diff --git a/src/map_type.h b/src/map_type.h
--- a/src/map_type.h
+++ b/src/map_type.h
@@ -17,14 +17,14 @@
  * Look at docs/landscape.html for the exact meaning of the members.
  */
 struct Tile {
-	byte   type;        ///< The type (bits 4..7)
+	byte   type;        ///< The type (bits 4..7), bridges (2..3), rainforest/desert (0..1)
 	byte   height;      ///< The height of the northern corner.
 	byte   m1;          ///< Primarily used for ownership information
 	uint16 m2;          ///< Primarily used for indices to towns, industries and stations
 	byte   m3;          ///< General purpose
 	byte   m4;          ///< General purpose
 	byte   m5;          ///< General purpose
-	byte   m6;          ///< Primarily used for bridges and rainforest/desert
+	byte   m6;          ///< General purpose
 };
 
 /**