Civil Engineering Reference
In-Depth Information
the curve from the two foci. Peg the ends of the cord at the focal points and use
a marker held against the inside edge of the cord to traverse the required ellipse.
h is practical method of describing an ellipse could be used in setting out an oval
plan, and probably was so used.
However when closely examined it seems that most oval building plans are not
true ellipses but are combined multi-centred arcs—either 3 centred or 5 centred.
Such plans are easy to set out on the ground. h e centres are located to give the
desired form, and the arcs swung from them directly by lengths of cord as radii
of the required lengths. It should be noted that where the circumference changes
direction the centres must lie on the same straight line so that the change in direc-
tion will be continuous (h eorem: the radius is perpendicular to the tangent at
point of contact, so that here the radius will be perpendicular to both curves).
Orienta-
tion
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D.
Setting Out Concerns
1.
Orientation
It is perhaps best to regard orientation together with location as conditions
precedent to the setting out of buildings. Given the location of a building the
predetermining factor with setting out is its required orientation (if any), i.e. its
directional axis.
Generally speaking the exact orientation of individual buildings was not a prime
consideration. For most buildings, particularly domestic buildings, the orientation
may be preset by e.g. street frontage, or the desideratum is approximate only: e.g.
a commanding prospect, exposure to the winter sun, shelter from the prevailing
wind, etc. However it was not unusual during antiquity for religious monuments
to be oriented rigorously according to considerations other than topographical—i.e.
they were oriented exactly according to a “true” bearing (azimuth). h is require-
ment has two distinct expressions which may be termed respectively Astronomical
and Geographical.
(a)
Astronomical Orientation
Numbers of religious monuments, particularly those of early date (signii cantly
megalithic monuments) were designed in the primary instance so that they were axi-
ally oriented on some astronomical phenomenon, e.g. sunrise at midsummer's day
(the summer solstice) or at the spring equinox etc. Such an orientation demanded
only that the monument be built where the astronomical phenomenon (i.e. the
specii c sunrise) was visible. h e phenomenon was observed by the naked eye, and
a line towards this direction was marked out on the ground to constitute the axis
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