Levelling
This booklet is written by
Mr. 0. Trutman and is offered with the
compliments of Wild Heerbrugg Ltd.,
Heerbrugg, Switzerland, optical
precision instrument makers.
Summary
1. Levelling page 4 (
2. Levels page 6 ~
3. Preparing for Use page 14
4. The Technique of Levelling page 18
5. Line Levelling page 20
6. Area Levelling ~ page 32
7!Conclusion page 43
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1. Levelling
Levelling is the process of determining the difference in elevation between two or
more points. The accuracy with which the measurements are made depends on the
scope and purpose of the survey and the means available.
The horizontal straight-edge is probably the simplest means by which differences
in elevation may be determined. A wooden straight-edge of say 3 m in length is set
horizontal with a spirit level, so that the vertical distance between the straight-edge
and the ground can be read with a levelling staff (fig. 1).
Levelling staff
Fig.1 wo,"
The horizontal straight-edge is used principally for the survey of cross-sections,
and if the distance across is greater than the length of the straight-edge then the
work must be done in stages, i.e. the straight-edge is set-up step by step (fig. 2).
For determining the difference in elevation between points that are far apart, the
straight-edge method is, however, inconvenient and inaccurate. If, for example, in
the cross-section in fig. 2 the difference in height between points B and C only is
required, it could be calculated from the differential readings (-0.80-1.15-1.50
+ 1.00+ 0.40 = -2.05 m), but a 3 m straight-edge would have to be set up 5 times.
An attempt could be made to shorten this procedure by setting the straight-edge
horizontal at point B and sighting along its upper face in an endeavour to take a
reading on a levelling staff at point C as indicated by the dotted line in the illustration.
Better results would be achieved by using a simple, non-optical sighting device
(similar in effect to open rifle sights), which could be set horizontal by means of a
spirit level. Such a device could be mounted on a tripod or held in the hand. In the
latter event the instrument must be so constructed that the spirit level can be
observed while making the pointing.
Even these methods are inaccurate, however, and are limited to distances which
allow the staff reading to be made with the unaided eye.
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