Throughout this chapter I have taken the mathematical or map gradient and not the engineer's gradient. The latter is generally used, I understand, to measure the gradients of roads, railways, etc.
To avoid confusion when Ski-ing, the gradient is usually named by the angle of the slope.
The gradient of slopes is shown by the contour lines, the drop between each being 30 metres or approximately 100 feet. The table on p. 92 was got out by Commander Merriman, R.N., and has proved very useful to me in setting tests as well as in judging whether slopes are comparatively safe from avalanche or not.
A slope showing eight 30-metre contour lines in one centimetre works out roughly at 27°, which is a steeper slope than most people care to take straight, running over unknown country. Anything steeper than this is apt to avalanche in certain conditions, though a 30° slope should usually be safe. (A 25° slope may be dangerous under some conditions.)
A comfortable slope is 5 contour lines in 1 centimetre, or a gradient of 17°. Taking English measurements as in Commander Merriman's scale, 16 contour lines in one inch on the map.
The beginner will probably content himself with slopes where 10 contour lines are shown in one inch, or a gradient of about 13°.
ROUGH TABLE OF GRADIENTS.
Assuming 30 metre contours to be equal to 100 feet contours (actually this is 98.4 feet). Natural Scale 1: 50,000.
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Drop per inch | Average angle | Gradient
on map. | of slope. | 1 in.
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100' | 1° 24' | 40.9
200' | 2° 45' | 20.8
300' | 4° 07' | 13.9
400' | 5° 29' | 10.4
500' | 6° 50' | 8.3
600' | 8° 12' | 6.9
700' | 9° 33' | 5.9
800' | 10° 52' | 5.2
900' | 12° 11' | 4.6
1,000' | 13° 30' | 4.2
1,100' | 14° 47' | 3.8
1,200' | 16° 04' | 3.5
1,300' | 17° 20' | 3.2
1,400' | 18° 34' | 3.0
1,500' | 19° 48' | 2.8
1,600' | 21° 00' | 2.6
1,700' | 22° 11' | 2.5
1,800' | 23° 22' | 2.3
1,900' | 24° 30' | 2.2
2,000' | 25° 39' | 2.1
2,100' | 26° 45' | 2.0
2,200' | 27° 50' | 1.9
2,300' | 28° 53' | 1.8
2,400' | 29° 56' | 1.7
2,500' | 30° 58' | 1.6
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Up till now I have only been describing the official Ordnance Maps.
There are several other maps which may also be useful.