diff --git a/clear_cmd.c b/clear_cmd.c
index 7ee1f6ef8c..90b70e5682 100644
--- a/clear_cmd.c
+++ b/clear_cmd.c
@@ -549,7 +549,7 @@ static void DrawTile_Clear(TileInfo *ti)
 	DrawClearLandFence(ti);
 }
 
-static uint GetSlopeZ_Clear(TileInfo *ti)
+static uint GetSlopeZ_Clear(const TileInfo* ti)
 {
 	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
 }
diff --git a/dummy_land.c b/dummy_land.c
index 93d9303ac6..66be1c42ff 100644
--- a/dummy_land.c
+++ b/dummy_land.c
@@ -14,8 +14,9 @@ static void DrawTile_Dummy(TileInfo *ti)
 }
 
 
-static uint GetSlopeZ_Dummy(TileInfo *ti) {
-	return GetPartialZ(ti->x&0xF, ti->y&0xF, ti->tileh) + ti->z;
+static uint GetSlopeZ_Dummy(const TileInfo* ti)
+{
+	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
 }
 
 static uint GetSlopeTileh_Dummy(const TileInfo *ti) {
diff --git a/industry_cmd.c b/industry_cmd.c
index 0a228343ac..385c10ace8 100644
--- a/industry_cmd.c
+++ b/industry_cmd.c
@@ -398,8 +398,9 @@ static void DrawTile_Industry(TileInfo *ti)
 }
 
 
-static uint GetSlopeZ_Industry(TileInfo *ti) {
-	return GetPartialZ(ti->x&0xF, ti->y&0xF, ti->tileh) + ti->z;
+static uint GetSlopeZ_Industry(const TileInfo* ti)
+{
+	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
 }
 
 static uint GetSlopeTileh_Industry(const TileInfo *ti) {
diff --git a/openttd.h b/openttd.h
index 4e2119fbe1..ed67eff48c 100644
--- a/openttd.h
+++ b/openttd.h
@@ -292,7 +292,7 @@ typedef struct {
 typedef int32 CommandProc(int x, int y, uint32 flags, uint32 p1, uint32 p2);
 
 typedef void DrawTileProc(TileInfo *ti);
-typedef uint GetSlopeZProc(TileInfo *ti);
+typedef uint GetSlopeZProc(const TileInfo* ti);
 typedef int32 ClearTileProc(TileIndex tile, byte flags);
 typedef void GetAcceptedCargoProc(TileIndex tile, AcceptedCargo res);
 typedef void GetTileDescProc(TileIndex tile, TileDesc *td);
diff --git a/rail_cmd.c b/rail_cmd.c
index e185d85295..47f70c66f5 100644
--- a/rail_cmd.c
+++ b/rail_cmd.c
@@ -1934,7 +1934,7 @@ void SetSignalsOnBothDir(TileIndex tile, byte track)
 	UpdateSignalsOnSegment(tile, _search_dir_2[track]);
 }
 
-static uint GetSlopeZ_Track(TileInfo *ti)
+static uint GetSlopeZ_Track(const TileInfo* ti)
 {
 	uint z = ti->z;
 	int th = ti->tileh;
diff --git a/road_cmd.c b/road_cmd.c
index 295c08bf10..21bbad1907 100644
--- a/road_cmd.c
+++ b/road_cmd.c
@@ -932,7 +932,7 @@ void DrawRoadDepotSprite(int x, int y, int image)
 	}
 }
 
-static uint GetSlopeZ_Road(TileInfo *ti)
+static uint GetSlopeZ_Road(const TileInfo* ti)
 {
 	uint z = ti->z;
 	int th = ti->tileh;
diff --git a/station_cmd.c b/station_cmd.c
index 66698a0736..e73abce068 100644
--- a/station_cmd.c
+++ b/station_cmd.c
@@ -2225,7 +2225,7 @@ void StationPickerDrawSprite(int x, int y, RailType railtype, int image)
 	}
 }
 
-static uint GetSlopeZ_Station(TileInfo *ti)
+static uint GetSlopeZ_Station(const TileInfo* ti)
 {
 	uint z = ti->z;
 	if (ti->tileh != 0)
diff --git a/town_cmd.c b/town_cmd.c
index ebf2e87192..3db23ece71 100644
--- a/town_cmd.c
+++ b/town_cmd.c
@@ -143,9 +143,9 @@ static void DrawTile_Town(TileInfo *ti)
 	}
 }
 
-static uint GetSlopeZ_Town(TileInfo *ti)
+static uint GetSlopeZ_Town(const TileInfo* ti)
 {
-	uint z = GetPartialZ(ti->x&0xF, ti->y&0xF, ti->tileh) + ti->z;
+	uint z = GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
 	if (ti->tileh != 0) z = (z & ~7) + 4;
 	return (uint16) z;
 }
diff --git a/tree_cmd.c b/tree_cmd.c
index efc180ebcd..6623ac5012 100644
--- a/tree_cmd.c
+++ b/tree_cmd.c
@@ -339,7 +339,8 @@ static void DrawTile_Trees(TileInfo *ti)
 }
 
 
-static uint GetSlopeZ_Trees(TileInfo *ti) {
+static uint GetSlopeZ_Trees(const TileInfo* ti)
+{
 	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
 }
 
diff --git a/tunnelbridge_cmd.c b/tunnelbridge_cmd.c
index b5bcfe3d53..d5b1fea4d8 100644
--- a/tunnelbridge_cmd.c
+++ b/tunnelbridge_cmd.c
@@ -1215,10 +1215,12 @@ static void DrawTile_TunnelBridge(TileInfo *ti)
 	}
 }
 
-static uint GetSlopeZ_TunnelBridge(TileInfo *ti) {
+static uint GetSlopeZ_TunnelBridge(const TileInfo* ti)
+{
 	uint z = ti->z;
 	uint x = ti->x & 0xF;
 	uint y = ti->y & 0xF;
+	uint tileh = ti->tileh;
 
 	// swap directions if Y tunnel/bridge to let the code handle the X case only.
 	if (ti->map5 & 1) uintswap(x,y);
@@ -1233,11 +1235,11 @@ static uint GetSlopeZ_TunnelBridge(TileInfo *ti) {
 		if ( ti->map5 & 0x80 ) {
 			// bridge ending?
 			if (!(ti->map5 & 0x40)) {
-				if (BRIDGE_FULL_LEVELED_FOUNDATION & (1 << ti->tileh))	// 7, 11, 13, 14
+				if (BRIDGE_FULL_LEVELED_FOUNDATION & (1 << tileh)) // 7, 11, 13, 14
 					z += 8;
 
 				// no ramp for bridge ending
-				if ((BRIDGE_PARTLY_LEVELED_FOUNDATION & (1 << ti->tileh) || BRIDGE_NO_FOUNDATION & (1 << ti->tileh)) && ti->tileh != 0) {
+				if ((BRIDGE_PARTLY_LEVELED_FOUNDATION & (1 << tileh) || BRIDGE_NO_FOUNDATION & (1 << tileh)) && tileh != 0) {
 					return z + 8;
 
 				} else if (!(ti->map5 & 0x20)) { // northern / southern ending
@@ -1251,7 +1253,7 @@ static uint GetSlopeZ_TunnelBridge(TileInfo *ti) {
 			// bridge middle part
 			} else {
 				// build on slopes?
-				if (ti->tileh) z+=8;
+				if (tileh != 0) z += 8;
 
 				// keep the same elevation because we're on the bridge?
 				if (_get_z_hint >= z + 8)
@@ -1264,12 +1266,12 @@ static uint GetSlopeZ_TunnelBridge(TileInfo *ti) {
 				// in the shared area, assume that we're below the bridge, cause otherwise the hint would've caught it.
 				// if rail or road below then it means it's possibly build on slope below the bridge.
 				if (ti->map5 & 0x20) {
-					uint f = _bridge_foundations[ti->map5&1][ti->tileh];
+					uint f = _bridge_foundations[ti->map5 & 1][tileh];
 					// make sure that the slope is not inclined foundation
 					if (IS_BYTE_INSIDE(f, 1, 15)) return z;
 
 					// change foundation type? XXX - should be const; accessor function!
-					if (f) ti->tileh = _inclined_tileh[f - 15];
+					if (f != 0) tileh = _inclined_tileh[f - 15];
 				}
 
 				// no transport route, fallback to default
@@ -1279,16 +1281,16 @@ static uint GetSlopeZ_TunnelBridge(TileInfo *ti) {
 		// if it's a bridge middle with transport route below, then we need to compensate for build on slopes
 		if ( (ti->map5 & (0x80 + 0x40 + 0x20)) == (0x80 + 0x40 + 0x20)) {
 			uint f;
-			if (ti->tileh) z += 8;
-			f = _bridge_foundations[ti->map5&1][ti->tileh];
+			if (tileh != 0) z += 8;
+			f = _bridge_foundations[ti->map5&1][tileh];
 			if (IS_BYTE_INSIDE(f, 1, 15)) return z;
-			if (f) ti->tileh = _inclined_tileh[f - 15];
+			if (f != 0) tileh = _inclined_tileh[f - 15];
 		}
 	}
 
 	// default case
 	z = ti->z;
-	return GetPartialZ(ti->x&0xF, ti->y&0xF, ti->tileh) + z;
+	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, tileh) + z;
 }
 
 static uint GetSlopeTileh_TunnelBridge(const TileInfo *ti) {
diff --git a/unmovable_cmd.c b/unmovable_cmd.c
index 505abd72c4..9b2f2e3d2c 100644
--- a/unmovable_cmd.c
+++ b/unmovable_cmd.c
@@ -180,9 +180,9 @@ static void DrawTile_Unmovable(TileInfo *ti)
 	}
 }
 
-static uint GetSlopeZ_Unmovable(TileInfo *ti)
+static uint GetSlopeZ_Unmovable(const TileInfo* ti)
 {
-	return GetPartialZ(ti->x&0xF, ti->y&0xF, ti->tileh) + ti->z;
+	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
 }
 
 static uint GetSlopeTileh_Unmovable(const TileInfo *ti)
diff --git a/water_cmd.c b/water_cmd.c
index 0a0076c1cb..a74807fba4 100644
--- a/water_cmd.c
+++ b/water_cmd.c
@@ -460,9 +460,9 @@ void DrawShipDepotSprite(int x, int y, int image)
 }
 
 
-static uint GetSlopeZ_Water(TileInfo *ti)
+static uint GetSlopeZ_Water(const TileInfo* ti)
 {
-	return GetPartialZ(ti->x&0xF, ti->y&0xF, ti->tileh) + ti->z;
+	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
 }
 
 static uint GetSlopeTileh_Water(const TileInfo *ti)