diff --git a/D03_PlanAnalysis/D03_PlanAnalysis_ZeroOrder.py b/D03_PlanAnalysis/D03_PlanAnalysis_ZeroOrder.py
index 8da3b6deb..b4f0c4e5c 100644
--- a/D03_PlanAnalysis/D03_PlanAnalysis_ZeroOrder.py
+++ b/D03_PlanAnalysis/D03_PlanAnalysis_ZeroOrder.py
@@ -78,17 +78,29 @@ def d8_accumulation(z: np.ndarray, valid: np.ndarray) -> np.ndarray:
             best = np.where(better, drop, best)
             receiver = np.where(better, index + dr * cols + dc, receiver)
         done = (receiver >= 0) | edge | ~valid
-        while True:
-            grew = False
-            for dr, dc in _D8:
-                take = ~done & _shift(done & valid, dr, dc, False)
-                take &= _shift(filled, dr, dc, np.nan) == filled
-                if take.any():
-                    receiver[take] = index[take] + dr * cols + dc
-                    done |= take
-                    grew = True
-            if not grew:
-                break
+        # 평지 이어 감 — 같은 높이 이웃 표는 한 번 · 아직 안 흐르는 후보 칸만 돌림(방향 · 갱신 차례 그대로)
+        eq = [_shift(filled, dr, dc, np.nan) == filled for dr, dc in _D8]
+    cand = np.flatnonzero(~done & np.any(eq, axis=0))
+    done_f, valid_f, rec_f = done.ravel(), valid.ravel(), receiver.ravel()
+    r, c = np.divmod(cand, cols)
+    nbr, ok = [], []
+    for k, (dr, dc) in enumerate(_D8):
+        inb = (r + dr >= 0) & (r + dr < rows) & (c + dc >= 0) & (c + dc < cols)
+        ok.append(inb & eq[k].ravel()[cand])
+        nbr.append(np.where(inb, cand + dr * cols + dc, 0))
+    while True:
+        grew = False
+        for k, (dr, dc) in enumerate(_D8):
+            take = ok[k] & ~done_f[cand] & done_f[nbr[k]] & valid_f[nbr[k]]
+            if take.any():
+                hit = cand[take]
+                rec_f[hit] = hit + dr * cols + dc
+                done_f[hit] = True
+                grew = True
+        if not grew:
+            break
+        keep = ~done_f[cand]
+        cand, nbr, ok = cand[keep], [n[keep] for n in nbr], [o[keep] for o in ok]
     down = receiver.ravel().tolist()
     acc = valid.astype(float).ravel().tolist()
     upstream = np.bincount(receiver[receiver >= 0], minlength=rows * cols).tolist()
diff --git a/resources/tester/test_617_1_d8_flat.py b/resources/tester/test_617_1_d8_flat.py
new file mode 100644
index 000000000..aa5f557cf
--- /dev/null
+++ b/resources/tester/test_617_1_d8_flat.py
@@ -0,0 +1,142 @@
+"""D 0차곡 D8 평지 이어 감 고침(PLAN 617-1) — 옛 꼴(통격자 바퀴)과 새 꼴(후보 칸 목록) 비트 동일 ·
+합성 격자(넓은 평지 · 계단 · 빈 칸 · 가장자리) + 창고 0차곡 도엽 다시 셈 = 저장본(읽기만).
+
+./venv/Scripts/python.exe -m pytest resources/tester/test_617_1_d8_flat.py -q
+"""
+
+import json
+import math
+
+import numpy as np
+import pytest
+
+from common_util import common_util_terrain_store as store
+from D03_PlanAnalysis import D03_PlanAnalysis_ZeroOrder as zo
+
+_D8 = zo._D8
+_shift = zo._shift
+
+
+def _old_d8(z: np.ndarray, valid: np.ndarray) -> np.ndarray:
+    """617-1 전 꼴 그대로(대조용)."""
+    from skimage.morphology import reconstruction
+
+    rows, cols = z.shape
+    height = np.where(valid, np.asarray(z, float), np.nan)
+    floor = np.nanmin(height) if valid.any() else 0.0
+    edge = valid & ~np.all([_shift(valid, dr, dc, False) for dr, dc in _D8], axis=0)
+    ground = np.where(valid, height, floor)
+    seed = np.where(edge | ~valid, ground, ground.max())
+    filled = np.where(valid, reconstruction(seed, ground, method="erosion"), np.nan)
+    index = np.arange(rows * cols).reshape(rows, cols)
+    best = np.zeros(z.shape)
+    receiver = np.full(z.shape, -1, np.int64)
+    with np.errstate(invalid="ignore"):
+        for dr, dc in _D8:
+            drop = (filled - _shift(filled, dr, dc, np.nan)) / math.hypot(dr, dc)
+            better = drop > best
+            best = np.where(better, drop, best)
+            receiver = np.where(better, index + dr * cols + dc, receiver)
+        done = (receiver >= 0) | edge | ~valid
+        while True:
+            grew = False
+            for dr, dc in _D8:
+                take = ~done & _shift(done & valid, dr, dc, False)
+                take &= _shift(filled, dr, dc, np.nan) == filled
+                if take.any():
+                    receiver[take] = index[take] + dr * cols + dc
+                    done |= take
+                    grew = True
+            if not grew:
+                break
+    down = receiver.ravel().tolist()
+    acc = valid.astype(float).ravel().tolist()
+    upstream = np.bincount(receiver[receiver >= 0], minlength=rows * cols).tolist()
+    stack = [i for i, n in enumerate(upstream) if n == 0]
+    while stack:
+        i = stack.pop()
+        j = down[i]
+        if j >= 0:
+            acc[j] += acc[i]
+            upstream[j] -= 1
+            if upstream[j] == 0:
+                stack.append(j)
+    return np.asarray(acc).reshape(rows, cols)
+
+
+def _grids():
+    rng = np.random.default_rng(617)
+    yy, xx = np.mgrid[0:60, 0:80]
+    flat = np.full((60, 80), 5.0)  # 넓은 평지(통째)
+    basin = flat.copy()
+    basin[10:50, 10:70] = 3.0  # 메운 웅덩이 = 넓은 평지
+    steps = np.floor(xx / 9.0) + np.floor(yy / 13.0)  # 계단(층마다 평지)
+    noisy = np.round(rng.normal(0, 1, (60, 80)) + yy * 0.05, 0)  # 같은 높이 많음
+    slope = 0.3 * xx + np.sin(yy / 5.0)
+    out = []
+    for z in (flat, basin, steps, noisy, slope):
+        out.append((z, np.ones(z.shape, bool)))
+        holes = rng.random(z.shape) > 0.1  # 빈 칸
+        holes[:, :3] = False  # 빈 띠 가장자리
+        out.append((z, holes))
+    one = np.zeros((1, 1))
+    out.append((one, np.ones((1, 1), bool)))
+    out.append((np.zeros((3, 5)), np.zeros((3, 5), bool)))  # 모두 빈 칸
+    return out
+
+
+@pytest.mark.parametrize("z,valid", _grids())
+def test_new_equals_old_synthetic(z, valid) -> None:
+    assert np.array_equal(zo.d8_accumulation(z, valid), _old_d8(z, valid))
+
+
+def _stored_tiles(limit: int = 10):
+    """열쇠가 지금 지형 파일과 맞는 창고 0차곡 도엽(고루 솎음)."""
+    found = sorted(store.TERRAIN_ROOT.glob("*/zero_acc_*_2m.key.json"))
+    step = max(len(found) // (limit * 3), 1)
+    for key_path in found[::step]:
+        key = json.loads(key_path.read_text(encoding="utf-8"))
+        if key.get("version") != zo.ZERO_ORDER_VERSION:
+            continue
+        epsg = int(key_path.name.split("_")[2])
+        dems = {}
+        for no, dem in key["dem"].items():
+            npz, dem_key, _ = store._names(no, epsg, zo.ZERO_ORDER_CELL_M)
+            try:
+                now = json.loads(dem_key.read_text(encoding="utf-8"))
+            except (OSError, ValueError):
+                break
+            if {k: v for k, v in now.items() if k != "seconds"} != dem or not npz.is_file():
+                break
+            dems[no] = npz
+        else:
+            yield key_path.parent.name, epsg, dems
+            limit -= 1
+            if limit == 0:
+                return
+
+
+def test_store_tiles_recompute_equal() -> None:
+    """창고 도엽 누적을 새 꼴로 다시 셈 = 저장본(읽기만 · 창고 안 씀)."""
+    tiles = list(_stored_tiles())
+    if not tiles:
+        pytest.skip("창고 0차곡 도엽 없음")
+    cell = zo.ZERO_ORDER_CELL_M
+    pad = math.ceil(zo.ZERO_ORDER_MARGIN_M / cell)
+    for no, epsg, dems in tiles:
+        with np.load(zo._acc_names(no, epsg, cell)[0]) as data:
+            want = np.asarray(data["acc"])
+            ix0, iy0 = int(data["ix0"]) - pad, int(data["iy0"]) - pad
+        rows, cols = want.shape[0] + 2 * pad, want.shape[1] + 2 * pad
+        z = np.full((rows, cols), np.nan, np.float32)
+        for npz in dems.values():  # load_terrain 이음과 같음(지형 다시 셈 길 없이)
+            with np.load(npz) as data:
+                px, py, pz = int(data["ix0"]), int(data["iy0"]), data["z"]
+            r0, c0 = max(py, iy0), max(px, ix0)
+            r1, c1 = min(py + pz.shape[0], iy0 + rows), min(px + pz.shape[1], ix0 + cols)
+            if r0 < r1 and c0 < c1:
+                src = pz[r0 - py : r1 - py, c0 - px : c1 - px]
+                np.copyto(z[r0 - iy0 : r1 - iy0, c0 - ix0 : c1 - ix0], src, where=np.isfinite(src))
+        acc = zo.d8_accumulation(z, np.isfinite(z))[pad:-pad, pad:-pad]
+        got = np.minimum(acc, zo.ACC_CAP).astype(np.uint16)
+        assert np.array_equal(got, want), no