diff --git a/src/terraform_gui.cpp b/src/terraform_gui.cpp
--- a/src/terraform_gui.cpp
+++ b/src/terraform_gui.cpp
@@ -649,8 +649,8 @@ struct ScenarioEditorLandscapeGeneration
 	{
 		if (widget != ETTW_DOTS) return;
 
-		int center_x = (r.left + r.right + 1) / 2;
-		int center_y = (r.top + r.bottom + 1) / 2;
+		int center_x = RoundDiv(r.left + r.right, 2);
+		int center_y = RoundDiv(r.top + r.bottom, 2);
 
 		int n = _terraform_size * _terraform_size;
 		const int8 *coords = &_multi_terraform_coords[0][0];