diff --git a/src/newgrf_commons.cpp b/src/newgrf_commons.cpp
--- a/src/newgrf_commons.cpp
+++ b/src/newgrf_commons.cpp
@@ -447,7 +447,7 @@ uint32 GetNearbyTileInformation(TileInde
 	/* Fake tile type for trees on shore */
 	if (IsTileType(tile, MP_TREES) && GetTreeGround(tile) == TREE_GROUND_SHORE) tile_type = MP_WATER;
 
-	uint z;
+	int z;
 	Slope tileh = GetTilePixelSlope(tile, &z);
 	/* Return 0 if the tile is a land tile */
 	byte terrain_type = (HasTileWaterClass(tile) ? (GetWaterClass(tile) + 1) & 3 : 0) << 5 | GetTerrainType(tile) << 2 | (tile_type == MP_WATER ? 1 : 0) << 1;