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3.
ESTABLISHING THE SITE DATUM
In some contexts, the site datum will have already been established and can be
occupied readily if there is some permanent marker indicating its location. If
there are benchmarks or other known points located on or near the site, then one
of these points should be occupied first because they are the most accurate. Even
if a benchmark is located several hundred meters or more from the site, but is
still sighted with the gun, then it can be used to set up the location of the datum.
In many situations, it will be necessary to establish the site datum as a new point,
particularly when recording the site for the first time, or in an area where no
permanent markers can be left because of potential looting and/or disturbance.
3.1
SELECTING THE SITE DATUM
Before selecting the site datum, the site boundaries should be determined and
ground characteristics assessed. The extent and location of natural features such
as bedrock exposures, gullies, cliff edges, and dense vegetation that obscures
ground visibility should be noted, and flagged if needed. Furthermore, all
significant cultural features, such as structures, different site loci, areas of
midden erosion, and dense concentrations of artifacts (e.g., flint knapping
stations) should be noted and flagged. Once the site characteristics have been
assessed and the ones that are going to be mapped are marked in some clear
fashion, then the location of the site datum can be determined.
Unlike conventional mapping, where the datum should be placed centrally in the
site, the datum or primary mapping station should be established where the line
of sight is the best. One mapping station may be sufficient for small and simple
sites. However, complex sites (i.e., large size, irregular topography, etc.) may
require several mapping stations to complete the job. Therefore, the site datum
should be placed in a location from which all of the mapping stations are visible.
Furthermore, it is recommended that the mapping stations should be situated
outside of the site boundaries to ensure that no points recorded are too close to
the gun. Contrary to one’s intuition, measurements taken at greater distances are
more accurate than those taken at close range.
If there are no constraints on leaving permanent markers, then there are several
options for establishing the site datum depending on how often the area is going
to be mapped. In cases where the site is going to be returned to again and again,
it is recommended that site datum are made of durable materials, such as sinking
rebar reinforced with concrete into the ground. However, more simple options
include pounding in a metal or wooden stake and clearly labeling it with the site
number.
If the datum marker must be removed after the mapping has been completed,
then use a large nail (i.e., PK nails). Then, mark the stake or nail with flagging
tape and pin flags to ensure that it can be relocated easily. It is important to
maintain as much accuracy as possible; it should be possible to relocate the
points for mapping stations down to the centimeter.
3-1