The term relief refers to the elevation of land areas. In other words, relief of an area refers to the position and character of the highlands and lowlands. Thus, the methods of representing such relief on maps include the following:
- Contour: These are lines drawn to join places of equal height level or altitude. The sea level is taken as the starting point in all measurement in metres or feet. The height of a particular point is written on the line.
- Contour Interval: In a particular map, contours are of equal intervals e.g. the above contour has contour interval of 100m.
- Form Lines/Form Contour: These are lines drawn on a map like contours but are based on estimations. They are not as accurate as contours and they are represented by broken lines.
- Contour Layering: As an aid to visual impression, the space between contours are often coloured or tinted. Different shades of colours are used to denote differences in height. Thus, green is used for lowlands, yellow and brown for highlands and white for snowcapped peaks. Waterbodies such as seas are represented with blue and the deeper the sea, the darker the blue colour.
- Hill Shading: This is a method of height representation used in hilly or mountainous area. The higher a hill or mountain is, the deeper the shade representing it.
- Hachures: These are short lines drawn down the slope in the direction of the steepest gradient. The steeper the slope, the heavier the lines which are used.
- Spot Heights: These are simply spots or points on the map whose height above the sea level has been accurately measured. The exact height is represented by a dot and the height is written beside it e.g. ● 1150, ● 1250, ● 1350.
- Trigonometrical Stations: These are simple points on the ground marking the angles of triangulation when mapping an area. They are usually indicated by a triangle and a dot in the middle with the height written by the side. There are three types of trigonometrical station. These are primary, secondary and minor trigonometrical stations.
- Bench Mark: This is a permanent mark made on objects like walls, buildings and bridges. It indicates the actual height which is usually written on the objects and in most cases, it is written along the road.
CONTOUR REPRESENTATION OF LANDFORMS
The following are some of the common landforms or relief features that can be represented on a map by using contour lines.
- Valley: A valley is lowland between two highlands. Valleys are represented by V-shaped contour lines with the apex of the V-shape pointing towards the highlands. When a valley contains water, it is called a river valley but when it contains no water, it is called a dry valley.
- Spur: Spurs are projection of highlands into low ground i.e. the V-shape points to the lowland. Spurs separate one valley from another and the contour numbering decreases outwards i.e. it decreases towards the apex.
- Conical Hill: The contour lines are usually circular in shape and become smaller and smaller towards the centre.
- Round Top Hill: The contours, showing round top hill are circular but do not taper to a point. The inner-most circle of contour is fairly large.
- Isolated Hill: This is a hill which stands apart or is far removed from other hills or highlands. It may either be rounded or conical in shape.
- Knoll: This is an isolated peak of few metres high, associated with highland.
- Ridge: A ridge is a strip of highland which is elongate and narrow. The contours run almost parallel to each other.
- Col or Saddle: A col or saddle is a stretch of lowland which separates two highland areas. On the ridge, two peaks are usually separated by a col. There is no much difference between a col and a saddle. The only difference is that the saddle is wider than the col.
- Pass or Gap: A pass or gap is also a col but is found at lower parts of highlands. A pass separates two highlands but at lower levels. A pass is always a col, but a col is not always a pass. A col becomes a pass when it is used or likely to be used as a route way, either for footpath, road or railway.
- Plateau: A plateau is an extensive and broad highland area with a comparatively level surface. Plateaux usually have steep sides. If a plateau is cut off by a number of river valleys, most of the original level surface is removed by erosion, leaving behind flat-top peaks. Such a plateau is called dissected plateau.
- Escarpment/Cuesta: Escarpment refers to steep slope or the precipitous face of a ridge. The contour lines are closely packed. Cuesta refers to a hill with steep slope (dip) on one side and a gentle slope (scarp) on the other side. Therefore, dip + scarp = Cuesta.
- Gorge: A gorge is a steep sided valley with river or stream.
- Watershed: This is a highland area which separates the headstreams of rivers flowing in different directions but rising from the same mountainous source.
- Undulating Lowland: This lowland gently rises and falls with low level and is usually shown by irregular and well-spaced contours.
- Flood Plain: This is lowland, bordering a river, formed by deposits of sediments carried down by a river. It is generally referred to as area liable to flooding. It is very good for the cultivation of swamp rice.
- Cliff or Precipe: This is a high and steep face of a rock. It is usually shown by a number of contour lines meeting together at a point. The point at which they meet together is called cliff. Whenever this feature occurs along a coastline, it is referred to as “Sea Cliff”.
- Water Gap: This is a depression which is often occupied by a river through a ridge of hard rock.
TYPES OF SLOPES
The nature of slope in any land surface can also be represented by contour lines. The various types include:
- Even or Uniform Slope: The contour lines in this slope are uniformly spaced. It means, the degree of steepness is the same throughout the slopes.
- Gentle Slope: This occurs when the contour lines are far apart or widely spaced.
- Steep Slope: This occurs when the contour lines are very close to one another or they are closely packed.
- Concave Slope: This is a combination of both steep and gentle slopes. When ascending a concave slope, one passes the area which has a gentle slope first before reaching the area with a steep slope. In other words, contour lines are widely spaced at the lower level and become closely packed at the higher level. Visibility of both ends is possible in concave slope i.e. the two ends are intervisible. In more practical terms; ‘Y’ can see ‘Z’; ‘Z’ can also see ‘Y’.
- Convex Slope: This is the opposite of concave slope. The contour lines are closely packed together at lower level and they become widely spaced at higher level. Visibility is not possible i.e. the two ends are not intervisible. Practically, ‘A’ cannot see ‘B’ and ‘B’ cannot see ‘A’.