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In contrast to line levelling, in area levelling all new points are measured from a single instrument position.
They can be entered in the form as intermediate sights (i). A benchmark with a known elevation serves as a reference, which must be entered as a backsight.
To check whether the height of the levelling device was accidentally changed during the measurement of the new points or whether the benchmark was read off incorrectly or inaccurately, the benchmark can be measured again at the end of the measurement (this time as a foresight).
Example 3 shows a corresponding survey. A misclosure of 1 mm was detected here.
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