Optional heading angle referenced by true North, clockwise specifying the direction of travel. The heading direction may help the routing algorithm to select the best direction, for example, when multiple directions are possible at a road junction. North is 0 degrees, East is 90 degrees, South is 180 degrees, and West is 270 degrees. The value must be in the range [0, 360] when specified. By default, or when null is set, heading is ignored for route calculation.