CONTOURS
Contours
Contours are imaginary lines on the map that join points of equal height above mean sea level. The height between the contour lines is the vertical height and this will always be defined in the key of the map you are using.
Contour interpretation has two aspects; the spacing of the contours indicating the steepness and the curves they make as they follow the hillside around indicating the shape of the land, known as releif.
These are two distinct elements in the representation of relief these are;
- Height .
- Shape.
Besides contours, spot heights and trig points record their height in metres above mean sea level.
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Numerical Representation of Height Bench Mark: Bench marks appear only on large scale maps and plans as a symbol with a height value stated to one or two places of decimals. The stated height is to the actual mark, not to ground level. However the marks are rarely more than a metre above sea level. Trigometrical Stations (Trig Points), See photo above). Trig points usually show the height of ground level to the nearest whole unit. Spot Heights:
Spot heights are located to indicate ground height in ruling positions
such as tops of hills, bottom of valleys, ridge points and saddles. |
Bathymetric Releif
Bathymetric relief, ie the showing of depths below sea or water level, when required on land maps is shown in a similar way to ground relief, by depth values and contours. Contours are similar to land contours except that they are usually shown in blue. Their values are usually related to mean sea level, but in inland water they are usually related to the mean surface level of the water; the datum in each case will be stated on the map.
To Summarise the Rules for Contours
- Contours and spot heights, trig points, show the height in metres above mean sea level.
- The vertical interval on a 1;50,000 landranger map is 10 metres.
- Every fifth contour line (index line) is printed more thickly, to help with counting between contours and the following of the lines around the feature.
- Where space permits, contour lines (index line) are numbered with their height, and they are always printed so the top of the number points uphill.
- On the 1;50,000 scale map, if the slope is to steep for the four intermediate lines to be drawn seperately, some or all of them are missed out. The thicker lines (index lines) are always present. When just one thin (intermediate line) is missing the slope steepness is about 1 in 3.
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Slopes
Concave Slope (See Above). The horizontal distance between the contours decreases with the increase of height, steeper at the top of the slope.
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Convex Slope (See Above). The horizontal distance between the contours increases in height, a lot steeper.
 | Uniform Slope (See Opposite) The horizontal distances between contours remains constant. If the horizontal distance between contours is zero, the uniform slope is vertical. Uniform slopes are sometimes called "even slopes". |
Mapyx Digital Mapping
Comparrison of map to actual terrain.
If you look at the maps and photographs below you can see the relation of the contours to the actual terrain.
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Look at the maps and photographs above and compare the actual map information to the actual terrain in the photograph.
Points to Remember
- When horizontal spacing of contours decreases with decreasing height the slope is convex and local visibility becomes restricted due to the proximity of dead ground.
- When horizontal spacing of contours increases with the decreasing height, the slope is concave and there is no dead ground locally.
- When contours follow no clearly defined pattern and the horizontal distance between them varies continuosly, this usually indicates undulating terrain. in these circumstances, the pattern of streams and rivers give a good indication of the general shape of the land.
- Irregular and closely spaced contours indicate rugged and broken slopes, smooth contours indicate smooth slopes.
- The closer together the contours the steeper the slope.
Other Landforms
There are a number of terms used to describe various landforms, which are encountered in map reading and land navigation. All these landforms are a combination of uniform, concave and convex slopes, and selected terms relating to them are as follows:
Ridge. A ridge occurs where two slopes are inclined upwards towards each other to form a long narrow hill top. Ridges are watersheds.
Valley. A valley is the low ground between two ridges and usually has a river or stream flowing along its lowest line , or floor.
Col. A col is a depression in the ridge line of a mountain chain sometimes, but not always providing a pass from one side to the other.
Spur. A spur is a projection or salient of high ground from a ridge into a valley. Succesions of spurs may occur alternately on opposite sides of a valley.
Saddle. A saddle is a ridge between two summits.
Escarpment. An escarpment is a long and very steep slope leading up from a plain to a plateau. Spurs and re-entrants may occur along an escarpment.
Knoll. A knoll is a small isolated hill on a plain or plateau. knolls are sometimes referred to as isolations.
How do you tell which way is uphill when looking at contours.
Actually it is quite simple the tops of the height numbers on the index contour line always points up hill. (See map below)
