Geoscience Reference
In-Depth Information
Table 10.3
List of landmarks
and corresponding bearings
used for initial triangulation
attempt.
Landmark feature
Grid reference
Corrected bearing
*
Summit of Morven
NJ 005285
028°
Corner of Wag Wood
NJ 018263
100°
Confl uence of streams
NJ 976255
238°
*Corrected for declination.
10
This initial triangulation is plotted on Figure
10.5a using three red lines. Perhaps because the
landmarks were not very close to the mapper's
location, there were some errors in the
measurements, and this results in the three lines
meeting not at a single point, but in a triangle of
error known as a 'cocked hat'.
this additional measurement, plotted as a green
line. This line intersects two of the red lines in a
much smaller triangle of error, but cuts the third
red line some distance further northwest. The
fact that the hut circle is closer to the mapper's
location than the other landmarks means that
this bearing is likely to be more accurate. The
mapper's location was therefore revised to the
centre of the smaller triangle of error, formed by
the green line and two of the red lines, giving a
location that is both more precise and more
accurate than that in Figure 10.5a.
A further bearing on a separate landmark (a hut
circle at grid reference NJ 001261) was
fortunately possible to resolve the uncertainty in
the location. Figure 10.5b shows the result of
10.4 Making a fi eld map
Mapping aims to
record
as much relevant information as
possible on the geology of an area using fi eld maps and a
notebook, and then
present
an interpretation of those
observations in the form of a fi nal fair copy map, which may
include interpretative cross-sections. This section summarizes
how to record information on the topographic base maps you
take into the fi eld.
10.4.1 Information to record on fi eld maps
Field maps are valuable records of observations and data
gathered in the fi eld, from which later geological interpretations
will be drawn. Your fi eld maps (and notebook) should be
comprehensible to any geologist wishing to make later
interpretations; they should not simply be personal records.
They should record the geology you see, and also evidence for
any geology inferred (Section 10.5.4), discriminating clearly
between directly observed and inferred features. The level of
detail on fi eld maps depends on the time available, and also
the scale of mapping. More detail (and data) can be recorded
for an area of 1 km
2
mapped in two days than a 100 km
2
area
mapped in the same time.
