diff --git a/book/chapters/dem.ipynb b/book/chapters/dem.ipynb
index 562240c..0ff64a8 100644
--- a/book/chapters/dem.ipynb
+++ b/book/chapters/dem.ipynb
@@ -248,7 +248,6 @@
   },
   {
    "cell_type": "markdown",
-   "id": "55ffbb9d",
    "metadata": {},
    "source": [
     "Extract various terrain features from the DEM, such as slope, aspect, curvature, northness, and eastness, and save the results."
@@ -444,7 +443,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 7,
    "id": "5c04ab81",
    "metadata": {},
    "outputs": [],
@@ -453,8 +452,8 @@
     "    with rasterio.open(dem_file) as dataset:\n",
     "        dem_data = dataset.read(1)\n",
     "        dy, dx = np.gradient(dem_data, dataset.res[0], dataset.res[1])\n",
-    "        northness = np.arctan2(dy , np.sqrt(dx**2 + dy**2))\n",
-    "        eastness = np.arctan2(dx , np.sqrt(dx**2 + dy**2))\n",
+    "        northness = np.arctan(dy / np.sqrt(dx**2 + dy**2))\n",
+    "        eastness = np.arctan(dx / np.sqrt(dx**2 + dy**2))\n",
     "    return northness, eastness\n"
    ]
   },