Archaeology Underwater: The NAS Guide to Principles and Practice

120 UNDERWATERSURVEY

used to securely attach plastic shower-curtain hooks to
cannons and anchors.
All survey points should be clearly marked in a way that
will last for as long as the points are needed. One of the
most common mistakes made when recording measure-
ments is to measure to the wrong control point. The mark-
ing method chosen for primary control points should be
designed so that it remains visible, even after a few years
of marine growth has accumulated. Each point needs a
label clearly identifying its name or number. For primary
control points, the labels should be large and securely fixed
on the point or very close to it. Labels made from plas-
tic sheet with the point’s name cut out seem to work well,
as the name is still readable even if covered in marine
growth. For secondary and detail points, less expensive
garden-tags can be used; if the point name is engraved
with a hot soldering iron then the label can be read after
several years under water.
Finding the points can be a problem, especially on
large sites or those with poor visibility. For points fixed
to rock, the area around the point can be cleared of
marine growth with a wire brush to make the point more
visible. Additional ideas that have worked include bright
plastic markers designed for survey points on land, or ani-
mal ear-tags, and using any durable tape or paint on the
point itself or a float above it if there is weed cover.
Features and artefacts can often benefit from having a
label attached, particularly cannons and anchors. When
positioning any artefact, a record should be made of the
point or points on the artefact that were actually positioned

• a simple sketch is usually enough. Where a surveyed point
  on a feature or artefact is to be repositioned later, the point
  should be marked. Techniques that have been used
  include: white map-pins for temporarily marking points
  and defining edges or corners of objects in photographs;
  silicone sealant for attaching survey points to corroded
  metal structures in the inter-tidal zone; yellow crayon to
  mark crosses for points or for adding labels to iron or con-
  creted items. Alternatively, the join in the cable-tie can be
  used as the point to be positioned.

The Principles of Survey

Survey is about depicting features in a symbolic way, and
showing the three-dimensional relationships between
them (though usually the result is shown on a flat sur-
face). This could, for example, involve the relationship
between a natural feature, such as a gully, and the
archaeological features and objects deposited within that
gully, but equally the relationships between the various
objects and features in the gully. The degree of accuracy
required relates to what is being shown, and what meth-
ods are employed to show it (including the scale).

All survey is based on measurements and bearings,

which can be combined to build up a complex picture.

There are two basic methods: offsets and ties/trilateration.

Offsets

Offsets are measurements that position features relative

to a tape baseline fixed between two control points. An

offset measurement positions a feature using a single

measured distance at right-angles to the baseline from a

known point, and is simple and effective over small

areas. Two divers are needed for these methods, one at each

end of the tape-measure. Like radial surveys, the positions

are usually only given in two dimensions although spot-

depths can also be recorded. Offsets are most frequently

used for assessment surveys, for recording detail or in

confined areas such as rock gullies.

The first step is to set out a tape-measure baseline

between two control points, which runs through the cen-

tre of the area to be recorded, and with a clear line of sight

between the two points. Distances are measured from

this baseline, horizontally or vertically, to features on the

site using a tape-measure or measuring rod. Vertical

offsets are often used for measuring profiles of ships’

timbers (figure 14.4 and plate 14.1). To position using

an offset, a measurement is made from the feature to the

point where the offset measurement meets the baseline

at a right-angle (figure 14.5(1)). The right-angle point

can be found by swinging the ‘offset’ tape-measure and

finding where the distance to the baseline is the shortest

(figure 14.5(2)). In poor visibility it may be useful to have

Figure 14.4 Recording the profile of a wreck using vert-

ical offsets from a horizontal datum. (Drawing by Graham

Scott)