Explain the Statements about the Differential Levelling
I. Bench mark (BM) - relatively permanent point of known elevation as indicated on the Contract drawings
II. Back sight (BS) - a sight taken to the level rod held at a point of known elevation (either BM or TP)
III. Height of Instrument (HI) - the elevation of the line of sight of the telescope
IV. Foresight (FS) - a sight taken on any point to determine its elevation