Geology Reference
In-Depth Information
2.4.1 Survey principles
A gravity survey consists of a number of
loops
, each of which begins and
ends with readings at the same point, the
drift base
(see Section 1.6). The
size of the loop is usually dictated by the need to monitor drift and will
vary with the transport being used and the accuracy desired; 2-hour loops
are common in very detailed work where the drift base is never more than
a short walk away. At least one station of the reference network must be
occupied in the course of each loop, and operations are simplified if this is
also the drift base for that loop. In principle, the network can be allowed to
emerge gradually as the work proceeds, but if it is completed and adjusted
early, absolute values can be calculated as soon as each field station has been
occupied, allowing possible errors to be identified while there is still time
for field checks to be made. There is also much to be gained from the early
overview of the whole survey area that can be obtained while the network is
being set up, and practical advantages in establishing bases while not under
the pressure to maximise the daily total of new stations that characterises
the routine production phase of most surveys.
A small survey may use an arbitrary base without any tie to an absolute
system. Problems will arise only if such a survey has later to be linked to
others or added to a national database. This often happens eventually, and
use of a purely local reference may be a false economy.
2.4.2 Base stations
The criteria used in selecting reference bases differ from those for normal
stations. Provided that exact reoccupation is possible, large terrain effects can
be tolerated. These may make it inadvisable to use the value interpretatively,
in which case the elevation is not needed either. On the other hand, since the
overall survey accuracy depends on repeated base readings, easy access and
quiet environments are important. Traffic noise and other strong vibrations
can invalidate base (or any other) readings. Also, the general principles
outlined in Section 1.6 apply to gravity bases, and descriptions should be
provided in the form of sketch plans permitting reoccupation exactly in
elevation and to within a few centimetres in position (Figure 2.7).
2.4.3 Station positioning
The sites of field stations must also be chosen with care. Except in detailed
surveys where stations are at fixed intervals along traverses, observers in
the field have some, and often considerable, freedom of choice. They also
have the responsibility for estimating terrain corrections within about 50
metres from the reading point, where features too small to be shown on any
