Geoscience Reference
In-Depth Information
15.2 Scale effects in interpretation of aerial photos
•
The recognition of instability with aerial photographs can only occur at a suitable
scale.
Table 15.2
Relative suitability of different scales of aerial photography (Soeters and
van Westen, 1996).
Recognition
Size (m)
Scale
1: 20,000
1: 10,000
1: 5,000
Instability
20 m
0
0
2
<
20-75 m
0
→
1
1
→
2
3
75 m
1
2
2
3
>
→
Activity of unstable area
20 m
0
0
1
<
20-75 m
0
0
→
1
2
75 m
1
1
2
3
>
→
Instability elements (Cracks,
20 m
0
0
0
<
steps, depressions, etc)
20-75 m
0
0
→
1
1
→
2
75 m
1
2
3
>
15.3 Development grades
•
The different types of developments require different grades. Typical grades for
various developments provided in the table.
Table 15.3
Grades required for development (part from Cooke and Doornkamp, 1996).
Development type
Grade %
Deg.
◦
Vert. : Horiz.
International airport runways
1
0.6
1V : 100H
Main line passenger and freight rail transport
2
1.2
1V : 50H
Local aerodrome runways
To minimize drainage problems for site development
Acceptable for playgrounds
Major roads
4
2.3
1V : 25H
Agricultural machinery for weeding, seeding
5
2.9
1V : 20H
Soil erosion begins to become a problem
Land development (construction) becomes difficult
Industrial roads
6
3.4
1V : 17H
Upper limit for playgrounds
Housing roads
8
4.6
1V : 12.5H
Acceptable for camp and picnic areas
Absolute maximum for railways
9
5.1
1V : 11.1H
Heavy agricultural machinery
10
5.7
1V : 10.0H
Large scale industrial development
(
Continued
)
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