Gradient: Gradient is the degree of slope between two points on an OS map.
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Concave slopes are gentle near the base but get steeper closer to the top. Concave slopes are represented on OS maps by contours that are widely spaced at the base and more closely packed near the top of the slope.
Convex slopes are steep near the base but more gentle closer to the top (a plateau). Convex slopes are indicated by contours that are closely packed near the base but more widely spaced near the top.
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Even or graded slopes have an even gradient and are shown by evenly spaced contours.
Stepped slopes have steps and plateaus. A number of contours are placed close together then contours are placed further apart. This pattern continues throughout the slope (Fig. 12.12).
Fig. 12.12 Slopes and contour lines.
Concave slope Convex slope
Even or graded slope
Calculating average gradient
Gradient is the degree of slope between two points on an OS map. The gradient is expressed as a ratio such as 1:10. It can be calculated very accurately using the following formula:
Average Gradient = 29 93
73m 1.6km
92 29 30 31 32 CHECK YOUR LEARNING
Examine the 1:50,000 Ordnance Survey map of Kenmare on page 194 and answer the following questions.
1. List two colours shown on the map extract and the heights they represent. 2. What is the total land area of the map over 200 m? 3. What is the height of the land at the following grid references?
a. V 904 765 180 b. V 857 771 c. V 884 738
4. What type of slope is found south of Knocknaguish (V 919 768)? 5. Calculate the average gradient between the spot height at V 931 996 to V 914 680.
92 30 31 32 93 Vertical Interval (Altitude 1 – Altitude 2) Horizontal Equivalent (Distance between the two locations) Example (Fig. 12.13):
188 m - 115 m 1.6 km
= 73 1,600 =
1 21.9
The gradient can be written as 1:21.9. This means there is a one-metre increase in the height of the land for every 21.9 metres travelled between the two points in question.
Fig. 12.13 Calculating average gradient. Stepped slope
