The Commissioners shall cause to be marked on the Map so procured or caused to be made by them a Series of Marks and Figures at convenient Distances on the said Map, denoting the Height of the Ground at every such Mark above or below the Level of a particular Spot within the Limits of the special Act, which may be easily found and identified, the Position of which Spot shall be described on the Map, and shall also cause to be drawn, wherever practicable, Lines of equal Altitude at every Four Feet of Elevation, or at such other Intervals as may appear, upon due Inquiry, to be the best adapted for the Guidance of Works of Sewerage and Drainage, for the Collection and Distribution of Water, and for other Purposes within such District for which a Knowledge of the Levels of the District may be necessary, and shall also cause proper Bench Marks for denoting the Levels to be inscribed and marked at convenient Distances and Places, at the Corners of Streets, on Posts, Houses, or other prominent Objects within such District.