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Measured Survey

- recording should be the first step in any conservation project, but despite representing one of the smallest investments in time and money, it seldom is. Throughout Victorian times, most drawings were produced to display an intention, while modern surveys are generally only "fit for purpose" which usually amounts to the obtaining of planning consents. Measured survey is however a long standing skill, despite the fact that Vitruvius did not provide drawings, the Romans actually had a building surveying profession . The basic measuring tool for land surveying until the change to metrication was the surveyors chain or "Gunterís chain" after Edmund Gunter (1581-1626). 66 feet or 22 yards it is the length of a cricket pitch. One chain square is one - tenth of an acre. The chain has 100 links and every ten links there is a brass tally.

The control of hand survey is usually achieved by measured lengths across and around rooms or areas, which are linked together by a traverse of fixed points. Heights are fixed by using a level datum line which is transferred to to the elevation or section being measured. Vaults and other overhead features require the use of a plumb line.

A large array of very sophisticated techniques and expensive instruments are now available, but all perform the same function as the traditional hand survey. They are more accurate, particularly over a wider area, and are also faster, but, the person undertaking a hand survey will learn far more about the building he is working on.

Glossary
Autoplumb - an optical instrument which basically does the work of a plumb bob, but eliminates problems of having to spend time at height, and, of wind sway.
Dumpy level - a survey instrument consisting of a telescope and spirit level which, used with a measuring staff, allows differences in height to be recorded.
Electronic distance measurer- (EDM)- a gadget which sends out an infra red beam to a reflector which returns it to a receiver. It measures distance very accurately. Reflectorless EDM's are now available. These reflect from any surface, but have less range, and, are slightly less accurate.
Optical square - a double prism used for setting out survey lines at right angles. It allows you to view an object straight ahead and another perpendicular to it.
Photogrammetry - a technique designed to overcome the problems of displacement caused by camera tilt, variations on the surface of the subject being photographed etc, in which stereo photography is used (stereo plotted) to provide a three dimensional image of the subject. In basic terms, it can be likened to eyesight, two eyes - one image.
Plane table - essentially a portable drawing table used in conjunction with a mounted sighting device and straight edge, which allows accurate setting out drawings to be made on sight.
Primary measured survey - (PMS)- an overall framework into which other information can be fitted.
Rectified photography - a cheaper but less accurate process than photogrammetry, where photos are taken of a facade to produce an accurate image. Only works properly when the facade is flat, and the camera is held exactly parallel to it. This is seldom achievable, and usually the process involves fitting a number of photos into an outline of the subject.
Theodolite - a precision instrument for measuring angles to vertical and horizontal planes. Consists in its most basic form, of a telescope which can rotate horizontally and vertically allowing readings to be taken fro m a calibrated circle. The instrument has to be centred over a fixed (control) point.
Traversing - a series of control points linked by lines known as legs, the length of which and the angle between them is accurately recorded.
Triangulation - using a network of triangles to accurately plot positions.
Total station - a combined theodolight and EDM which allows horizontal and vertical angles, and, distances to be recorded.