imations of the earth’s shape are known as spheroids (or

ellipsoids). Different national and international mapping

systems use different spheroids that best fit the area to be

mapped. In the UK, the National Grid is based on an ellips-

oid defined by Mr George Biddell Airy, the Astronomer

Royal, in 1840, while the American satellite-navigation

network known as the Global Positioning System (GPS –

see below) calculates position on the World Geodetic

System 1984 (WGS84) which was defined by the US

military specifically for satellite positioning. There are

many spheroids, each developed for a specific purpose

by a different sovereign state. India, for example, was

mapped by the British in the early nineteenth century on

the Everest spheroid. Devised specifically for the task,

this spheroid was named after Everest, the principal

surveyor at the completion of the survey, and is still in

use today.

It is essential to be aware of the spheroid on which

any chart and set of positions are based. This is especially

significant when plotting or using positions from an

electronic positioning system (e.g. a GPS receiver), which

gives a set of coordinates. Using the wrong spheroid can

have serious consequences; including the loss of sites,

ships running aground and even territorial disputes. A posi-

tion in the WGS84 ellipsoid plotted on a chart in the UK

based on the Airy spheroid, can be 164 m (533 ft) away

from the intended position. There is software readily

available for transformations between different systems,

but it is essential to know which system provided the posi-

tion and on which system the chart or site-plan is based.

P

osition-fixing at sea relies on the same basic

principles as on land. Offshore, however, the en-

vironment often requires alternative methods for

obtaining position. Position is found through the meas-

urement of distances and angles.

Position-fixing is essential in archaeology for:

• pinpointing the exact location of a site;


• establishing relative locations of sites; and


• obtaining positional data during geophysical survey.

This chapter will outline some of the fundamental

principles involved in position-fixing and give a summary

of position-fixing equipment, including optical instruments,

electronic systems and satellite-navigation systems. The first

step is to establish exactly what a position is.

Position is normally expressed in terms of coordinates

which can be depicted on a map, chart or plan. However,

to use these numbers correctly it is important to under-

stand how the numbers were obtained and this is more

complex than it may at first appear. The map or chart is

a scaled representation of the ground or the earth’s sur-

face. The earth has been identified, after much debate

through the centuries, as being neither flat nor round, but

an irregular shape resembling a rounded pear.

The problem faced by all map-makers is how to rep-

resent the shape of the earth on a flat piece of paper. To

do this, the shape of the earth must first be defined

mathematically and then projected onto a flat piece of paper

or, more specifically, a flat plane. Mathematical approx-

Position-Fixing

Contents

u Geographical coordinates

u Accuracy

u Methods of position-fixing

u Equipment

11

Underwater Archaeology: The NAS Guide to Principles and Practice Second Edition Edited by Amanda Bowens
© 2009 Nautical Archaeological Society ISBN: 978-1-405-17592-0