What is a Map Scale
A scale is the relationship or
ratio between the distance on the map and the true distance on the earth’s
surface.
— Scale = Distance on map/distance on earths surface
Ways of Scale Representation
We can identify three ways of reprinting scale:
i.
Statement scale,
ii.
representative fraction
scale and
iii.
linear scale.
Statement Scale
This is the map scale
stated in words. The scale may be stated verbally for example ‘one centimetre
to one kilometre’.
Representative Fraction (R.F)
i.
This is a means of
expressing the relative size of a map or drawing in terms of a fraction.
ii.
R.F scale is frequently
expressed as a fraction with the numerator as one.
iii.
The ratio means that one
unit on the map represents a given number of units on the ground for instance,
R.F 1/100,000 or as a ratio 1:100,000.
iv.
This means that one unit
on the map represents 100,000 units on the ground.
v.
If the unit used is
centimetres, it means that one centimetre on the map represents 100,000
centimetres on the ground (1 km. =1000m*100 cm)
Conversion can be done as
follows:
One centimetre to two kilometres
—
1 cm: 2 km
—
1 cm: 2 x 100,000 or 1/200,000
—
R.F =1:200,000
Linear Scale or Line Scale
a)
Is a line showing the
distance on the map that represents a given distance on the ground.
b)
In many cases a linear
scale is placed at the bottom of the map. It is divided into two sections.
c)
The large section to the
right is divided into equal units from 0 towards the end of the scale to the
right. e.g. 0 1 2 3 4 km.
d)
This is also called the
primary section.
The small section or secondary section:
e)
This is a small section
placed to the left of 0.
f)
It is subdivided into
fractions indicating smaller units of measurements such as 0, 250, 500, 750
metres.
g)
m-1000-----500-----0----------1----------2-----------4----------5-km
Secondary section Primary
section
Conversion of One Scale into Another
According to McMaster (1978:6) a scale can be
converted into another as follows:
i.
Given the R.F or ratio,
to find the number of centimetres to the kilometre divide by 100,000 (the
actual number of cm in a km) by the denominator of the fraction e.g. 1/250.000
write 250,000/100,000 which gives 40 cm to a kilometer.
ii.
Given the number of
kilometres to a centimetre, to find the R.F: Multiply 100,000 by the number of
kilometres to the centimetres and you will have the denominator of R.F e.g. two
kilometres to the centimetre, write 2 x 100,000 which gives 1/200,000.
iii.
Given the number of
centimetres to the kilometre, to find the R.F: divide 100,000 by the number of
centimetres to the kilometre in the scale. What you get is the denominator of
the R.F. For instance five centimetres to the kilometre: Write 5/100,000, which
gives 1/20,000.
