Page 145 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
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that it transits accurately on the two outermost base line marks. The plumb bob
on the theodolite gives the mark for the co-ordinate point, a round headed nail
being inserted on this point. Distances at right angles to the base line are then set
out with theodolite and steel tape. The advantage of this method is that the
theodolite can sight down into column bases which are usually set deeper than
the general formation level. For the assistance of bricklayers and formwork
carpenters, sight boards can be provided, with the cross-arm fixed at a given level
above formation level and with saw cuts exactly on the lines of sight to be used.
A builder’s line can then be fixed through such saw cuts. An alternative to the
foregoing is to set out two base lines at right angles to each other and use theodo-
lite right angle settings from these to give centres for such column bases, etc.
The introduction of EDM equipment has, however, meant that accurate dis-
tance and angle measurements can now be made from a single point set up.
The instruments work by measuring the time of a wave in travelling from the
transmitter to a reflector and back. Readings may be automatically repeated
to improve accuracy. Built-in or add-on equipment allows for automatic data
logging, reduction of distances to horizontal and vertical components and for
downloading to a computer. Accuracy over short distances is good. Over longer
distances corrections may need to be made for atmospheric conditions which
vary from the manufacturers’ setting. The improved accuracy available has
meant that setting out on site or general survey work is often done by some
form of traversing. By this method the position of two known points is
extended by noting the angle to a third point and its distance from the instru-
ment set up over one of the points. Extended traverses should be closed onto
another known point to check for errors.
Even with EDM equipment, setting out of regular structures is probably best
done using a marked baseline as described above. The equipment also has major
advantages in ground surveying since the location and elevation of any point in
the area to be surveyed can usually be determined directly from just one or two
positions of the instrument. Data from the instrument can then be downloaded
into a computer and with the use of appropriate software, contoured plans of
the area can be produced for design or for earthworks measurement purposes.
Acertain amount of planning is necessary to produce the best results by ensuring
a regular grid of locations is used for targeting and that any individual feature,
such as sharp changes in slope are picked up. As an alternative ranging poles
can be used to set out a rough grid and readings at say 20m intervals between
these should give sufficient coverage for accurate plotting.

12.4 Setting out verticality, tunnels and pipelines

As a building rises the vertical alignment must also be controlled. This can be
done by extending building centre lines at right angles to each other out to
fixed points clear of the structure. These lines can then be projected up the
building and marked, allowing accurate measurements from these marks at