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Practical
skills (part 4)
Marjorie Henry, Contributor
I
have started to review some of the
practical skills you must have as
you prepare for the examinations.
I have already looked at directions
both by compass points and bearings.
This week the focus will be on the
'use of scale to measure distance'
- specific objective 1.2. In any accurate
plan or map, everything must be drawn
exactly to scale.
Through
usage, the word 'scale' has come to
be employed in two distinct senses.
In the first place, it denotes the
relationship or ratio which the distance
between any two points on the map
bears to the corresponding true distance
on the ground. This is expressed either
in words or as a representative fraction
(RF). Used in the second sense, the
word 'scale' denotes a line scale
which enables distances on the map
to be directly measured and read off
in terms of distances on the ground.
All maps should bear a line scale.
Let
us expand a bit on the three ways
that map scales are represented.
(i)
In words or a statement - for example,
'one centimetre to one kilometre'.
This is stating that one centimetre
on the map is equal to one kilometre
on the actual ground.
(ii)
As a fraction or representative fraction
- for example, 1:50,000 or written
1/50,000. This is indicating that
one unit of measurement on the map,
for example one centimetre, is equal
to 50,000 units on the actual ground.
(iii)
A linear scale - this is a line that
is divided into sections to represent
a unit of measurement, for example,
one kilometre. The first section is
subdivided into smaller units, for
example, metres.
The
diagram below illustrates this.
The
map extracts you are given to use
in the examinations are from the Caribbean
territories. If you can access one,
look at it and identify the types
of scales shown. You will notice that
there are two types, namely, the linear
scale and the representative fraction.
Of
the three ways of representing the
scale, the linear scale is the easiest
way to find the distance between two
places on a map. With the other scales,
a little calculation may be needed.
Using one of the diagrams given in
the last lesson, here are the steps
to be taken to find the distance between
two points A and B on a map in a straight
line.
Step
1
Lay
the straight edge of a piece of paper
so as to join A and B. Please note
that to ensure that you have a straight
edge, you should fold the paper and
give it a crease.
Step
2
Mark
these points on the straight edge
of the paper.
Step
3
Move
the paper on to the line scale so
that the point B is on a complete
kilometre to the right, and the point
A on the divided part at the left
of the scale.
The
answer would be 2.5km.
If
the distance along a winding road
is required, the best way to calculate
this is to lay a piece of thread or
cord along the road. With the aid
of a divider or a sharp-pointed pencil,
guide it along the road as it curves
and bends. Mark the points where you
start and end. Measure the distance
between the two points as indicated
above. Remember to state your unit
of measurement in your answer. Read
your question carefully as sometimes
you are asked to:
(i)
measure in a straight line
(ii)
give your answer in the metric unit
or
(iii)
give your answer to the nearest kilometre
or 100 metre
When
asked to do the latter, if you have
.5 (a half) or more of the unit of
measurement after you have arrived
at an answer, then increase it to
the whole unit. In the example above,
the answer is 2.5km. The nearest km
would be 3 and not 2. If you were
asked to give your answer to the nearest
100 metre, then your answer would
be 2km 500m.
Marjorie
Henry is an independent contributor.
Send questions and comments to kerry-ann.hepburn@gleanerjm.com
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