This glossary of common surveying terms can be accessed either directly from the main selection screen, or by selecting one of the highlighted words in italics found throughout the various bodies of text.
Backsight
In levelling, the first
reading taken on a staff after setting up the Level. A backisight is
always taken on a point of known or assumed height.
Benchmark
A reference point or mark of known height. An OBM, ordinance bench
mark, is a known height (can be found on an OS map) marked onto a
building or other permanent structure. A TBM, temporary benchmark, is a
point of assumed height on a site, to which all levels taken on that
sight are relative.
Catenary
The shape of curve that a tape suspended at each end adopts. The
curve adds to the measured lenght and so catenary (or sag) correction is
applied.
Change Point
In levelling, a point at which two readings are taken on the
staff, one before moving the instrument to a new position, and one after
setting the instrument on the new position.
Coefficient of Expansion of Steel
A
factor applied to steel to quantify the effects of temperature on its
expansion. Its value is 0.000012 / 'c.
Constant Tension Handle
A device which, by using roller grips, will allow a tape to be
pulled to a specified tension. Ideally, this is the same as the
standard tension of the tape, so that no tension correction will be
necessary.
Elevation
A measure ment in height relative to some benchmark.
Engineering Plans
The scale of such plans is usually much greater than in other
surveying. Scales from 1:500 to 1:50 are often used. As well as plans,
cross sections and longitudinal sections are also produced - often with
the vertical scale distorted.
Focusing
The adjustment of an optical instrument to allow the user to see
the image clearly. A wide range is required since the instrument will
be used at many different ranges, different users will have different
focusing depths.
Foresight
In levelling, the last reading to be taken from one setting of the
instrument. A foresight must be taken on a fixed point, which then
becomes a benchmark.
Face Left
Position of a theodolite in which the vertical circle is to the
left of the user, and all of the main controls are on the right.
Face Right
Position of a theodolite in both the vertical and horizontal planes
have been rotated 180 degrees, so that the controls are onm the left,
and the horizontal circle is on the right. See face left, above.
Height of Plane of Collimation (HPC)
The height of the central reticle of a level. It is given by
taking a known or assumed benchmark, placing a staff on it, reading the
staff, and adding the resulting reading to the benchmark value.
Sighting from here to a staff held on any other point can give the
height of the new point, by subtracting the new staff reading from the
HPC.
Intermediate Sight
In levelling, a sight which is neither a backsight or a foresight.
Parallax
The change in apparent position of one object with respect to a
further one. A source of possible error in many kinds of observation.
Point
A feature of interest. Where no permanent feature exists, usual
practice for short term use is to securely hammer a soft wood peg 50mm
square into the ground. A small nail is then hammered into the peg, and
forms the feature to which all sighting and measurement takes place.
Reference Object (RO)
A choice of sighting point for Theodolite sighting. It should be
selected as the most reliable, and preferably most distant, of all the
points to be sighted. A recorded angle for a given point is hence the
angle from the RO to that point.
Steel Tape
Usually a 20, 30 or 100 metre length of flexible steel, graduated
in mm. They are usually contained within a plastic or metal box with a
winding handle, for ease of use. Their calibrations are designed to be
accurate at a given temperature and tension, which can usually be found
printed on the first metre of tape.
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