Global Positioning System Reference
In-Depth Information
satellite positioning technology. Modern theodolites incorporate electro-
optics, plus digital data storage and display. Some modern instruments
incorporate electronic distance-measuring devices, typically infrared laser
rangefinders that can ascertain the distance to a target up to 20 km away, to
within a few millimeters. The combination of an electro-optic theodolite
and a rangefinder is termed a
total station
. (For a 1950s theodolite and a
total station, see fig. 3.16.) Perhaps surprisingly, the best theodolites are
slightly more accurate than the best total stations (at the time of publica-
tion), and total stations are more accurate than the GPS system. It is a
matter of ease and speed of use. A total station can measure angles and
distances very quickly and then download the data into a database to be
integrated with other measurements. It can be operated remotely and can
interface with other systems such as GPS. And with an error of two parts
per million (up to a range of 1,500 m, where the angular error is equivalent
to 3 mm), we have reached an accuracy that is plenty good enough for most
FIGURE 3.15.
''The Great
Theodolite.'' This heavy-
weight instrument was state
of the art in 1836 when it was
acquired by the U.S. Coast
Survey, headed up by Ferdi-
nand Hassler. Hassler de-
signed the theodolite him-
self and had it manufactured
in England. The Great Theo-
dolite was used constantly
between 1836 and 1873,
when it was damaged be-
yond repair in Georgia dur-
ing a tornado.
Image courtesy
of the National Ocean and At-
mospheric Administration. This
organization can trace its roots
back to the Coast Survey.
