Levelling is the most widely used method of obtaining the elevations of ground points relative to a reference datum and is usually carried out as a separate procedure to those used in fixing planimetric position.
The basic concept of levelling involves the measurement of vertical distance relative to a horizontal line of sight. Hence it requires a graduated staff for the vertical measurements and an instrument that will provide a horizontal line of sight.
A level line or level surface is one which at all points is normal to the direction of the force of gravity as defined by a freely suspended plumb-bob. As already indicated in Chapter 1 during the discussion of the geoid, such surfaces are ellipsoidal in shape. Thus in Figure 2.1 the difference in level between A and B is the distance A'B.
A horizontal line or surface is one which is normal to the direction of the force of gravity at a particular point. Figure 2.1 shows a horizontal line through point C.
A datum is any reference surface to which the elevations of points are referred. The most commonly used datum is that of mean sea level (MSL). In the UK the MSL datum was fixed by the Ordnance Survey (OS) of Great Britain, and hence it is often referred to as Ordnance Datum (OD). It is the mean level of the sea at Newlyn in Cornwall calculated from hourly readings of the sea level, taken by an automatic tide gauge over a six-year period from 1 May 1915 to 30 April 1921.
Bench mark (BM)
In order to make OD accessible to all users throughout the country, a series of permanent marks were established, called bench marks. The height of these marks relative to OD has been established by differential levelling and is regularly checked for any change in elevation.