What Is Plane Surveying? How Plane
Surveying Works?
In the past, most survey work depended
on triangulation from known fixed points using a theodolite and this
may still be a suitable method for smaller sites. Again it is
necessary to ensure the instrument is in good condition and that its
base is truly horizontal.
Readings taken on both faces of the
instrument may reduce residual errors. Setting out by taping along a
line given by the theodolite may also still be the clearest way of
providing centre lines or points, particularly for regular structure
layouts such as building columns.
The appropriate time for this is when
blinding concrete has been placed to column and wall foundations. The
base line, which is either the centre line of the building, or a line
parallel to it but clear of the building, should have been set out
previously by end pegs sited well clear of the work.
It is usual to work from co-ordinates
along this base line from some fixed zero point, and measuring right
angle distances out from them. In this way lines of walls and column
centres can be marked on the blinding concrete.
Distances may be measured by steel or
fibreglass tape pulled horizontally, so it is a great convenience if
the site is level. If not a plumb bob has to be used to transfer
distances. Distance co-ordinates along the base line from the zero
peg are set out, using the steel tape and marking a pencil line
across the peg.
The theodolite is set out over the
pencil line, and its position is adjusted laterally so 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 theodolite right
angle settings from these to give centres for such column bases, etc.
The introduction of EDM equipment has,
however, meant that accurate distance 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 instrument 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 20 m
intervals between these should give sufficient coverage for accurate
plotting.
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