HSIE K–12

How cartographers create topographic maps using points of reference on the land. Topographic maps allow geographers to identify landscape, the gradient of a slope, relief and aspect.

This video:

• details how cartographers create maps using points of reference on the landscape
• explains how contour lines show the height of the land above sea level
• how understanding contours can teach geographers what is present at a location, for example, hills, ridges or valleys
• details how to use contour lines and scale to calculate gradient, relief and aspect of the landscape.

Watch 'Contours, gradient, relief and aspect' (3:20).

Topographic maps allow geographers to identify landscape, gradient of a slope, relief and aspect.

Transcript of Contours, gradient and aspect

[Music playing]

[Screen shows a blue sky with clouds. Text on the screen reads, ‘Curriculum Secondary Learners – HSIE. Contours, gradient, relief and aspect. Presented by Melissa Ellis’.]

Melissa Ellis

Because maps are flat and the land isn't, lines and symbols are used to indicate topographic features.

[Presenter is standing in front of a decorative background. In the bottom right-hand corner of the screen, the text reads, ‘Melissa Ellis. HSIE Curriculum Support Project Officer’.]

The lines on a topographic map that join areas of equal height are called contours.

[Screen shows an illustration of a topographic map hovering above a mountain range. The features of the mountain range match the contour lines of the topographic map. The map is labelled, ‘Topographic map’. The mountain range is labelled, ‘Landscape’. On the topographic map, there are 4 black dots, one red triangle and a range of curved lines. One of the curved lines is circled. This line is labelled, ‘Contour’.]

Cartographers reduce clutter by only recording measurements for a set contour interval.

[Screen zooms in to show just the topographic map. Two of the adjacent contour lines are labelled, ‘Contour interval’.]

For example, this map only shows every increase or decrease in height by 20 metres.

[All of the contour lines on the left-hand side of the map are labelled with their heights above sea level. The heights range from 200 metres to 280 metres.]

The black dots on topographic maps represent spot height.

[A circle appears around one of the black dots on the topographic map. It is labelled, ‘Spot height’.]

Where there is a triangle, it means that there is a triangulation station where the height has been specifically measured and marked by a concrete block linking the map to the real world.

[A circle appears around the red triangle on the topographic map. The screen zooms out slightly to show the tip of the mountain range illustration. There is now a red box sitting on top of the mountain range. This is also circled.]

Relief refers to variations in the height of the land.

[Screen shows the presenter standing in front of a decorative background. Text on the screen matches the presenter’s spoken words.]

Local relief is calculated by finding the difference in height between two points in an area, or the highest minus the lowest point.

[Screen shows a simple topographic map with 3 contour lines. The lines have the heights 700, 800 and 900 written on them. The 900 line has a dot on it that is labelled, ‘A’. The 700 line has a dot on it that is labelled, ‘B’. A straight line connects the 2 dots. In the bottom right-hand corner, there is a box that reads, ‘Contour Interval: 100m’.]

In this example, it is simply the difference between the height at point A, which is 900 metres, and the height at point B, which is 700 metres. Thus, the relief is 200 metres.

[Screen shows an animated illustration of a mountain range. Text on the screen matches the presenter’s spoken words.]

Gradient is a measure of how steep a slope is.

[The illustration moves to the right-hand side of the screen. On the left-hand side, a ruler appears along the height and base of the mountain. The height of the mountain is labelled, ‘Rise – 20m’. The base of the mountain is labelled, ‘Run – 5km’. The screen shows the formula that is used to calculate a gradient. It reads, ‘Gradient = rise divided by run’.]

It is calculated as a fraction of the increase in height, the rise, divided by the distance it covers, run.

[The formula on the screen changes. It now reads, ‘Gradient = 20m divided by 5km’.]

Thus, in this example, the gradient is calculated by first getting the rise and the run into the same units.

[The formula on the screen changes. It now reads, ‘Gradient = 20m divided by 5000m’.]

So we start by converting the 5-kilometre run into 5,000 metres. Then rise over run is 20 over 5,000.

[The formula on the screen changes. It now reads, ‘Gradient = 1 divided by 250’.]

Breaking this down further, it becomes 1 over 250.

[The formula on the screen changes. It now reads, ‘Gradient = 1:250.]

In geography, we state gradient as a ratio. So the gradient is 1 to 250.

[An animated red line extends along the screen, just above the formula. It goes 250 units horizontally, followed by 1 unit vertically.]

This means that for every 250 metres you travel along, you go up 1 metre.

[Screen shows presenter standing in front of a decorative background.]

Sometimes we need to know from a map what direction a slope or object is facing. This is called the aspect. The aspect is simply the direction from the highest point to the lowest point at a site. This will show where the slope is facing downhill.

[Screen shows a video of a hill. The video is being filmed from ground level, with the peak of the hill in the background.]

A good tip to remember this is, if you stood on the slope and dropped a ball, what direction would it roll.

[An arrow appears on the left-hand side of the hill. It is pointing downwards along the slope.]

That is the direction the slope is facing.

[Screen shows the simple topographic map from earlier in the video. There is now a triangle at the highest point of the map. It is labelled, ‘1000’. Point A is now on the south-most part of the 800 line. A vertical blue arrow starts at the triangle, runs through point A and extends towards the base of the map. Point B is now next to the northwest part of the 700 line. A diagonal blue arrow starts at the triangle, runs through point B and extends to the top-left part of the map.]

In this example, the aspect of point A is south, and the aspect of point B is northeast.

[Screen shows presenter standing in front of a decorative background.]