2 Mapping techniques
2.4 T h e field map and aerial photographs
The field map, aerial photographs and field notebook are the most important rec
ords of your field observations. Take great care of them. They must be:
1 labelled with your name and address, 2 neatly and carefully drafted and leg
ible,
3 fully annotated with legends, symbols and scales, and contain all the necess
ary location information,
4 completed whilst you are in the field.
The importance of completing your map whilst in the field cannot be overemphasised, as it is only then that you can carry out an ongoing
Fig. 2.13 Measurement of the plunge of a minor fold axis by placing the edge of the Frei
berg compass on the hinge line.
Fig. 2.14 Measurement of the plunge of a minor fold axis by using a pencil to extend the axis.
interpretation, construct cross-sec
tions and identify key and problem areas that warrant further work.
2.4.1 Styles of mapping
The style of mapping used is largely controlled by the map scale, the degree of structural complexity and the degree of exposure. If good detailed topographic maps are avail
able then field data may be plotted directly onto these. Where insufficient detail is shown on the
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topographic map, aerial photographs must be used in the field for accurate location of outcrops, and to map in lithological boundaries and struc
tural trends. The data is subsequently transferred onto the base map (Barnes, 1981). Barnes (1981) reviews various mapping styles, and these are summarised below.
1 Traversing is used mainly for regional mapping at scales of 1:250,000 to 1:50,000.
2 Contact mapping is used mainly for more detailed mapping at scales of 1:50,000 to 1:15,000.
3 Exposure mapping is detailed map
ping during which the location and size of each outcrop are rec
orded, usually at scales of 1:15,000 to 1:1000.
4 Baseline mapping involves detailed mapping using a measured base
line (or compass and pacing) at scales of 1:10,000 to 1:500.
5 Grid mapping or plane table mapping are techniques used for detailed exposure mapping at scales of 1:1000 to 1:1.
Traversing: In structurally com
plex areas this is the best method of quickly establishing basic strati- graphic and structural relationships.
It can be achieved by traversing per
pendicular to the strike of the domi
nant structural trend and by con
structing sketch cross-sections in the field. Fig. 2.15 shows a sketch section through a simple antiformal struc
ture (note the use of bedding/cleav
age relationships to locate the hinge region of the antiform).
Contact Mapping: This technique
involves following lithological or structural contacts in order to estab
lish 3D structural relationships. For example, it may be necessary to determine if a fault cuts up or down stratigraphy or to establish the out
crop pattern in a polyphase defor
mation terrane.
Exposure Mapping: This method is essential for detailed structural studies in complexly deformed areas.
In particular it is used to establish structurally homogeneous sub-areas and to establish interference relation
ships in areas of complex folding.
An example of exposure mapping is shown in Fig. 2.16.
Baseline, grid and plane table map
ping: Detailed mapping using these techniques is essential in establishing detailed relationships in one outcrop or in a group of closely-spaced out
crops. Key structural relationships are illustrated by these methods. An example of baseline mapping is shown in Fig. 2.17.
2.4.2 Map scales
A detailed structural map may be produced at any scale from 1:250,000 to 1:1. The same types of structural data should be collected at every locality, irrespective of the scale at which you are mapping. Failure to measure all available structural elements may seriously hinder your future interpretations. Avoid using base maps which have been excess
ively enlarged from a large scale topographic map: they are no more accurate than the maps from which they come.
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F i g 2.15 Field sketch t h r o u g h an antiform, illustrating the use of bedding/cleavage relationships to determine the position of the hinge of the fold. Structural data are plotted at the base of the section so that the section can be related to the map more readily. Symbols as in Table 2.1 (p. 42).
F i g . 2.16 A n example of exposure mapping in an area with t w o cleavages developed.
Fig. 2.17 An example of detailed baseline mapping with the locations measured off from the baseline (e.g. Barnes, 1981).
2.4.3 Aerial photographs
In many situations, you will map directly onto overlays on aerial photographs (Fig. 2.18). Provided the central region of the aerial photo
graph is used, then problems due to excessive distortion can be minimised. Structural data, outcrop boundaries, formation boundaries, major fold axes and fault traces, and locality numbers are plotted directly onto the photograph overlay (Fig.
2.18). These are subsequently trans
ferred onto your base maps using the
techniques outlined in Barnes (1981).
Aerial photographs are particularly useful for accurate outcrop location, mapping lithological boundaries, and identifying and mapping struc
tural features. In many areas the structural grain can be readily seen on the aerial photograph but difficult to pick out on the ground. Use of aerial photographs is a skill that is acquired through practice and pati
ence. Great care needs to be taken in accurately locating yourself and becoming accustomed to the scale of the photograph.
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Fig. 2.18 Overlay of an aerial photograph, showing the exposures mapped, and traces of lithological boundaries and structures plotted on the overlay.