Geoscience Reference
In-Depth Information
representation of the geology at the appropriate scale. The fi nal
fair copy map (with legend, etc.) should allow the geology of
the area to be interpreted to a degree with no other aid -
although maps are commonly published with an accompanying
summary report outlining the features of the geological units
shown on the map, along with horizontal and vertical cross-
sections. This chapter will introduce the basic skills needed for
producing geological maps and cross-sections. The fundamental
principles of mapping apply whatever tools are used - be they
pencils and paper, or the handheld computers that are
increasingly favoured by geological surveys. The specifi c
details of such electronic mapping methods are beyond the
scope of this topic.
10
10.2 Preparation and materials
Guidance on basic preparation and fi eld equipment is provided
in Chapter 2. The following sections look in more detail at how
to prepare for geological mapping.
10.2.1 Base maps and other aids
Topographic maps
Good geological mapping depends on good base maps. You
should fi nd out what is available for the mapping area well
before you start, so that you can assess which maps to use (and
at what scale). Many governments produce good quality
topographic maps based on a 'National Grid' designed
specifi cally for that country; increasingly these are available
digitally (e.g. for the UK, OS 1:10,000 Landplan maps; for the
USA, 1:24,000 quadrangle maps). Digital topographic maps can
be downloaded free, for instance from the U.S. Geological
Survey website, for much of the USA. However, for some
countries or regions good maps may be costly or impossible to
obtain. In this case, a handheld GPS unit (Sections 2.6 and
10.2.2) is essential for recording your location in the fi eld,
supplemented by any maps that exist, and either aerial
photographs or satellite images.
Base maps are generally drawn using a grid, typically a national
grid defi ned for a particular country. In most cases, grid north
on the map is nearly equal to true north (Earth's rotation axis),
but there may be a small difference, as described in Section 2.3
and shown in Figure 2.4 (p. 10). To compare any compass
readings taken in the fi eld with bearings on the map, the
angular difference between grid and magnetic north must be
corrected for, as this difference may be substantial depending
on the area and year. Section 2.3 explains how to adjust your
compass-clinometer for this before you use it for mapping.
