diff --git a/src/viewport_type.h b/src/viewport_type.h
--- a/src/viewport_type.h
+++ b/src/viewport_type.h
@@ -85,28 +85,28 @@ DECLARE_ENUM_AS_BIT_SET(ViewportPlaceMet
 /** Drag and drop selection process, or, what to do with an area of land when
  * you've selected it. */
 enum ViewportDragDropSelectionProcess {
-	DDSP_DEMOLISH_AREA,
-	DDSP_RAISE_AND_LEVEL_AREA,
-	DDSP_LOWER_AND_LEVEL_AREA,
-	DDSP_LEVEL_AREA,
-	DDSP_CREATE_DESERT,
-	DDSP_CREATE_ROCKS,
-	DDSP_CREATE_WATER,
-	DDSP_CREATE_RIVER,
-	DDSP_PLANT_TREES,
-	DDSP_BUILD_BRIDGE,
+	DDSP_DEMOLISH_AREA,        ///< Clear area
+	DDSP_RAISE_AND_LEVEL_AREA, ///< Raise / level area
+	DDSP_LOWER_AND_LEVEL_AREA, ///< Lower / level area
+	DDSP_LEVEL_AREA,           ///< Level area
+	DDSP_CREATE_DESERT,        ///< Fill area with desert
+	DDSP_CREATE_ROCKS,         ///< Fill area with rocks
+	DDSP_CREATE_WATER,         ///< Create a canal
+	DDSP_CREATE_RIVER,         ///< Create rivers
+	DDSP_PLANT_TREES,          ///< Plant trees
+	DDSP_BUILD_BRIDGE,         ///< Bridge placement
 
 	/* Rail specific actions */
-	DDSP_PLACE_RAIL,
-	DDSP_BUILD_SIGNALS,
-	DDSP_BUILD_STATION,
-	DDSP_REMOVE_STATION,
-	DDSP_CONVERT_RAIL,
+	DDSP_PLACE_RAIL,           ///< Rail placement
+	DDSP_BUILD_SIGNALS,        ///< Signal placement
+	DDSP_BUILD_STATION,        ///< Station placement
+	DDSP_REMOVE_STATION,       ///< Station removal
+	DDSP_CONVERT_RAIL,         ///< Rail conversion
 
 	/* Road specific actions */
-	DDSP_PLACE_ROAD_X_DIR,
-	DDSP_PLACE_ROAD_Y_DIR,
-	DDSP_PLACE_AUTOROAD,
+	DDSP_PLACE_ROAD_X_DIR,     ///< Road placement (X axis)
+	DDSP_PLACE_ROAD_Y_DIR,     ///< Road placement (Y axis)
+	DDSP_PLACE_AUTOROAD,       ///< Road placement (auto)
 };
 
 #endif /* VIEWPORT_TYPE_H */