diff --git a/src/heightmap.cpp b/src/heightmap.cpp
--- a/src/heightmap.cpp
+++ b/src/heightmap.cpp
@@ -352,7 +352,7 @@ static void GrayscaleToMapHeights(uint i
  * This function takes care of the fact that land in OpenTTD can never differ
  * more than 1 in height
  */
-static void FixSlopes()
+void FixSlopes()
 {
 	uint width, height;
 	int row, col;