Archaeology Underwater: The NAS Guide to Principles and Practice

84 POSITION-FIXING

Geographical Coordinates

Conventional marine chart coordinates are geographical
coordinates and are described in terms of latitude and
longitude. These are expressed as angular units (degrees,
minutes, and seconds). A line of latitude is defined as
a line or (more specifically, a plane) that bisects the
spheroid at right-angles to the line between the poles.
Latitude is measured as the angle subtended by an ima-
ginary line (the normal) running perpendicularly through
the point to be located to the equatorial plane (figure 11.1)
and is expressed as a value north (N) or south (S) of
the equator.

A line of longitude, or meridian, is defined as a line

that describes the shape of the spheroid passing through

the poles. Longitude is the angle subtended from the point

to be located to the prime meridian in the equatorial

plane. The prime meridian is an arbitrary meridian

chosen as zero, in most cases the line that passes through

Greenwich, UK. Longitude is expressed as either east

(E) or west (W) of the prime meridian. On a marine

chart these coordinates appear as a grid. This grid is

not regular or orthometric (a grid formed of squares)

but changes depending on the scale and the projection.

This is because lines of longitude (meridians) converge

at the poles.

The projection: This is the method by which positions

on the spheroid are represented on a flat piece of paper.

In simple terms, different projections are alternative

ways of mathematically wrapping a piece of paper (or

mapping-plane) around the spheroid. The projection is

fundamental to mapping and caused the geodesists of

the past great difficulties. It wasn’t until Mercator devised

his projection in the sixteenth century that mapping

truly developed.

Figure 11.2 shows the basis of the Universal Transverse

Mercator (UTM) projection. This is the most common

projection used for nautical charts and mapping. It is a

set of grid-projection parameters devised to apply to any

and all spheroids. In general cases when working offshore

with GPS position-fixing systems, UTM parameters are

applied to the WGS84 spheroid. UTM was devised in an

attempt to create a truly international mapping system.

It divides the spheroid into six-degree segments and

applies the grid parameters to each segment. These

segments are called zones and in effect each zone is a

North pole

South pole

Equator

Line of

latitude

(parallel)

Line of

longitude

(meridian)

Axis of

rotation

Figure 11.1 The earth, showing latitude, longitude and
equator

Figure 11.2 The basis of the Universal Transverse Mercator (UTM) projection