Methods of Representing Relief on Maps
There are different methods of showing relief on maps. The methods include contour lines, hachuring, hill shading or layer colouring, spot heights and trigonometric stations.
The relief features of a place are usually shown on topographical maps using contour lines. A contour is a line joining points of equal height value.
Hachures are short lines drawn to show the shape of the land. As shown the lines are usually drawn following the direction of the slope or gradient.
Contour layering and colouring
This method of relief representation involves dividing an area into height zones with each zone representing a range of heights. For example, if the height of an area ranges from 0 to 500m, the land can be divided into any convenient height zones such as 0 – 100m, 100 – 200m, 200 – 300m, 300 – 400m, 400 – 500m. Then different shades of colour are used to represent each height zone or contour layer (Figure 5.2). Conventionally, blue is used to represent water bodies, green for lowlands, yellow for middle grounds, brown for highlands and white for snow capped hill or mountain tops.
A spot height is a point whose height above mean sea level has been accurately determined through land surveying techniques. On a map a spot height is indicated with a dot and the actual height value written beside the dot.
These are points on the ground indicating where the angles of triangulation have been measured when mapping an area using land survey methods. On the map the location of a trigonometrical station is shown by an equilateral triangle with a dot inside it and the height of that location written beside the triangle e.g. 1125. There are three types of trigonometrical stations namely:
A Bench Mark (BM) is a permanent land survey mark inscribed on an object such as wall, building, roadside, or bridge to indicate the exact height above sea level of that spot. On a map they are shown by the symbol (or ). The height is usually written beside it.
Landforms on Contour Maps
There are different types of landform. One of the tasks of relief analysis is to identify the various relief features on a map. The commonest means of representing landforms or relief features on maps is by the use of contour lines. Some of the common landforms that can be represented on a contour map are shown below.
Hill or Mountain
This is a piece of land which rises above the surrounding environment
Valleys and Spurs
A valley is a long, narrow depression in-between two highlands. If a valley has water flowing through it, it is known as a river valley, while the one without water is called a dry valley. As indicated , on contour maps valleys are represented by V-shaped contour lines with the V being inverted hence the apex pointing upwards. On the other hand, a spur is a piece of upland stretching out from a hill and having a length that is greater than its width. Like valleys, spurs are indicated on contour maps with V-shaped contour lines. However, unlike in the case of valleys, the apex of the V-shaped contours of spurs point downwards towards the lowland . Hence, spurs can be said to be the opposite of valleys.
This is a long stretch of highland or ridge with a very steep (scarp) slope on one side and a very gentle (dip) slope on the other side.
A ridge is a narrow long chain or range of highlands . The highlands are usually separated from one another by openings known as col (saddle) or pass (gap).
Col or Saddle
A col or saddle is a low land separating two highlands. The major difference between a col and a saddle is that a saddle is usually wider than a col.
Pass or Gap
Like a col, a pass or gap is also lowland that separates two highlands (see Figure 5.6); it is a way through a mountain range. However, whereas a col appears at a high altitude, a pass occurs at lower altitudes. Consequently, a pass is usually deeper with the land on both sides being very much higher than what obtains in the case of a col or saddle.
A plateau is large area of highland with an almost flat or table top
Slopes, Cross-sections and Intervisibility
Slope refers to the angle or inclination of any side of a highland (hill or mountain). There are different types of slope. For instance, a slope can be gentle or steep. It can also be a combination of both gentleness and steepness, in which case it can be described as being either a concave or a convex slope, depending on the arrangement of gentle and steep portions of the slope.
On a contour map a gentle slope is shown with widely spaced contour lines. On the other hand, the contour lines of a steep slope are close to one another. A concave slope when the slope of a hillside is steep at the upper level and gentle at the lower level. Conversely, if the slope of a hillside is gentle at the upper level but steep at the lower level, this results into a convex slope.
A cross-section is usually drawn to show the shape of the ground represented by a series of contour lines on a map. In other words, cross-sections give us a general idea of the topography or nature of slopes in various places.
A cross-section is usually drawn between two places of interest on a map. Consider the contour map of a hypothetical island. We can draw a cross-section showing the topography between points A and B.
In map analysis, intervisibility is a measure that is used to determine if one location on a contour map can be physically seen from another location given clear weather conditions. Determining the intervisibility of two places on a map requires drawing a cross-section between the two places. (The procedures for drawing a cross-section have already been discussed above). After drawing the cross-section, a straight line is drawn between the two places; this is known as line of sight. If the line of sight cuts across a high-rise physical obstruction such as a hill, trees, tall buildings and so on, then the two points are not intervisible from each other. But if there is no such obstruction it then means that the two points are intervisible. Points A and B are not intervisible while points C and D are intervisible.
Calculating Average Gradient
Gradient refers to the amount of slope. It is usually expressed as the ratio between the height and length of the ground between two places on a contour map. In reality slope between two places is hardly even or smooth; it is usually undulating and, hence, of different steepness. For this reason, we normally talk about the average slope or gradient between two places.
The following procedures are followed in calculating average gradient:
- On the map identify the two points you want to determine the average gradient between them.
- Identify and record the height of each of the places.
- Determine the difference in height between the two places by subtracting the value of the lower height from that of the higher height; this gives you the vertical interval (V.I.).
- Measure the horizontal distance between the two places on the map. (This measurement will be in centimeters). However you will need to convert the paper measurement (that is measurement on the map) to its ground or horizontal equivalent (H.E.) using the map scale.
- Calculate the gradient using the following formula:
|Gradient||= Difference in height (metres)|
|Horizontal distance (metres)|