Measurement of Direction
i.
Geography deals with
places in relation to others.
ii.
In order to determine
the direction of a place, we need to choose one direction from which we can
measure other directions.
iii.
So, for this purpose we
normally use the North and the needle of the compass used to measure direction
points approximately towards the north.
iv.
This direction is
usually indicated at the top of a sheet of map or in the border.
v.
Where more than one
north is given, stick to the true north or simply north.
Determining the geographical position
The geographic position of a place on a map may be
shown using the following:
a)
Place names
b)
Compass bearing,
c)
Latitude and longitude,
d)
Grid reference
Place names
i.
Names of places on maps
are commonly used to locate position of an area or places.
ii.
In Tanzania names of
places such as Kigoma, Bukoba, Dar es Salaam are clearly indicated.
iii.
However these place
names have their disadvantages in that one name can appear in two places.
iv.
Moreover the name
occupies much space on the map than the area actually represented and thus
difficulty to locate a place precisely.
The North Direction and
Position of Places:
i.
Direction is best given
using a compass.
ii.
A compass is an
instrument used to find direction.
iii.
It consists of a
free-swinging magnetised needle which points to the North and South magnetic
poles.
iv.
Using a geographic or
true north, Magnetic North or a grid north may show the north direction on a
map.
The Geographic or true
north
i.
The Geographic or true
north is the direction towards the 90° North latitude.
Magnetic North
a)
Magnetic North is the
direction shown by magnetic compass, which always points to the magnetic north
pole.
b)
This is situated to the
left or right of true north and varies from year by year in relationship to the
true north.
The greed north
a)
The grid north is the
direction towards the north in maps drawn to grid system.
b)
In most cases the true
north is similar to greed north
Compass
Directions:
a)
The compass directions
are measured from the North along a 360° circle.
b)
The eight compass
directions are: North, North East, East, South East, South, South West, West
and North West. Each of these is 45° degrees from the next.
Using the compass direction,
i.
The direction is given
as an arc in 360° beginning from north, swinging right in a clockwise direction
until you reach the point the point you want to measure.
ii.
Therefore the direction
of north is 0°, East is 90°, South is 180° and West is 270°.
iii.
By using this method you
are able to give an accurate measurement of direction to a degree or even a
fraction of it.
iv.
The direction of a place
can be given with respect to another.
Bearing of a
Compass (or Direction):
i.
Compass bearing shows
the direction of a place in relation to another point measured clockwise from
0° to 360°.
ii.
The position of a point
is given in degrees which can further be divided into minutes and seconds.
Finding the bearing
To find the bearing of a place from another place you need to follow the
following procedure:
i.
Join the two points. In
our example below, join x and y with a straight line.
ii.
At point x draw a
line parallel to the north-south line given on the map.
iii.
Using a protractor,
measure the angle of y from north towards line AB.
iv.
The bearing of point y
from x is 130° or y bears 130° from x or y is South
East of x.
v.
Up to this point you are
now aware that some of the problems encountered when using place names to
determine location of places can best be solved by combining names with bearing
and distance.
For example hypothetically, Kibaha is twenty
kilometres West of Dar es Salaam City.
Latitude and Longitude: .
a)
Latitudes and longitudes
are angular measurements from the centre of the earth
b)
The position of a place
can also be given using latitudes and longitudes.
c)
Indeed, latitudes and
longitudes provide the international reference system that locates any place on
the earth’s surface.
d)
These are the most
geographical way of giving position.
e)
For that matter, these
measurements are always needed in making accurate maps.
f)
Longitudes and latitudes
are indicated on map margins.
g)
Since Latitudes and
longitudes are angular measurements from the centre of the earth, they are
given in degrees, minutes and seconds of arc.
Latitudes
i.
Latitudes are measured
northwards and southwards from the centre of the earth.
ii.
They describe how far
north or south of the equator a place is.
iii.
A circle joining places
of the same latitude at the earth’s surface is called a parallel of latitude.
Longitudes
i.
A longitude on the other
hand is an angular measurement eastwards or westwards from the centre of the
earth.
ii.
A meridian of longitude
is the shortest line that can be drawn on the surface of the earth.
iii.
It joins the North Pole
and the South Pole.
iv.
Many countries have
accepted the meridian of the Greenwich as the Prime meridian from where other
meridians are measured. It is 0°.
Points to note
a)
The Greenwich is just a
convention. A country is at liberty to choose any other meridian as its prime
meridian for its measurements
b)
Any point on the earth’s
surface can be accurately pinpointed using these lines as there is only one
point on earth that corresponds to any one set of figures.
c)
To be more precise, the
N, S must be added to latitudes and E, W to longitudes.
d)
When stating the
position you must first give the latitude and then the longitude.
e)
It is common to write
minutes (´) and seconds (´´). For example the location of Dar es
Salaam is 6°48´S 39°12´E.
f)
Large areas on the Atlas
map can also be located by using lines of latitude and longitudes.
Grid Reference:
Unlike latitudes and longitudes,
i.
Grids belong to the map
and have no relationship to the ground.
ii.
The network of gridlines
forms perfect squares.
iii.
In East Africa usually
these squares have sides of 100 km, 10km and 1km.
iv.
The grid lines are
numbered from a particular point, usually the South-Western corner of the whole
country.
v.
This is named the grid
origin.
vi.
From the origin all
vertical lines (eastings) are numbered eastwards.
vii.
In contrast, all
horizontal lines are numbered northwards and they are called northings. The
numbers at the top and bottom of a map refer to vertical lines i.e. the
eastings. The numbers along the left and right hand borders refer to the edges
of a grided square.
viii.
The reading in a Grid
system is referred to as a grid reference and is given in a six figure number.
ix.
Always remember to give
the eastward direction and then the northward direction.
x.
The vertical lines are
called easting because their numbers increase towards east.
xi.
The horizontal lines are
called northing because their numbers increase towards north.
xii.
Grid reference is
written in six digits. E.g.: 105605
Relief
a)
The relief of an area is
the surface form of the ground, which shows the size, shape, slope, etc., of
the highlands and lowlands.
b)
The relief of an area
may be represented in many ways on a map.
c)
These methods include:-
d)
indication of spot
height,
e)
trigonometric points,
f)
hill shading,
g)
layer colouring and
h)
contouring.
i)
Our attention is drawn
to contouring. Nonetheless you are free to look for details about other
methods.
