92 POSITION-FIXING
data-logger (plate 11.1). The data is stored on an electronic
medium, or downloaded onto a PC, or indeed both.
In essence a theodolite is a telescope that can be
swivelled both horizontally and vertically against fixed
scales for measuring the angle. The instrument is levelled,
using spirit bubbles, so that the scales are precisely
aligned in relation to the earth’s magnetic field. Within
the telescope there is a graticule along which sights are
taken to the objects to be surveyed. In the case of the total
station, a staff with a reflector for the electronic distance
measurement is used to survey the relevant points. Like
many electronic surveying systems, EDMs rely on the near-
constant speed of radiated electromagnetic energy close
to the visible end of the electromagnetic spectrum, com-
monly the infra-red wavelength. The time taken for a pulse
of energy to reach a target and return to the instrument
is measured and, using the known velocity of the energy,
the distance is measured. With the combination of mea-
suring angles and distances, several methods of determining
position can be used. The use of a theodolite or total
station requires specialist training but once mastered it
is a simple and effective tool that can be used in many
different situations. There are many texts available des-
cribing survey methods using the theodolite: Surveying
for Archaeologistsby F. Bettes is a good starting point.
Theodolites and total stations are used widely in land
surveying and terrestrial archaeology for mapping survey
detail on sites. Total stations have been used to measure
boats and hulks on the beach and accurately to posi-
tion survey datums for sites in the inter-tidal zone (figure
11.11). Millimetric accuracy is possible with these systems.
Angles and distances can be directly plotted on paper
or, in the case of the total station, the instrument can be
set to a local or the national grid. Theodolites are rela-
tively cheap to hire and are readily available as they are
used widely in the construction industry. The accuracy of
these instruments is high. This is because a series of mea-
surements is taken for each fix over a short period of time,
usually between about 0.5 and 5 seconds depending on
the accuracy required. A statistical average is then con-
verted into a distance with an accuracy in the order of about
5 mm over an approximate maximum range of 4 km.Electronic position-fixing and GPS
With the development of the offshore oil industry in
the 1970s, electronic position-fixing (EPF) systems were
developed to give adequate positional accuracies for sur-
vey and rig, and vessel and pipeline positioning. Some of
these navigation systems, such as Decca Navigator andFigure 11.11 Surveying a submerged site in shallow water using a shore-based EDM. (Based on original artwork by Ben
Ferrari; after Morrison, 1985, fig. 5.2)