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Levelling

Levelling measures height differences with an instrument that gives a horizontal line of sight. You read a staff on a known point (backsight) and on the new point (foresight). The difference between the readings is the height difference.

\Delta h = B - F

height difference from back and fore readings

H_{new} = H_{known} + B - F

height of the new point

Symbols

$B$	backsight (known point)	m
$F$	foresight (new point)	m
$H$	height above sea level	m

Example

Known point 45.200 m, $B = 1.520$ and $F = 0.840$ :

$H = 45.200 + 1.520 - 0.840 = 45.880$ m.

A large reading means the staff stands low, because the line of sight hits higher up the staff.

Practise surveying for free →

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Part of Building and Structures: Surveying.