ABSTRACT
Figure 21.2: There are a variety of levelling instruments available and ranking high among these are the modified and improved versions of the popular traditional dumpy level. These instruments include the quickset, precise, and automatic levels. The automatic level represented above, operates by means of integral selflevelling glass prisms. Once the instrument has been attached to the tripod and levelled by means of a circular centre bubble, it is ready to be used. The principle
Figure 21.1: It is essential in all site-levelling operations that the various levels required should have reference to a fixed datum. Wherever possible this should be the Ordnance Bench Mark (OBM), which is a chiselled-out arrow head with a horizontal recess above it, as illustrated above. OBMs are carved in stone blocks, usually found set in the walls of public buildings and churches. The inverted horizontal centre line over the arrow is a
zontal plane, encompassing the whole site when the head of the instrument is pivoted through a 360 circle. This theoretical plane is referred to as the height of collimation, from which various levels can be measured and/or established. These levels are taken from a measuring rod known as a levelling staff.
