diff --git a/dem.py b/dem.py
index 19a636f..36a6c26 100644
--- a/dem.py
+++ b/dem.py
@@ -275,7 +275,7 @@ class Dem:
         side_width = toml_dict["parameter"]["side_width"]  # 边线宽度
         _line_angel = math.atan((point_y_e - point_y_s) / (point_x_e - point_x_s))
         if point_x_e < point_x_s:
-            line_angel = _line_angel + math.pi / 2
+            line_angel = _line_angel + math.pi
         else:
             line_angel = _line_angel
         left_offset_x = side_width * math.cos(line_angel + math.pi / 2)