diff --git a/src/ground_vehicle.hpp b/src/ground_vehicle.hpp
--- a/src/ground_vehicle.hpp
+++ b/src/ground_vehicle.hpp
@@ -134,7 +134,7 @@ struct GroundVehicle : public Specialize
 	 */
 	inline void UpdateZPositionAndInclination()
 	{
-		this->z_pos = GetSlopePixelZ(this->x_pos, this->y_pos);
+		this->z_pos = GetSlopePixelZ(this->x_pos, this->y_pos, true);
 		ClrBit(this->gv_flags, GVF_GOINGUP_BIT);
 		ClrBit(this->gv_flags, GVF_GOINGDOWN_BIT);
 
@@ -143,7 +143,7 @@ struct GroundVehicle : public Specialize
 			 * direction it is sloped, we get the 'z' at the center of
 			 * the tile (middle_z) and the edge of the tile (old_z),
 			 * which we then can compare. */
-			int middle_z = GetSlopePixelZ((this->x_pos & ~TILE_UNIT_MASK) | (TILE_SIZE / 2), (this->y_pos & ~TILE_UNIT_MASK) | (TILE_SIZE / 2));
+			int middle_z = GetSlopePixelZ((this->x_pos & ~TILE_UNIT_MASK) | (TILE_SIZE / 2), (this->y_pos & ~TILE_UNIT_MASK) | (TILE_SIZE / 2), true);
 
 			if (middle_z != this->z_pos) {
 				SetBit(this->gv_flags, (middle_z > this->z_pos) ? GVF_GOINGUP_BIT : GVF_GOINGDOWN_BIT);
@@ -200,7 +200,7 @@ struct GroundVehicle : public Specialize
 		if (HasBit(this->gv_flags, GVF_GOINGUP_BIT) || HasBit(this->gv_flags, GVF_GOINGDOWN_BIT)) {
 			if (T::From(this)->HasToUseGetSlopePixelZ()) {
 				/* In some cases, we have to use GetSlopePixelZ() */
-				this->z_pos = GetSlopePixelZ(this->x_pos, this->y_pos);
+				this->z_pos = GetSlopePixelZ(this->x_pos, this->y_pos, true);
 				return;
 			}
 			/* DirToDiagDir() is a simple right shift */
@@ -220,7 +220,7 @@ struct GroundVehicle : public Specialize
 			this->z_pos += HasBit(this->gv_flags, GVF_GOINGUP_BIT) ? d : -d;
 		}
 
-		assert(this->z_pos == GetSlopePixelZ(this->x_pos, this->y_pos));
+		assert(this->z_pos == GetSlopePixelZ(this->x_pos, this->y_pos, true));
 	}
 
 	/**