diff --git a/src/tile_cmd.h b/src/tile_cmd.h
--- a/src/tile_cmd.h
+++ b/src/tile_cmd.h
@@ -70,7 +70,7 @@ struct TileDesc {
  * @param ti Information about the tile to draw
  */
 typedef void DrawTileProc(TileInfo *ti);
-typedef uint GetSlopeZProc(TileIndex tile, uint x, uint y);
+typedef int GetSlopeZProc(TileIndex tile, uint x, uint y);
 typedef CommandCost ClearTileProc(TileIndex tile, DoCommandFlag flags);
 
 /**
@@ -133,7 +133,7 @@ typedef Foundation GetFoundationProc(Til
  * @param tileh_new Slope after terraforming.
  * @return Error code or extra cost for terraforming (like clearing land, building foundations, etc., but not the terraforming itself.)
  */
-typedef CommandCost TerraformTileProc(TileIndex tile, DoCommandFlag flags, uint z_new, Slope tileh_new);
+typedef CommandCost TerraformTileProc(TileIndex tile, DoCommandFlag flags, int z_new, Slope tileh_new);
 
 /**
  * Set of callback functions for performing tile operations of a given tile type.