What are contours?
A contour is a line on a map connecting all
places of equal height above sea level. When you interpret contour lines you
are able to get the size and shape of highlands and lowlands. Contour lines
never cross each other because no one point can be at two different heights
above the Mean Seal Level (M.S.L).
a)
On
very steep slopes they are close to each other but they cannot cross each
other.
b)
Contours
are drawn in regular steps measured vertically from M.S.L.
c)
These
steps are called Vertical Interval (V. I) such as 100, 150 or 200 metres.
d)
The
V.I is normally kept constant on a map and it is indicated in the margin of the
map.
e)
It is
common to thicken the contour lines at a given interval in order to make them
easily identifiable.
f)
In
many cases they are coloured in brown, orange and red.
g)
Contour
lines are numbered along them to indicate their height above M.S.L. in such a
way that higher ground lies above the figures.
h)
From
the numbering of contours you can determine the direction of the slope and the
height of the numbered contour lines correctly.
Some Facts about Contour Lines
i.
Contour
lines close or join around hills, basins and depressions.
ii.
In
hills the higher contours are in the middle.
iii.
Contour
lines never cross each other.
iv.
Contour
lines form a V-shape pointing upstream to denote a valley and a
v.
V
pointing down to denote a spur.
vi.
In
contour maps all contour lines close or extend to the map edge.
Land Forms on Contour Maps
Highland landforms:
These include:-,
i.
Plateaus
ii.
hills,
iii.
ridges,
iv.
spurs,
v.
slopes,
vi.
scarps,
vii.
passes,
viii.
saddles
and
ix.
Watersheds.
Lowland landforms:
These
include gorges, levees, deltas, flood plains, and V-shaped valleys.
Coastline landforms:
They
include:,
i.
estuaries,
ii.
cliffs
and corals,
iii.
fringed
coastlines.
iv.
Since
you have already learnt how to represent these features using contour lines in
your O-level secondary education we will only remind ourselves how to draw a
cross section and calculate the vertical exaggeration and gradient.
Cross Section or Profile
i.
Maps
show relief in plan.
ii.
It is
important to visualise the appearance of the features as they are seen from the
ground.
iii.
Constructing
relief sections helps to do this.
Steps considered in relief construction
The following are steps to be followed when
constructing a relief section (cross section).
i.
Identify
the two end points of the required section on a map.
ii.
Note
their positions and heights and the vertical Interval.
iii.
Mark
them as A and B and join them with a pencil.
iv.
Place
the straight edge of a plain paper along the drawn line and mark the end points
A and B.
v.
Mark
along the edge of this paper, the positions and heights of contours, water
features and important places that cut the line.
— Remove the paper from the map and place it where
you intend to draw the cross section. The width of the cross section will be
the distance between A and B. Draw a line equal to the width.
— Find the appropriate vertical scale to show the
heights of contours such as 1cm to 100m.
— Construct a frame for the relief-section by
drawing perpendicular lines from A to B and divide the heights into equal parts
according to the vertical scale you have chosen.
— Place the marked paper along the base line so
that AB on the paper lies on AB on the framework.
— Mark each contour line along the horizontal line
and proceed with others according to their heights.
— Connect all the points with a pencil. You should
smoothen hills and valleys with a smooth curve.
— Mark and label the required information.
Indicate the North; add a title to the cross section, as well as the vertical
and horizontal scale.
The point to note
The horizontal scale is the map scale. The vertical scale is usually
exaggerated
The Vertical Exaggeration
i.
This
is the amount or number of times by which the vertical scale or height is larger
than the horizontal scale or distance.
ii.
This
relationship is important as it determines the shape and size of features shown
such that they are not too small or too large.
iii.
If the
map scale is 1:50,000 and the vertical scale is 1 cm to 100m,
