Global Positioning System Reference
In-Depth Information
hairs, and these were positioned a fixed angular distance apart, such as
0.01 radians. The stadia rods were marked prominently in such a way that,
when a rod was fixed into the ground some distance away from the tele-
scope, the length of the rod that lay between the stadia hairs could be
estimated—the idea is shown in figure 3.14—and the distance to the rod
could be calculated.
Meet the Theodolites
Land surveying is both easier and more di≈cult than surveying or navigat-
ing at sea. It is easier because the surveyor does not have to deal with a
pitching and rolling deck, and because he can use larger and more sophisti-
cated instruments. It is more di≈cult because he is asked to do more: he
must measure in three dimensions—azimuth (horizontal angle), elevation
(vertical angle), and distance—and usually without a horizon to help him.
Yet such was the inclination (now that's almost a pun) to survey tracts of
land with precision, that the development of surveying instruments led,
even before the electronic age, to accuracies that were astounding.
In essence, a theodolite consists of a pair of perpendicular sextants on a
tripod, from which a plumb line is suspended to the survey point. Consider
a tripod that provides a horizontal, circular platform around which a sex-
tant can rotate, measuring azimuth angles. A second sextant is fixed atop
this platform and is free to rotate in the vertical direction to measure
elevation angles. The theodolite shown in figure 3.15 is a 300-pound mon-
ster that dates back to 1836, when it was the pride and joy of the United
States Coast Survey.
9
Its base circle has a diameter of 30 inches; this
dimension was used in those days to describe a theodolite because size was
an indication of accuracy. The theodolite of figure 3.15 produced an error
of one part in 100,000.
The theodolite concept is quite old, belonging to the sixteenth century.
As technology improved, so did theodolites, which continued to progress
until the end of the twentieth century, when they began to give way to
9. This organization was inaugurated by President Thomas Je√erson in the first decade
of the nineteenth century to perform an accurate survey of the U.S. east coast, though it
took a decade or two to get going. In the United States and other countries at that time,
coastal surveys were carried out mainly to decrease the number of shipwrecks. Je√erson was
very interested in surveying—he commissioned the Lewis and Clark expedition, one pur-
pose of which was geodetic surveying—and he owned a theodolite that had been made in
England by Jesse Ramsden, of dividing engine fame.
