Making a geological map - Geological Field Techniques - page 210

Aerial photographs
Aerial photographs may be used for mapping, either using a
transparent overlay (e.g. acetate, Mylar®), or mapping directly
onto photographs enlarged to a suitable scale (e.g. 1:7000).
Many are high enough resolution to be used at large scales
(e.g. 25 cm resolution photographs are appropriate for 1:2500
scale mapping). Satellite images may also provide coverage
of areas that are otherwise poorly mapped, but are generally
lower resolution (e.g. 60 cm pixels for Quickbird; 15-30 m
pixels for Landsat). Coverage and resolution of such imagery
will no doubt improve, although availability (and cost) may
remain subject to political and commercial pressures. Any map
data drawn on the images must be carefully transferred to a
map on return from the fi eld, and will be subject to the
various types of distortion inherent in such images. Table 10.1
assesses the relative merits of aerial photographs and satellite
images.

10

Satellite images
Some low-resolution imagery (e.g. Landsat) is freely available
online, but this is of limited use for detailed mapping, although
it may help provide a regional context (Section 3.2.1).
However, a range of data (maps, high-resolution satellite
imagery, aerial photographs) is increasingly being displayed
online, both in bird's eye view and three-dimensional
perspective, via interfaces such as GoogleMaps™, Google
Earth™, Microsoft Bing Maps™. These systems make available
some of the visualization capabilities of a GIS to any internet
user, and represent an invaluable resource when planning a

Table 10.1 Advantages and disadvantages of aerial photographs and satellite images for
geological mapping.

Method Advantage Disadvantage

Aerial photograph Clear depiction of landscape features

May show more detail than map
Landforms may be interpreted before
mapping
Clear features in photographs make it
easy to confi rm your location

Limited global coverage
Costly; rarely available for free
May be old: landscape may have changed
May be diffi cult to obtain for some areas
Distortions must be corrected by processing
No contours; slopes may not be obvious

Satellite image Tend to show larger areas

More readily available (e.g. Landsat)
More sources of free images
More information in different spectral
bands: allows interpretation before
mapping
Newer images constantly being
acquired

Most are lower resolution than photographs
Limited coverage of high-resolution images
High-resolution images are costly
Relief often less clear than in an aerial
photograph
May be old: landscape may have changed
Distortions must be corrected by processing
Clouds may obscure area