POSITION-FIXING 85
different datum with a different central meridian value
and there is no overlap between zones. It is important
to note that Great Britain is covered by three UTM zones
- zones 29, 30 and 31. The zone extents are as shown
in table 11.1.
It is imperative that the zone is stated whenever grid
coordinates are quoted and that online navigation or
tracking systems are configured to the correct zone for the
area of operation. The transverse Mercator projection
has the advantage that distances and angles are represented
by their scaled values; areas, however, are distorted.
Grid coordinates on land maps are often described
in terms of eastings and northings. These coordinates
describe a position on a flat plane, and orthometric
grid. This is the case for both national and international
mapping systems. The advantages of grid coordinates
are that measurements, both distances and angles, can
be related from the map to the ground in regular units,
most commonly metres.
How the grid relates to the projection: The grid is over-
laid onto the projection (discussed above). The method
and parameters used to overlay the grid onto the projec-
tion are collectively called the map datum. There are
many different map datums: for example the National Grid
of the UK is based on the Ordnance Survey of Great Britain
1936 (OSGB36) datum. This datum uses the Airy 1840
spheroid and a transverse Mercator projection. It contains
parameters defining the origin of the grid position on
the spheroid and orientation of the projection. On an
Ordnance Survey map this datum will be eastings and
northings in metres. On old UK Admiralty charts, the
datum may be quoted as OSGB36 but the coordinates will
actually be longitude and latitude. In this case, the grid
has not been applied; thus the positions have only been
projected from the spheroid. It is critical when reading maps
that the user knows which map datum is being used.
Vertical reference: So far only a two-dimensional
position has been discussed. The third dimension that also
needs to be considered is height or depth. Height can be
expressed as a distance above the spheroid but it is more
commonly related to a separate datum. On an admiralty
chart the depths will be related to chart datum. Chart
datum is a plane that defines zero height, therefore a
distance below the plane is expressed as a depth and a
distance above the plane is expressed as a height. Chart
datum can change for each individual chart and is norm-
ally derived from tidal observations at a local point from
which the lowest astronomical tide (LAT) is calculated.
LAT is the lowest tide predicted from known tidal con-
stants. The tide can go below this level on occasions when
influenced by meteorological effects.
For national mapping systems a height datum is
chosen that is constant for the total area mapped. For
example, all the heights that appear on an ordnance
survey map in the UK are related to Ordnance Datum
Newlyn (ODN). This is a survey point at the Ordnance
Survey observatory in Newlyn, Cornwall that has been
assigned zero height. The point was derived through
tidal observations in 1911. The difference between the indi-
vidual chart datums around the coast of the UK and ODN
are noted in nautical almanacs.Archaeology and vertical reference: From the prehistoric
period to the present day, there have been significant
fluctuations in sea-level. Indeed, the process is continuing
today. These changes have obviously had a pronounced
effect on shoreline settlements, navigation, the viability
of harbours, etc. The existence of extensive tracts of
prehistoric forests, now only visible around coasts and
estuaries at very low tides, and even earlier landscape
features permanently submerged on the seabed, is testi-
mony to the profound rise in sea-level relative to the
land. In some coastal areas, for example, there has been
rise of over 20 m (65 ft) relative to the land over the past
10,000 years. The chronology of these complex changes
can be charted by the careful observation and record-
ing of ancient features, whether natural or artificial, in
relation to a fixed datum point. The absolute level of
altitudinal benchmark used might be Ordnance Datum
(OD) in the UK or Normaal Amsterdams Peil (NAP) in
continental Europe. Such information can then be com-
pared with the body of data being collected by institu-
tions such as the International Geoscience Pro-gramme
(formerly the International Geological Correlation
Programme), which was established in 1974 to process such
data globally.Table 11.1 UTM zone extents (Great Britain)
Zone Western Central Eastern False False Scale
extent meridian extent easting northing factorsZone 29 012 W 009 W 006 W 500,000 0 0.9996
Zone 30 006 W 003 W 000 500,000 0 0.9996
Zone 31 000 003 E 006 E 500,000 0 0.9996