6.0 Methodology II: Air Photo Interpretation
The objective of this procedure, which is commonly referred to as "pretyping", is to delimit terrain polygons by air photo interpretation. Photos are viewed stereoscopically, polygon boundaries are drawn, and each polygon is labeled using an appropriate combination of letters following Howes and Kenk (1988) (see also Section 4). On-site symbols should be added wherever appropriate. Pretyping must be completed before field work so that mappers can plan effective use of field time and design traverses that include representative terrain polygons and any ambiguous features.
Terrain polygons are marked on alternate photos along a flight line. The area that is mapped on each photo is limited by the conjugate principle points along the trend of the flight line, and by the midpoints between the principle points of photos in adjacent flightlines (Fig. 6). Thus mapping is carried out on the central part of each photograph, thereby avoiding marginal areas where distortion may be severe. Mappers should ensure that boundary lines and labels match between the edges of adjacent photos in the same flightline and between flightlines.
Mappers may wish, or be required, to define that part of each photo that is to be mapped by drawing a "box" around the central part of image. Boxes can be constructed either by drawing straight lines through conjugate principle points and principle points (noted above), or by using and/or drawing lines between obvious landmarks, such as roads, streams and peaks. In either case, polygon boundaries may need to extend slightly outside the box in order to join with those on the adjacent photo.
On-site symbols should be marked on the intervening photos to avoid confusion with terrain boundary lines. Mapping is usually done directly on the air photos, using a drafting pen and black ink or other fine-tipped pen. (Drawing ink can be easily erased using a damp Q-tip.) Mappers should ensure that letters and lines (including dotted boundary lines) are thick enough to be clearly legible to the technicians who later transfer terrain data to the base map.
The competence with which air photo interpretation is carried out is by far the most important factor that influences the reliability of the final terrain map (see Section 12.0 below). Thus it is essential that either the air photo interpreter have appropriate training and be experienced in the interpretation of terrain similar to that in the project area, or that she/he be closely supervised by a suitably experienced person.
The mapper must also be familiar (see Section 5.4 above) with the geological and geomorphological history of the study area, particularly the glacial history. As air photo interpretation proceeds, the mapper should work out the specific sequence of events that led to the deposition of surficial materials and development of landforms. ln other words, she/lie should develop a model for evolution of the terrain, and use and modify this model continually during subsequent air photo interpretation and field work. For example, if ice downwasted at the end of Fraser Glaciation, a different suite of landforms and materials might be expected than if deglaciation occurred by frontal retreat. If the former existence of a glacial lake is established, knowledge of ice-dam location and lake surface elevation can be used to assist recognition of terrain underlain by lake sediments and hence the location of potentially unstable slopes.
During pretyping, exposures of surficial materials, sites for ground checks, routes for traverses, helicopter landing sites, etc. should be identified and marked on the same photos as the terrain polygons. These can be marked using a coloured wax pencil, (e.g., omnichrome); such marks are clearly visible yet can be very easily removed with an eraser or a damp Q-tip.
If project objectives include assessment of slope stability and/or other geological hazards, such as river bank erosion, air photos that predate (or postdate) those being used for ~e mapping should be examined in order to provide a historic perspective on geomorphological processes.
Minimum Requirements:
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