Global Positioning System Reference
In-Depth Information
FIGURE 3.3.
A groma. A vertical rod holds two perpendicular horizontal rods, offset as
shown. From the end of these are suspended four plumb bobs. The main rod is set
vertically in the ground, enabling the surveyor to sight along horizontal lines at right
angles.
laying out a military camp or a building. One stunning example of Roman
construction that survives to the present day is the Pont du Gard aqueduct
in France, a World Heritage site that was constructed around the year
19 BCE. The distance from Roman Ucetia to Nemausus (modern Uzès to
Nîmes) was 50 km, and the drop in elevation of the aqueduct was only
17 m, corresponding to a mean gradient of
1
⁄
3000
. Thus, the Romans must
have accurately measured di√erences in elevation to within a minute of
arc. An aqueduct is unforgiving of errors in layout or construction: water
will not flow uphill, should the surveyors make a mistake or the founda-
tions settle. Clearly, a great deal of accurate surveying must have been
performed prior to constructing this aqueduct, yet the Romans used only
the groma and water level. Several such aqueducts exist across Europe
(fig. 3.4); they are as much a testament to Roman surveying skills as to
their engineering prowess.
3
ALIDADE
An
alidade
is an instrument so basic that you may be surprised to learn that
it has a name. At its simplest, an alidade is a straight rod such as a ruler,
with a raised sight at each end—like a rifle gunsight. The ancient Greeks
wrote about it in the third century BCE, calling it a
dioptra
. Our name
3. The Pont du Gard aqueduct has for centuries been held up as an icon of precision
engineering. See Holmes (2010, pp. 354-55).
