Civil Engineering Reference
In-Depth Information
by way of a straight forward recital of facts—i.e. this proceedure was used in
Mesoptamia, that in Egypt and another in Greece etc. h e best that can be done
is to raise logical questions and adduce such information bearing on them as can
be found, whatever the context.
First it is advisable to make a preliminary observation. Almost inevitably setting
out is connected in terms with surveying. And there may be a connection in fact,
since on occasion it is necessary to survey an area of land in order to position the
building to be set out e.g. to tie it into some sort of town plan (R. Taylor,
Roman
Builders
, pp. 62-63). However the two operations are essentially distinct. Survey-
ing is operative over a far greater area than setting out, and partly for that reason
may use dif erent proceedures (and dif erent instruments!).
Since surveying is thus only marginally connected with setting out, it is not in
place here to treat substantively ancient surveying and topography, which is an
extended subject. Only a few remarks will be made to relate surveying to setting
out. Surveying consists of i xing the position of various points by accurate mea-
surement in relation to the position of some other points already determined. To
i x any position it is necessary to ascertain two such measurements concerning it.
h ese measurements may be either distances or bearings (angles) or one bearing
and one distance. On the whole it is a simpler matter to measure distances than
to measure bearings, and it is probably fair to say that early surveying proceeded
by way of measured distances—which is called chaining in traditional modern sur-
veying. And it may well be that very little use was made of angular measurement
until Roman times. Two distances can be measured from two points, one at each
end of a base line of known length and the distances can be expressed as the radii
of two arcs swung one from each end of the base line. h e intersection of the two
arcs subtended by these radii then gives the position of the point surveyed. h is is
called triangulation. h e alternative is to drop perpendiculars from each point to
a base line (= of sets), and to measure the distance intercepted along the base line
together with the length of the of set. h is is the method more proper to chaining.
Using either of these proceedures or a combination of them, any feature on the
ground can be accurately recorded without recourse to measuring angles.
A New Kingdom mural decoration of a h eban tomb (v Arnold, p. 252, i g 6.2)
shows a surveying party at work in cultivated i elds. h ey are using a calibrated
rope to lay down a line, and measure distances along it. In essentials the scene
illustrates the proceeding of any chain survey. On the other hand, whereas from
earliest times men were familiar with angular measure, there is no record of men
conducting surveys by measuring angles before Roman times.
With the expansion of the Roman Empire a great increase supervened in the
requirements for land surveying. h is was necessary to provide for communica-
tions, services (viaducts, aquaducts etc.) and above all for land settlement and
Survey-
ing &
Setting
out
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