 사용자 지시). 실패하면 빈 목록 —
     호출측은 분석을 계속한다(도엽 미확보 지역 폴백).
 
-    `route_xy`: (N, 2) 노선 정점 XY(사업지 CRS, m).
+    `route_xy`: (N, 2) 노선 정점 XY(사업지 CRS, m), `crs`: 그 좌표계(pyproj 입력 문자열).
     """
     started = time.monotonic()
-    features = _load_features_metric(processed_dir, epsg)
+    features = _load_features_metric(processed_dir, crs)
     if not features:
         return []
 
@@ -456,22 +456,20 @@ def build_sheet_surface_from_route(
     `methods`를 주지 않으면 `SHEET_SURFACE_METHODS` 전체를 만든다 — 관리자가 화면에서
     한 방식을 요청할 때 그 목록만 넘긴다.
     """
-    from common_util.common_util_route_geometry import (
-        find_planned_route_file,
-        read_planned_route,
-    )
+    from B04_PreProcess.B04_PreProcess_Engine_Extent import project_epsg_from_prj
+    from common_util.common_util_route_geometry import load_design_route
 
-    route_file = find_planned_route_file(project_root / "B03_FileInput" / "input")
-    if route_file is None:
-        logger.warning("도엽 서피스: 계획 노선 파일이 없습니다.")
-        return []
-    planned = read_planned_route(route_file)
+    # 노선 CSV의 `crs_epsg` 열은 표시용 라벨이라 LAS(.prj) 좌표계와 다를 수 있다
+    # (2026-09-01 실측: 라벨 5179, 실제 5176 — 도엽 서피스만 딴 자리에 만들어졌다).
+    # `load_design_route()`가 .prj 좌표계로 재투영해 주므로 라이다 지표면과 한 자리에 선다.
+    planned = load_design_route(project_root)
     if planned is None or len(planned.vertices) < 2:
-        logger.warning("도엽 서피스: 계획 노선 파일을 읽지 못했습니다: %s", route_file.name)
+        logger.warning("도엽 서피스: 계획 노선 파일을 읽지 못했습니다.")
         return []
+    crs = planned.crs_input or project_epsg_from_prj(project_root)
     route_xy = np.array([(v.x, v.y) for v in planned.vertices], dtype=np.float64)
     return build_sheet_surface_model(
-        project_root, processed_dir, models_dir, route_xy, planned.epsg or 5186, methods
+        project_root, processed_dir, models_dir, route_xy, crs, methods
     )
 
 
@@ -527,7 +525,9 @@ def run_sheet_surface_analysis(
     )
 
     _report(70, "surface_model", "도엽등고선 3D 서피스 생성 중")
-    models = build_sheet_surface_model(project_root, processed_dir, models_dir, route_xy, epsg)
+    models = build_sheet_surface_model(
+        project_root, processed_dir, models_dir, route_xy, f"EPSG:{epsg}"
+    )
     if not models:
         raise ValueError("도엽등고선으로 지표면을 만들지 못했습니다 — 도엽 확보를 확인하세요.")
 
diff --git a/B04_PreProcess/B04_PreProcess_Router_Inflow.py b/B04_PreProcess/B04_PreProcess_Router_Inflow.py
index ab520978..9d72165a 100644
--- a/B04_PreProcess/B04_PreProcess_Router_Inflow.py
+++ b/B04_PreProcess/B04_PreProcess_Router_Inflow.py
@@ -25,10 +25,7 @@ from B04_PreProcess.B04_PreProcess_Engine_Watershed_Export import STAGES, draina
 from B04_PreProcess.B04_PreProcess_Engine_Watershed_Flow import polygonize_labels
 from B04_PreProcess.B04_PreProcess_Engine_Watershed_Grid import GridSpec
 from B05_Profile.B05_Profile_Repository import get_surface_crs_epsg
-from common_util.common_util_route_geometry import (
-    find_planned_route_file,
-    read_planned_route,
-)
+from common_util.common_util_route_geometry import load_design_route
 from common_util.common_util_storage import resolve_stored_project_path
 from config.config_db import get_db_pool
 
@@ -74,15 +71,25 @@ def _load_routing(stored_path: str) -> dict[str, Any] | None:
 
 
 async def _resolve_epsg(project_id: UUID, stored_path: str) -> str:
-    """분석에 쓰인 좌표계를 그대로 되찾는다(노선 파일이 명시하면 그 값 우선)."""
+    """분석에 쓰인 좌표계를 그대로 되찾는다.
+
+    격자 산출물은 `load_design_route()`가 맞춘 **사업지(.prj) 좌표계**에 있다. 노선 CSV의
+    `crs_epsg` 열은 표시용 라벨이라 그 값을 쓰면 좌표가 딴 곳으로 간다
+    (2026-09-01 실측: 라벨 5179, 실제 5176 — 유입 폴리곤이 1,500km 밖에 찍혔다).
+    """
+    from B04_PreProcess.B04_PreProcess_Engine_Extent import project_epsg_from_prj
+
+    project_root = Path(resolve_stored_project_path(stored_path))
+    planned = load_design_route(project_root)
+    if planned is not None and planned.crs_input:
+        return planned.crs_input
+    prj_crs = project_epsg_from_prj(project_root)
+    if prj_crs:
+        return prj_crs
     pool = get_db_pool()
     async with pool.acquire() as connection:
         epsg = await get_surface_crs_epsg(connection, project_id, 0)
-    project_root = Path(resolve_stored_project_path(stored_path))
-    route_file = find_planned_route_file(project_root / "B03_FileInput" / "input")
-    planned = read_planned_route(route_file) if route_file else None
-    source = (planned.epsg if planned else None) or epsg or 5186
-    return f"EPSG:{source}"
+    return f"EPSG:{epsg or 5186}"
 
 
 def _collect_inflow(
diff --git a/B07_DesignDetail/B07_DesignDetail_Router_Support.py b/B07_DesignDetail/B07_DesignDetail_Router_Support.py
index da5d0526..6309b34b 100644
--- a/B07_DesignDetail/B07_DesignDetail_Router_Support.py
+++ b/B07_DesignDetail/B07_DesignDetail_Router_Support.py
@@ -206,7 +206,7 @@ def watershed_source(context: Any) -> dict[str, Any]:
 
     저장본은 전부 WGS84라 여기서 미터 좌표로 되돌린다(B04가 저장할 때와 반대 방향).
     """
-    to_metric = Transformer.from_crs("EPSG:4326", f"EPSG:{context.epsg}", always_xy=True)
+    to_metric = Transformer.from_crs("EPSG:4326", context.crs, always_xy=True)
 
     def metric(point: tuple[float, float]) -> tuple[float, float]:
         x, y = to_metric.transform(point[0], point[1])
diff --git a/common_util/common_util_drainage_context.py b/common_util/common_util_drainage_context.py
index 364704b7..bb71200f 100644
--- a/common_util/common_util_drainage_context.py
+++ b/common_util/common_util_drainage_context.py
@@ -52,7 +52,9 @@ class DrainageContext:
     project_root: Path
     vertices: list[RouteVertex] = field(default_factory=list)
     z_source: str = Z_SOURCE_CSV
-    epsg: int = 5186
+    # 사업지 좌표계 — pyproj 입력 문자열(`EPSG:n` 또는 .prj WKT). 노선 CSV의 `crs_epsg`
+    # 열은 표시용 라벨이라 실좌표계와 다를 수 있다(2026-09-01 실측: 라벨 5179, 실제 5176).
+    crs: str = "EPSG:5186"
     route_id: int | None = None
     to_lonlat: Callable[[float, float], tuple[float, float]] = lambda x, y: (x, y)
 
@@ -96,15 +98,17 @@ async def load_drainage_context(project_id: UUID) -> tuple[DrainageContext | Non
         sampler=sampler,
     )
 
-    epsg = int(planned.epsg or db_epsg or 5186)
-    transformer = Transformer.from_crs(f"EPSG:{epsg}", "EPSG:4326", always_xy=True)
+    # 노선을 실제로 담고 있는 좌표계를 쓴다 — `load_design_route()`가 .prj 좌표계로
+    # 재투영하며 `crs_input`만 갱신하고 `epsg` 라벨은 CSV 값 그대로 남긴다.
+    crs = planned.crs_input or f"EPSG:{planned.epsg or db_epsg or 5186}"
+    transformer = Transformer.from_crs(crs, "EPSG:4326", always_xy=True)
     return (
         DrainageContext(
             stored_path=stored_path,
             project_root=project_root,
             vertices=vertices,
             z_source=z_source,
-            epsg=epsg,
+            crs=crs,
             route_id=int(route["id"]) if route else None,
             to_lonlat=lambda x, y: transformer.transform(x, y),
         ),