diff --git a/B06_Section/B06_Section_UI_Cross_Ford_Geom.ts b/B06_Section/B06_Section_UI_Cross_Ford_Geom.ts
index c33ac5554..1d21de895 100644
--- a/B06_Section/B06_Section_UI_Cross_Ford_Geom.ts
+++ b/B06_Section/B06_Section_UI_Cross_Ford_Geom.ts
@@ -30,7 +30,7 @@ import { designInterpolator, groundInterpolator } from "./B06_Section_UI_Cross_C
 
 /** 측벽 한 매의 조작값 — 집수정 9키와 같은 축(높이·좌우·상하 대각). */
 export interface FordWallAdjust {
-  /** 벽 높이(m, 노견 상단 ~ 바닥판 상면). null = 자동(바닥판이 계류 하상에 놓임). */
+  /** 벽 높이(m, 노견 상단 ~ 바닥판 상면). null = 기본 = 관경 + 최소 토피(원지반 안 봄 · 107-2). */
   heightM: number | null;
   /** 좌우(m, + = 노견 바깥). 안쪽으로는 못 간다. */
   lateralM: number;
@@ -103,7 +103,8 @@ export interface FordLayout {
   adjust: FordAdjust;
 }
 
-/** 측벽 높이 하한 = 관경 + 최소 토피. 관이 노면 위로 솟지 않게 막는다. */
+/** 측벽 높이 하한 · 기본 = 관경 + 최소 토피 — 바닥 = 노면 − (관경 + 토피)(박스 「노면 − 복토」 꼴 ·
+ *  원지반 안 봄 · 깊은 계류면 ▲▼ 로 내림 · 107-2). 관이 노면 위로 솟지 않게 막는다. */
 function minWallHeight(ford: FordSet): number {
   return ford.diameter_m + ford.min_cover_m;
 }
@@ -149,15 +150,6 @@ export function computeFordLayout(
   const inletOnLeft = uphill === "left";
   const roadTopAt = roadTopFactory(section, edges.left, edges.right);
 
-  // 바닥판이 놓일 계류 하상 — 두 노견 사이 원지반 최저점.
-  const bedElevation = (() => {
-    let low = Math.min(groundAt(edges.left.offset_m), groundAt(edges.right.offset_m));
-    for (let offset = edges.right.offset_m; offset <= edges.left.offset_m; offset += 0.25) {
-      low = Math.min(low, groundAt(offset));
-    }
-    return low;
-  })();
-
   const buildSide = (role: FordWallRole): FordSideLayout | null => {
     const onLeft = role === "inlet" ? inletOnLeft : !inletOnLeft;
     const edge = onLeft ? edges.left : edges.right;
@@ -166,8 +158,7 @@ export function computeFordLayout(
     const wing = role === "inlet" ? ford.wing_in : ford.wing_out;
     const wanted = role === "inlet" ? adjust.inlet : adjust.outlet;
     const lateralM = Math.max(wanted.lateralM, 0);
-    const autoHeight = roadTopAt(edge.offset_m) - (bedElevation + ford.slab_thickness_m);
-    const heightM = Math.max(wanted.heightM ?? autoHeight, minWallHeight(ford));
+    const heightM = Math.max(wanted.heightM ?? minWallHeight(ford), minWallHeight(ford));
     // 벽 상단이 노견에 붙도록 바닥판 상면을 역산한다. 상하 이동은 buildBasin이 얹는다.
     const anchor: OffsetPoint = {
       offset: edge.offset_m,
diff --git a/resources/tester/helper_107_ford_free.cjs b/resources/tester/helper_107_ford_free.cjs
index 59aef59ac..c8c132c23 100644
--- a/resources/tester/helper_107_ford_free.cjs
+++ b/resources/tester/helper_107_ford_free.cjs
@@ -69,4 +69,88 @@ bodies.ford.control.select(173.09, null);
 bodies.box.control.select(352.14, "left");
 out.released = released;
 
+// 107-2 — 원지반을 바꾼 사본: 세월교 바닥 = 노면 − (관경 + 토피) · 박스 본체 · 끝 · 날개 · 흙막이 자리 그대로
+const { computeStoredLayouts } = loadTs(B06("B06_Section_Structure_Layouts.ts"));
+const fixture = JSON.parse(
+  fs.readFileSync(path.join(__dirname, "fixtures", "b06_73_4_body_guard_sections.json"), "utf8"),
+);
+const sections = fixture.cross_sections;
+const at = (c) => sections.find((x) => Math.abs(x.chainage_m - c) < 0.6);
+const clone = (v) => JSON.parse(JSON.stringify(v));
+const r3 = (v) => Math.round(v * 1000) / 1000;
+const pts = (list) => (list ?? []).map((p) => [r3(p.offset), r3(p.elevation)]);
+/** 원지반 사본 — [from, to] 안 샘플을 dz 만큼 옮김. */
+const shiftGround = (x, dz, from = -Infinity, to = Infinity) => {
+  for (const p of x.samples)
+    if (p.offset_m >= from && p.offset_m <= to && p.elevation_m != null) p.elevation_m += dz;
+};
+const GROUNDS = {
+  base: () => undefined,
+  bedDown2: (x) => shiftGround(x, -2, -2, 2),
+  allUp05: (x) => shiftGround(x, 0.5),
+  allDown1: (x) => shiftGround(x, -1),
+};
+const walls = (culvert) =>
+  (culvert?.walls ?? []).filter((w) => w.role !== "extra").map((w) => pts(w.points));
+function fordShape(base, edit, ground) {
+  const x = clone(base);
+  edit(x);
+  ground(x);
+  const { ford, culvert } = computeStoredLayouts(
+    x,
+    sections.map((y) => (y === base ? x : y)),
+  );
+  return {
+    sides: ford.sides.map((side) => ({
+      role: side.role,
+      parts: side.parts.map((part) => pts(part.points)),
+      slabTop: r3(side.slabTopElevation),
+      // 노면(벽 상단) − 바닥판 상면 = 관경 + 최소 토피
+      depth: r3(side.top.elevation - side.slabTopElevation),
+    })),
+    pipeCover: r3(ford.ford.diameter_m + ford.ford.min_cover_m),
+    slab: pts(ford.slabBridge),
+    pipe: pts(ford.pipe.outer),
+    walls: walls(culvert),
+  };
+}
+function boxShape(base, edit, ground) {
+  const x = clone(base);
+  edit(x);
+  ground(x);
+  const { box, culvert } = computeStoredLayouts(
+    x,
+    sections.map((y) => (y === base ? x : y)),
+  );
+  return {
+    top: pts(box.topSlab),
+    bottom: pts(box.bottomSlab),
+    barrel: pts(box.barrel),
+    sides: box.sides.map((side) => ({
+      role: side.role,
+      offset: r3(side.offset),
+      top: r3(side.topElevation),
+      bottom: r3(side.bottomElevation),
+      apron: pts(side.apron),
+    })),
+    walls: walls(culvert),
+  };
+}
+const FORD = at(173.087);
+const BOX = at(352.14);
+const keep = () => undefined;
+const inletWing = (x) => {
+  x.culvert.inlet = { role: "inlet", structure: "" };
+};
+// 날개벽 박스 — 흙막이 없이 양쪽 날개벽(에이프런 1.414 m)
+const noGuard = (x) => {
+  delete x.culvert;
+  for (const wing of [x.box.wing_in, x.box.wing_out])
+    Object.assign(wing, { installed: true, slab_extend_m: 1.414 });
+};
+const run = (shape, base, edit) =>
+  Object.fromEntries(Object.entries(GROUNDS).map(([k, g]) => [k, shape(base, edit, g)]));
+out.ford = { guards: run(fordShape, FORD, keep), inletWing: run(fordShape, FORD, inletWing) };
+out.box = { guards: run(boxShape, BOX, keep), bare: run(boxShape, BOX, noGuard) };
+
 process.stdout.write(JSON.stringify(out));
diff --git a/resources/tester/helper_91_ground_free.cjs b/resources/tester/helper_91_ground_free.cjs
index 3a1e864a9..5cb2a0ec8 100644
--- a/resources/tester/helper_91_ground_free.cjs
+++ b/resources/tester/helper_91_ground_free.cjs
@@ -68,7 +68,7 @@ const layoutsOf = (s, base) =>
     sections.map((x) => (x === base ? s : x)),
   );
 
-// ① 91-2 세월교 바닥 — 자동은 하상을 따라감 · 손 값(측벽 높이)은 원지반을 바꿔도 그대로 · 흙막이 기본 높이도
+// ① 91-2 · 107-2 세월교 바닥 — 기본 · 손 값(측벽 높이)은 원지반을 바꿔도 그대로 · 흙막이 기본 높이도
 const FORD = at(173.087);
 function fordFloor(edit) {
   const s = clone(FORD);
diff --git a/resources/tester/test_107_ford_free.py b/resources/tester/test_107_ford_free.py
index 9fb762f2a..0872cb6eb 100644
--- a/resources/tester/test_107_ford_free.py
+++ b/resources/tester/test_107_ford_free.py
@@ -51,3 +51,24 @@ def test_벽을_고르면_세월교_박스_고름을_풂() -> None:
     # 카드 안 — 벽을 고르면 본체 창을 닫고 · 본체를 고르면 벽 창을 닫음
     assert "bodies.clearSelection(); // 세월교 · 박스 고름을 풂" in view
     assert "if (activeRevet !== null) toggleRevet(activeRevet);" in view
+
+
+@pytest.mark.parametrize("variant", ["guards", "inletWing"])
+def test_세월교_원지반을_바꿔도_바닥_그대로(out: dict, variant: str) -> None:
+    """107-2 — 바닥 = 노면 − (관경 + 최소 토피) · 하상 −2 m · 전체 +0.5 / −1 m 사본도 같은 모양."""
+    runs = out["ford"][variant]
+    for name, shape in runs.items():
+        assert shape == runs["base"], name
+    base = runs["base"]
+    assert all(side["depth"] == pytest.approx(base["pipeCover"]) for side in base["sides"])
+
+
+@pytest.mark.parametrize("variant", ["guards", "bare"])
+def test_박스_원지반을_바꿔도_본체_끝_날개_흙막이_그대로(out: dict, variant: str) -> None:
+    runs = out["box"][variant]
+    for name, shape in runs.items():
+        assert shape == runs["base"], name
+    if variant == "bare":
+        assert all(side["apron"] for side in runs["base"]["sides"])
+    else:
+        assert len(runs["base"]["walls"]) == 2
diff --git a/resources/tester/test_91_ground_free.py b/resources/tester/test_91_ground_free.py
index 29d5db9de..821be3e19 100644
--- a/resources/tester/test_91_ground_free.py
+++ b/resources/tester/test_91_ground_free.py
@@ -29,9 +29,9 @@ def fit() -> dict:
     return json.loads(done.stdout)
 
 
-def test_세월교_바닥_자동은_하상을_따라감(fit: dict) -> None:
-    for before, after in zip(fit["fordAuto"], fit["fordAutoBedDown"]):
-        assert after["slabTop"] < before["slabTop"] - 1.0
+def test_세월교_바닥_기본도_원지반을_바꿔도_그대로(fit: dict) -> None:
+    """107-2 — 기본 바닥 = 노면 − (관경 + 토피) · 하상을 안 따라감(옛 91-2 「자동은 하상」 뒤집음)."""
+    assert fit["fordAuto"] == fit["fordAutoBedDown"]
 
 
 def test_세월교_바닥_손_값은_원지반을_바꿔도_그대로(fit: dict) -> None: