UNDERWATERSURVEY 127
PROCESSING MEASUREMENTS AND DRAWING
UP THE SITE-PLAN
Chapter 19 summarizes information that must be included
on each survey drawing and site-plan. The site-plan is
created using sketches and measurements collected
from a site. The plan may be drawn by hand or on a
computer. The advantage of computer-generated plans
is that printed copies can easily be made, they can be
printed at different scales, and the level of detail shown
can vary as the work progresses. If the site has not been
recorded before, the sketches or assessment survey results
can be turned into a basic site-plan. In doing this,
missing information is often identified and additional
work can be planned.
If a recording survey is being undertaken, then the first
step is to add the control points that were installed
around the site to the site-plan. If a previous site-plan has
been created from an assessment survey, then measure-
ments may be needed from the new control points to a
couple of the main features on the site, to align the pre-
vious site-plan with the control points. Once the control
points have been established, the features can be added
to the plan as they are recorded and processed.
The advantage of drawing up site-plans on paper or
plastic film is that it requires few tools other than paper,
pencils, scale ruler and dividers/compasses. Measure-
ments made under water can be scaled down and drawn
directly onto the paper. There are, however, a number of
drawbacks with using paper:
- only one ‘fair’ copy of the site-plan exists so its loss
or damage can be catastrophic (although this can
be overcome by regular digitization); - the plan has to be redrawn if the site changes due
to environmental effects or excavation; unless a fair
copy is made, this destroys the previous site-plan; - large pieces of paper are usually involved and these
require the use of large drawing boards or tables; - if the site is extended the new drawing may be off
the edge of the existing paper plan; - paper shrinks and stretches as humidity changes,
though drafting film is more stable.
The reasons listed above mean that it is preferable to
draw site-plans on a computer. Personal computers are now
very common, as are the computer-aided design (CAD)
programs used to draw site-plans. These can be drawn in
two or three dimensions, can cover any area and can
be plotted at any scale. Layering facilities in these pro-
grams allow plans to be plotted with selected subsets
of the information visible. Processing the measurements
made on site also becomes easier and the positions of points
calculated by survey processing programs can be readily
imported in bulk. A number of suitable CAD programs
exist and deciding which one to use is largely a matter of
personal choice. It is recommended that one of the more
common programs is used or one that uses a standard file
format such as ‘drawing exchange format’ (DXF). This
allows the site-plan to be shared easily and minimizes the
risks associated with ‘future proofing’ (see chapter 8).
Drawing the site-plan in one computer program sep-
arates it from the rest of the information recorded about
the site. To keep all of the information together in one
place requires a geographic information system (GIS). GIS
programs run on personal computers and provide the
combined capabilities of a CAD drawing program and
a database. The GIS program can be used to draw the
site-plan and to record information about finds, control
points, measurements, dive-logs and anything else rele-
vant to the site itself. As with CAD drawing programs, there
are various GIS programs available. Any of these programs
can be used to record information about a site but cost
and complexity vary widely. To date there is only one GIS
designed specifically for marine archaeology work and
this is the ‘Site Recorder’ program from 3H Consulting
Ltd. Like other GIS programmes, Site Recorder can record
information about the site and be used to draw site-plans
but it also includes archaeology-specific tools such as the
ability to process survey measurements (plate 14.6).
It is essential that the site-plans and site information
be copied and stored in a secure archive. If the site is
destroyed accidentally or deliberately (by excavation),
then the plans may be the only record of the site itself.
Paper records should be copied photographically or dig-
itally and deposited separately with team members and
the authorities responsible for the site. Digital records
can also be deposited with professional archive organiza-
tions such as the Archaeology Data Service in the UK
(see chapter 19).THREE-DIMENSIONAL COMPUTER-BASED
SURVEY
Sites with little height variation can be recorded by add-
ing contours to the two-dimensional plan. When dealing
with a very three-dimensional site, such as substantial
remains of a ship’s hull, then three-dimensional record-
ing is needed, and is probably best done by using a com-
puter program. Three-dimensional (3-D) trilateration or
‘direct survey measurement’ (DSM) uses direct distance
and depth measurements to position features on a site.
Distances are measured directly from control points to
features and any difference in depth is dealt with in the
processing (figure 14.11). The 3-D trilateration technique
has a number of advantages: it can be very accurate, it can