Bases and Base Networks

Bases and Base Networks

Bases (base stations) are important in gravity and magnetic surveys, and in
some electrical and radiometric work. They may be:
1. Drift bases – Repeat stations that mark the starts and ends of sequences
of readings and are used to control drift.
2. Reference bases – Points where the value of the field being measured has
already been established.
3. Diurnal bases – Points where regular measurements of background are
made whilst field readings are taken elsewhere.
A single base may fulfil more than one of these functions. The reliability
of a survey, and the ease with which later work can be tied to it, will often
depend on the quality of the base stations. Base-station requirements for
individual geophysical methods are considered in the appropriate chapters,
but procedures common to more than one type of survey are discussed below.
 Base station principles
There is no absolute reason why any of the three types of base should coincide,
but surveys tend to be simpler and fewer errors are made if every drift
base is also a reference base. If, as is usually the case, there are too few
existing reference points for this to be done efficiently, the first step in a
survey should be to establish an adequate base network.
It is not essential that the diurnal base be part of this network and, because
two instruments cannot occupy exactly the same point at the same time, it
may actually be inconvenient for it to be so. However, if a diurnal monitor
has to be used, work will normally be begun each day by setting it up and
end with its removal. It is good practice to read the field instruments at a drift
base at or near the monitor position on these occasions, noting any differences
between the simultaneous readings of the base and field instruments.
 ABAB ties
Bases are normally linked together using ABAB ties (Figure 1.13). A reading
is made at Base A and the instrument is then taken as quickly as possible

to Base B. Repeat readings are then made at A and again at B. The times
between readings should be short so that drift, and sometimes also diurnal
variation, can be assumed linear. The second reading at B may also be the
first in a similar set linking B to a Base C, in a process known as forward
looping.
Each set of four readings provides two estimates of the difference in field
strength between the two bases, and if these do not agree within the limits
of instrument accuracy (±1 nT in Figure 1.13), further ties should be made.
Differences should be calculated in the field so that any necessary extra links
can be added immediately.

 Base networks
Most modern geophysical instruments are accurate and quite easy to read,
so that the error in any ABAB estimate of the difference in value between
two points should be trivial. However, a final value obtained at the end of
an extended series of links could include quite large accumulated errors. The
integrity of a system of bases can be assured if they form part of a network in
which each base is linked to at least two others. Misclosures are calculated by
summing differences around each loop, with due regard to sign, and are then
reduced to zero by making the smallest possible adjustments to individual
differences. The network in Figure 1.14 is sufficiently simple to be adjusted

by inspection. A more complicated network could be adjusted by computer,
using least-squares or other criteria, but this is not generally necessary in
small-scale surveys.
 Selecting base stations
It is important that bases be adequately described and, where possible, permanently
marked, so that extensions or infills can be linked to previous work
by exact re-occupations. Concrete or steel markers can be quickly destroyed,
either deliberately or accidentally, and it is usually better to describe station
locations in terms of existing features that are likely to be permanent. In any
survey area there will be points that are distinctive because of the presence
of manmade or natural features. Written descriptions and sketches are the
best way to preserve information about these points for the future. Good
sketches are usually better than photographs, because they can emphasize
salient points.
Permanence can be a problem, e.g. maintaining gravity bases at international
airports is almost impossible because building work is almost always
under way. Geodetic survey markers are usually secure but may be in isolated
and exposed locations. Statues, memorials and historic or religious buildings
often provide sites that are not only quiet and permanent but also offer some
shelter from sun, wind and rain.

Global Positioning Satellites

Small, reasonably cheap, hand-held GPS receivers have been available since
about 1990. Until May 2000, however, their accuracy was no better than
a few hundred metres in position and even less in elevation, because of
deliberate signal degradation for military reasons (‘selective availability’ or
SA). The instruments were thus useful only for the most regional of surveys.
For more accurate work, differential GPS (DGPS) was required, involving
a base station and recordings, both in the field and at the base, of the estimated
ranges to individual satellites. Transmitted corrections that could be
picked up by the field receiver allowed real-time kinetic positioning (RTKP).
Because of SA, differential methods were essential if GPS positioning was
to replace more traditional methods in most surveys, even though the accuracies
obtainable in differential mode were usually greater than needed for
geophysical purposes.
 Accuracies in hand-held GPS receivers
The removal of SA dramatically reduced the positional error in non-differential
GPS, and signals also became easier to acquire. It is often now possible to
obtain fixes through forest canopy, although buildings or solid rock between
receiver and satellite still present insuperable obstacles. The precision of the
readouts on small hand-held instruments, for both elevations and co-ordinates,
is generally to the nearest metre, or its rough equivalent in latitude and longitude
(0.00001◦). Accuracies are considerably less, because of multi-path
errors (i.e. reflections from topography or buildings providing alternative
paths of different lengths) and because of variations in the properties of the
atmosphere. The main atmospheric effects occur in the ionosphere and depend
on the magnitude and variability of the ionization. They are thus most severe
during periods of high solar activity, and particularly during magnetic storms
(Section 3.2.4).
Because of atmospheric variations, all three co-ordinates displayed on a
hand-held GPS will usually vary over a range of several metres within a
period of a few minutes, and by several tens of metres over longer time
intervals. Despite this, it is now feasible to use a hand-held GPS for surveys
with inter-station separations of 100 m or even less because GPS errors,
even if significant fractions of station spacing, are not, as are so many other
errors, cumulative. Moreover, rapid movement from station to station is, in
effect, a primitive form of DGPS, and if fixes at adjacent stations are taken
within a few minutes of each other, the error in determining the intervening
distance will be of the order of 5 metres or less. (In theory, this will not
work, because corrections for transmission path variations should be made
individually for each individual satellite used, and this cannot be done with
the hand-held instruments currently available. However, if distances and time
intervals between readings are both small, it is likely that the same satellite
constellation will have been used for all estimates and that the atmospheric
changes will also be small.)
 Elevations from hand-held GPS receivers
In some geophysical work, errors of the order of 10 metres may be acceptable
for horizontal co-ordinates but not for elevations, and DGPS is then still
needed. There is a further complication with ‘raw’ GPS elevations, since
these are referenced to an ellipsoid. A national elevation datum is, however,
almost always based on the local position of the geoid via the mean sea level
at some selected port. Differences of several tens of metres between geoid
and ellipsoid are common, and the source of frequent complaints from users
that their instruments never show zero at sea level! In extreme cases, the
difference may exceed 100 m.
Most hand-held instruments give reasonable positional fixes using three
satellites but need four to even attempt an elevation. This is because the
unknown quantities at each fix include the value of the offset between
the instrument’s internal clock and the synchronized clocks of the satellite

constellation. Four unknowns require four measurements. Unfortunately, in
some cases the information as to whether ‘3D navigation’ is being achieved
is not included on the display that shows the co-ordinates (e.g. Figure 1.15),
and the only indication that the fourth satellite has been ‘lost’ may be a
suspicious lack of variation in the elevation reading.