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several bench marks would normally be established by levelling from
the datum. If heights are based on an arbitrary datum these are known
as Temporary Bench Marks or TBMs.
Level (or horizontal) lines are always at right angles to the

direction of gravity. The direction of gravity is generally towards the
centre of the earth. Over large areas, as the Earth is curved, level
surfaces will also be curved. For these, a height difference is measured

along a vertical between two curved level surfaces.
One of the most popular ways to measure heights is using a level.
It is called a level because it gives a true horizontal line. The height of
each point is then measured by its vertical distance above or below

this horizontal line.

Figure 12.1. Optical level

The following steps are taken when using it:
1. Set up the tripod.

2. Ensure the top is level.
3. Push legs firmly into the ground.
4. Attach level.

5. Use foot screws to centralize the circular bubble.

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