Civil Engineering Reference
In-Depth Information
All this means that if great accuracy is required in marking out a straight line
of exact length, the path of the line must be established by using a cord and the
length of the line by using an accurately calibrated measuring rule/rod. Since
these are inevitably short, then usually a succession of rods must be placed end
to end without any disturbance to their position during the operation. h is is, of
course, a delicate operation but the one method available of accurately measur-
ing out a length using simple devices. It should be noted here that the base line
for the Ordonnance Survey of England made in the 18th century was laid out on
Salisbury Plain by placing wooden measuring rods end to end in this manner.
(For a consideration of accurate measurement of length with cords or with rods v
J.J. Coulton,
ABSA
, 70, 1975, pp. 89-93.)
Laying out
angles
3.
Angle
h e lines of any designed building meet or intersect at specii c angles—generally
at right angles but on occasions otherwise. It is thus a basic requirement of set-
ting out that the designed angle is exactly marked out on the ground. In modern
times there have always been available precision “bearing” instruments so that any
horizontal angle can be laid of to an accuracy of, say, 1 minute of arc. However
the availability during antiquity of an instrument with a horizontal circle gradu-
ated in degrees of arc permitting horizontal angles to be recorded or laid of at any
point on the ground is a doubtful matter. Such an instrument (a dioptre) may have
existed in Roman times. Even so it is certain that such instruments were not avail-
able as a matter of course to those setting out the lines of projected buildings (i.e.
they may have been available for special “engineering surveys”). h is reproduces
the situation in modern times where a theodolite is not to be found normally on
building sites. In short, setting out buildings with exact angles as required by their
design of necessity made use of other (simple) methods.
h e angles incorporated in building design during antiquity were limited. In the
vast majority of instances 90°, but on occasions 45° / 135° (for an octagon), 60° and
30°. As has been noted all of these angles could be laid of by simple geometrical
construction using only pieces of cord of determined length. h e procedure was,
in ef ect, to bisect an existing angle. h e construction for erecting a perpendicular
is simply to bisect an angle of 180° (i.e. a straight line). h is is done by swinging
equal arcs from points equidistant from the apex of the angle, and joining the apex
to the point of intersection (the principle is to establish similar triangles). h is
procedure is so simple that it was understood from Neolithic times.
Proceeding beyond this it was observed that a triangle with sides 3, 4 & 5 units
of length was a right angled triangle (as also, fairly closely, was a triangle with
sides 5, 5 & 7 units). h us some device forming a triangle with sides in this propor-
tion provided a right angle without the requirement of geometrical construction.
22.3
22.1
22.2
