44 GEOLOGICAL FIELD MAPPING National Geological Surveys publish district geological maps at scales of : 50 000 or : 25 000, based on field mapping typically performed at larger scale, for instance : 10 000 in the case of the British Geological Survey (BGS) District maps hold a large amount of detail from routine fieldwork, plus data from boreholes, water wells, underground workings, and similar Compilations give rise to regional and national maps, and the maps are also used by professionals requiring information on soils, economic deposits, land use, water supply, and hazard potential Hence the field geologist needs to record all aspects of the geology Two points require emphasis First, field investigations must be rigorous, making careful, comprehensive examination of the geology and recording the results accurately at the correct location Inadequate or wrongly positioned data will result in incomplete or incorrect deductions Second, a geological field map is partly interpretative It contains ‘factual’ data, studied and measured in the field, but incomplete exposure means that the nature, position, or even existence of various boundaries depends on judgement by the geologist in the light of field observation, experience, and existing concepts Further information may dictate the need for revision Basic equipment comprises a mapping board with plastic cover, base map, compass, clinometer, notebook, black and coloured pencils, sharpener, and eraser Air photographs can be extremely useful A hammer and hand lens enable fresh rock to be obtained and studied A fine-line pen with waterproof black ink will be needed to ink-in the map and notebook, and it is helpful to record some map features in coloured ink Base Map When topographical base maps are obtainable at different scales, the choice will be guided by the nature of the investigation Scales of : 100 000 to : 50 000 are appropriate for regional exploration and reconnaissance More detailed mapping is generally performed on scales of : 25 000 to : 5000 Very detailed work in connection with mines, quarries, and engineering sites may require scales of : 1000 to : 50, often available as company plans The : 10 000 Ordnance Survey sheets used by the BGS show contours at or 10 m intervals, the positions of streams, buildings, fences, and roads, and a numbered grid It is relatively easy to locate a position on the map, record it as a grid reference, and identify morphology by ‘reading’ the contours Accurate contoured maps can be made from air photographs using specialized photogrammetric techniques Where suitable topographical maps have not been produced, have poor accuracy, or are unavailable because of political or military restriction, the geologist must find an alternative Small-scale maps can be enlarged photographically, or scanned or digitized and redrawn by computer, but only have the detail of the original Satellite images, obtainable for all the Earth’s surface, can be processed to give a coarse resolution base map suitable for reconnaissance work However, the creation of contoured maps is restricted to satellites which ‘see’ in three dimensions, through overlapping images Air Photographs The most commonly used air photographs are those made on black and white panchromatic film from level flight with the camera axis vertical Approximate scales of contact prints are generally on the order of : 8000 to : 40 000 Typically, there is 60% photograph overlap along the line of flight and 30% overlap of adjacent lines, allowing the area to be viewed as a three-dimensional image through a stereoscope As a result of spacing, the vertical scale is markedly exaggerated, enhancing topographical features that might not otherwise be obvious Air photographs are not maps Only the centre (principal point) is viewed from directly above, comparable to a map Away from the centre, the top of a vertical object appears displaced radially outwards relative to its base The scale varies according to terrain height relative to flying height and to camera tilt Despite such limitations, air photographs can be immensely useful for locating position and tracing boundaries Data recorded on overlays in the field are subsequently transferred to the base map In three-dimensional view, photographs display relationships between terrain, drainage, and geology over a broader area than can be seen from a ground position Variations in tone and texture, plus the dip of strata and patterns of fractures, can be interpreted to yield a photogeological map showing solid and superficial units and the size, shape, and orientation of the principal structures Field investigation is essential to identify rock types and measure structures, but photo-interpretation is strongly recommended prior to fieldwork and each evening to give a rapid, detailed overview and allow optimum planning of traverses Methods of interpretation are described in relevant books Photogeological information is recorded on the base map in a colour that distinguishes it from field data