5.0 Methodology I: Initiating The Project
Project objectives should be clearly defined by the project proponent and made clear to the terrain specialist at the start of a project. It is also desirable to identify the end products of the terrain analysis at this stage (e.g., terrain map, specific derivative map(s), report), and the medium of presentation of the maps (e.g., mylar, overlay, air photo mosaic, etc.; see Section 9.0).
The purpose for which a terrain map is to be used determines the scale at which it should be presented. Thus the scale of the presentation (i.e., final) map(s) and survey intensity level should be defined at the same time as project objectives. For example, 1:5000 scale terrain and slope stability mapping is carried out for potential forestry cut blocks on steep slopes prior to road location and designation of yarding systems; mapping at 1:20,000 scale (I cm to 200 m) is appropriate for delimitation of avalanche hazards along a highway, whereas a 1:250,000 scale terrain map (I cm to 2.5 km) is appropriate for preliminary comparisons of terrain between potential provincial parks.
Survey intensity level should be selected according to the purpose of the mapping. Where the cost of any mistakes on the terrain map could be high, much ground checking is necessary. Where mapping is for reconnaissance purposes, and is later to be followed by detailed survey, then field checking can be reduced.
Other factors should also be considered. As noted above (Section 4.6), certain kinds of terrain are more easily mapped by air photo interpretation than others, and thus require less field checking. Dense tree cover hinders air photo interpretation, so more field checks per unit area are desirable in forested terrain than in treeless alpine and arid areas. For any given survey intensity level, an experienced mapper will generally produce a more reliable map than a mapper with less experience.
Specification of TSIL also has significant implications for the project costs. Although there is generally a direct relation between amount of field checking and the cost of field work, it should also be noted that for any TSIL specified, the checking of areas with ground access (roads) will be less expensive than checking of inaccessible areas. The latter require either helicopter transport or lengthy foot traverses, which take a long time, and so both these options add to project costs.
Once the scale of the finished map has been determined from project objectives, air photos should be selected according to the following considerations. In general, it is desirable to use air photos at a scale slightly larger than that of the finished map. This ensures that polygons of appropriate size can be delimited, and that transfer of terrain data from photos to map will minimize any errors in the placement of boundary lines. Suitable relations between map scale and air photo scale are indicated in Table 8. Air photos with a smaller scale than the finished map should not be used unless no photos of suitable scale are available. Interpretation of air photos at a scale significantly larger than that of the presentation map may assist in the recognition of some features, but it can result in delimitation of unnecessary details, more time is spent handling the many photos and matching polygons from one photo to the next, and the cost of the photos is greater.
When carrying out terrain mapping at very large scales, e.g., for maps at 1 :10,000 or 1:5000, the general rule for suitable air photo scale cannot be followed, for two reasons. First, air photos at a larger scale than the final map are often not available. If this is the case, it is necessary to use the smaller scale air photos, and to compensate for lack of visible terrain detail on the air photos by carrying out extra ground checking. Second, air photo interpretation of forested terrain on large scale air photos is difficult because the visual image is dominated by the trees. Features such as slope steepness, slope breaks, and landform morphology are commonly masked by the forest or not recognizable within the small area that is covered in a single stereoscopic view. In this case, it is useful to examine smaller scale photos of the map area.
Landforms that are obscured or incomplete on the large scale photos may be recognizable on the small scale photos, and information then transferred from one set of photos to the other.
Air photo scales are commonly referred to as "x chain", meaning that the scale is "x chains to one inch". A "chain" is an old imperial measure of distance -- the length of a cricket pitch, or 22 yards. Thus "20 chain air photos" have a scale of 20 x 22 yards to I inch, which works out to 1 :15,840 (at sea level) (Table 9).
Other characteristics of air photos that should be considered are noted below. To assess some of these, it is necessary to view the air photos, or at least, to request viewing and advice about specific characteristics from staff of an air photo library.
Scale: In densely forested landscapes, landforms with distinctive morphology, such as river terraces and drumlins, are most easily recognized on medium-scale air photos (about 1:25,000 to 1:50,000). On larger scale photos (e.g., 1 :15,000), details of the tree canopy obscure slope breaks and topographic features that are visible on smaller scale photos. When carrying out large scale mapping, it is commonly useful to refer to small scale air photos for clarification of indistinct features.
Snow cover Snow cover sufficient to seriously hinder air photo interpretation is a moderately common problem. If alpine areas are to be mapped, it is well worth reviewing selected flightlines for this potential problem before ordering specific air photos.
Date of air photos: For most projects, it is desirable to obtain the most recent photos available at the appropriate scale. The chief advantage of recent photos is that they show the current road network and other artificial features, and thus provide information about access. Old air photos may show roads and bridges that are no longer passable. Recent air photos also show the effects of recent geological processes such as landslides. Where recent development (e.g., urban or industrial development) has masked the natural features of the ground surface, however, it is easier to carry out air photo interpretation using older photos. When a project is concerned with active geomorphological processes, such as river channel migration or frequency of events such as debris flows, air photos of several different dates should be reviewed.
Technical quality of the photos: Sharpness of image and degree of contrast (range of grey tones) are usually adequate on recent air photos, but should be assessed if old air photos are to be used.
Black and white air photos vs. colour air photos: Colour photos allow recognition of features that are differentiated by their distinctive colour, such as some rocks and their weathering products, and some vegetation types. Characteristics such as soil moisture and age of substrate in disturbed areas, that are indicated by grey tones on black and white air photos, are commonly less readily distinguished on colour photos. On large scale colour photos of forested terrain, foliage patterns dominate the visual image; landforms may be more clearly defined. In general, both types of photos have advantages and disadvantages. Selection of colour or black and white photos should be based on project objectives, photo scale, and prior experience.
Black and white prints from colour negatives: Reducing costs by this means is generally a false economy because the prints have a narrower range of grey tones than prints from black and white negatives.
Shadows: When photos were not taken at the optimal time of year or time of day, shadows may be sufficiently extensive and dark that significant terrain features and details are obscured This problems most common where photography of rugged terrain was carried out in winter, for example, to record the effects of a specific flood or landslide.
Vertical exaggeration: Slope steepness is usually exaggerated in stereo view. If the overlap of stereo pairs is greater or less than about 60%, a misleadingly gentle or steep appearance of slopes can result. Vertical exaggeration also varies with camera focal length and photo scale.
Minimum Requirements:
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A brief review of all previous mapping should be carried out at the time of project initiation in order to avoid unnecessary duplication of mapping (c./, Bobrowsky et al., 1992). At the start of a project, the mapper should obtain and review previous mapping (e.g., smaller scale mapping) of terrain, surficial geology and soils in or near to the study area. The objective of this review is to familiarize the mapper with the geomorphological characteristics (especially the glacial history) of the region, so that this knowledge can be used to assist air photo interpretation (see also Section 6.0). Information about bedrock geology should be reviewed because some bedrock reports include observations about Quaternary geology. Bedrock may also influence landforms, and characteristics of surficial materials such as texture. Government indexes (e.g., Geological Survey of Canada, B.C. Ministry of Energy, Mines and Petroleum Resources, federal and provincial Ministries of Environment, B.C. Ministry of Forests) and local government (e.g., regional district) sources should be checked. Drill hole records (e.g., water wells) may also provide useful information, but logs should be interpreted with caution.
Minimum Requirements:
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Table 8. Guide to the selection of air photo scale.
Scale of Presentation Map |
Suitable Air Photo Scales
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Scale on same photos at 1500
|
1 :20,000 |
1 :15,OOO to 1 :20,000 |
1 :10,000 to 1 :15,000 |
1 :50,000 |
1 :35,000 to 1 :50,000 |
1 :30,000 to 1 :45,000 |
1 :125,000 |
1 :80,000 to 1 :125,000 |
1 :75,000 to 1: 120,000 |
Table 9. Common Air Photo Scales.
Description of Air Photo Scale |
Scale at sea level |
20 chain |
1: 15,840 (4" to 1 mile) |
40 chain |
1: 31,680 (2" to 1 mile) |
80 chain |
1: 63,360 (1" to 1 mile) |