Common Surveying Terms

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.

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.

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.

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.

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.

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.

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.

The change in apparent position of one object with respect to a further one. A source of possible error in many kinds of observation.

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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