What this lesson teaches
I can read and choose the right maps, graphs, photographs and other visual resources.
Syllabus 9173, Fieldwork, Additional Notes on Data. Students are required to be familiar with the following types of data: Maps: contour maps, choropleth maps, isoline maps, dot maps, flow-line maps, proportional symbols maps and cartograms; Graphs: pie charts, bars, histograms, scatter graphs, radar charts, triangular graphs and line graphs; Photographs: landscape photographs, aerial photographs and satellite images; Others: tables, schematic diagrams, illustrations and cartoons.
Make a guess
On a map, contour lines are packed very close together on one side of a hill and spread far apart on the other. Which side is steeper?
- You cannot tell without visiting the hill.
- Both sides are equally steep.
- The side with widely spaced contours.
- The side with close contours.
Show the answer
The side with close contours.
Close contours mean height changes quickly over a short horizontal distance, so the slope is steep.
Gradient compares height gained with horizontal distance, and close contour lines show steeper ground.
A contour line joins places at the same height. Check the height units and contour interval. Close contours mean steep ground. Widely spaced ones mean gentle ground. Hilltops and hollows both show as closed loops, so check the heights.
Measure horizontal distance on the map using the scale. Gradient = height change divided by horizontal distance. If A is 40 m high, B is 60 m high and they are 100 m apart, the gradient is 20 / 100 = 0.2, or 20%. As a ratio, that is 1 in 5.
A 20% gradient is not an angle of 20 degrees. Use horizontal distance, not the distance along the slope. An average gradient can hide short, steeper sections.
In fieldwork, contours help plan routes, compare slopes and trace where runoff goes. Check the real path too, because maps may miss small steps or drains.
- Contour
- A line joining places of equal height.
- Gradient
- Height change divided by horizontal distance.
- 1 in 5
- Same as 20%, not 20 degrees.
Step by step
Find the rise
Subtract the two heights.
Find the distance
Measure horizontally using the scale.
State the answer
As a decimal, percentage or 1-in-N ratio.
Worked example: Use the same units
A path rises 15 m over 0.3 km. Convert 0.3 km to 300 m. Then 15 / 300 = 0.05 = 5%, or 1 in 20. Mixing metres and kilometres gives the wrong answer.
Watch out for this
A contour line shows a river at that height.
A contour joins points of equal height. Rivers flow downhill across contours.
Check your understanding
What is the gradient of a 10 m rise over 200 m?
- 5%, or 1 in 20.
- 10%, or 1 in 10.
- 20%, or 1 in 5.
Show the answer
5%, or 1 in 20.
Yes. 10 / 200 = 0.05.