Maps and map interpretation

MAP SCALE

Maps are all generally, much smaller in size as it an be compared to the actual areas represented which are much larger in size. Hence; a map scale is a relationship of map distance to ground represented. This is what understood as map scale. It is thus ; map scale defined as the constant relationship between the shorter lengths on the map and larger ground distances represented.

Or

The ratio of distance between the map and the actual ground represented and the fundamental application for this is as follows:-

CATEGORIES OF MAP SCALES

Scales of maps are not all the same. They are extremely varied in size, expression methods and units of corresponding distance expression. It is thus; map scale are classified according to such terms of variation.

MAP SCALES ACCORDING TO SIZE.

1. Large scale :-

It is a type of scale which gives larger representation on a map and permits limited details to appear.

2. Small scale:-

This makes smaller representation of very wider part of the earth’s surface on a map permit limited details to appear.

3. Medium scale:-

This provides a medium representation of a part of the earth’s surface on a map and permits details in moderate level to appear.

There is no general agreement of the quantitative limits of the terms, small, large and medium scales. However most of the cartographers agree that:-

A map with reduction ratio 1;50,000 or less would be large e.g. 1:25,000

A map with reduction ratio of 1:500,000 or more is considered as small scale e.g. 1:1,000,000.

A map with reduction ratio of in between of the two above, considered medium scale. E. g. 1:200,000.

Scales according to the forms of expression.

It is widely considered on how a scale has been indicated on the map. This generates three types of scales and include the following.

1. Statement scale.

This is also known as verbal scale. It is considerably to a scale that is expressed on a map by being stated in words. E.g. 1 cm on a map represents 1 km on the ground.

2. Representative fraction scale:-

It is considerably to a expressed on a map in ratio or fraction with numerator and denominator appearing in a similar smaller unit e.g. 1:50,000

3. Linear scale.

This shows the ratio of distance between the map and ground along the line.

Map scales according to expression units used

Scale according to this consideration, broadly categorized into two and include the following.

4. Metric scale.

It is a scale system in which the units of expression for the corresponding distances between the map and ground are of metric e. e. 1 cm represents 5km.

5. Imperial scale:-

It is scale system in which the units of the corresponding distances between the map and ground are of imperial. E.g. 1 inbch represents 2.5 miles.

SCALE SIZE OF MAPS

Scale size varies considerably from map to maps as some drawn on large scale, while others may be drawn on medium or small scale. Variation in map scale size is made by some determinant factors.

The main determinant factors for map scale size include:-

1. Size of the ground represented by the map

2. The size of the map drawn

3. Amount of the ground details shown on the map

Size of the ground represented.

Maps which represent varied grounds in size differ in scale size. Smaller ground is made have larger representation on a map and thus; the map is subjected to have large scale. Conversely; wider area is made to have smaller representation and thus; the map subjected to small scale.

Map A; is of large scale as the represented area is smaller in size; while map B is of small scale as much wider area represented.

Size of the map.

Maps of varies size differ in scale size. A map of large size makes large representation of an area and thus, subjected to large scale size. Conversely to a map of small size makes smaller representation of an area and thus; subjected to small scale.

Amount of details.

This controls scale size indirectly. Maps constructed to show varied levels of details differ in scale size. A map that planned to show few details is designed to be of small size and thus, subjected to small scale as makes smaller representation. Conversely; a map planes to show numerous details is designed to be of large size and thus subjected to large scale as makes larger representation.

METHODS OF SHOWING SCALE ON MAPS.

On a map, scale can be represented by one or more of the following useful methods.

1. Statement scale.

2. Representative fraction scale (RF Scale)

3. Linear scale

1. Statement scale.

This shows the relationship of distance between the map and the actual ground represented by being stated in words. E.g. one centimeter on a map represents one kilometer on the ground; 1 cm represents 1 km; or 1 cm to 1km.

Statement scale is also known as verbal scale.

Properties of statement scale.

The scale on the map is expressed by being stated in words.

Map distance is given in smaller unit of like centimeter and inch; while, the ground distance given in larger units of meter, feet, kilometers and miles, But it has to be noted that, cm

corresponds to meters and kilometers to give metric statement scale, while inch goes corresponds to feet and miles to give imperial statement scale.

The amount of map distance in the scale is always 1.

Representative fraction scale.

It is a form of scale expression in which the relationship of distance is given in ratio or fraction. It is established with special emphasize of showing the relationship of distance in similar unit. E.g. 1:50,000

Properties of RF Scale.

The scale is expressed in ratio fraction with numerator and denominator.

The numerator stands stands for map distance; while the denominator stands for ground distance.

The numerator and denominator treated in similar smaller units.

The amount of numerator is 1. For instance; scale should not be expressed as 2:50,000; instead it should be 1:25,000.

It is neutral to metric and imperial scales.

2. Linear scale.

Linear scale is alternatively known as graph plain bar, divided and open scale. It is a form of scale expression in which the line on the map shows the relation ship of distance between the map and ground represented.

Categories of linear scales.

Linear scales are not all the same. They are extremely varied in appearance and nature of the measurement units used. Thus, linear scales are categorized in basis of the appearance and measurement units used.

According to the form of appearance; linear scales are categorized into two and include:

Long linear scale.

It is a linear scale system commonly used to obtain direct accurate ground measurements especially of distance from the topographical map. It is very common on large scaled maps.

Short linear scale.

It is a linear scale system which gives a general idea on the relationship of distance between the map and actual ground represented.

According to the nature of the measurement units used, linear scales categorized into three types. These include the following:-

Metric linear scale.

It is linear scale system which uses metric units to express the relationship of distance between the map and actual ground represented.

Imperial linear scale.

It is linear scale system which uses imperial units to show the relationship of distance between the map and actual ground represented.

Double linear scale.

It is linear scale system with both metric and imperial units. Commonly on top metric or imperial units. Commonly on top metric or imperial measurements may appear, and at below the measurements in imperial or metric units may appear.

Properties of linear scale

The relationship of distance between the map and ground expressed a long a line.

The line scale is divided into equal parts, and each part shows the ground distance in regular interval.

Most of the linear scales have two major portions of primary and secondary sections.

The primary section is larger and placed at the right hand side of the line scale . This shows larger ground distances commonly known as primaries, and given in larger units of kilometers along the metric scale, and miles along the imperial linear scale.

The secondary section is smaller and placed at left of the line scale. This section is divided into fractions to show smaller ground distances known as secondary’s. These given in smaller units of merits along the metric scale, and feet along the imperial linear scale.

Note:-

Along the imperial linear scale, the larger measurements of miles indicated a long the top of the line scale; while the smaller measurements indicated along the bottom of the line scale.

DRAWING OF THE LINEAR SCALE.

The drawing of a linear scale should involves the following fundamental steps.

take into consideration on the relationship of distance between the map and the actual ground represented. Assume the scale is 1 cm represents 0.5 km.

Determination (consideration) of the unit length for the linear scale drawing. It is principle for the unit length amount should measure ground distance of the whole number.

E.g.

As    1cm = 0.5 km.

X    = 1km?

1cm x 1 km = 1cm

0.5 km    0.5

Hence; 2cm measure 1 km along line

Draw the line to have length that evenly divided by the amount of unit length. The length. The length of the line determined by the space of the map and sometimes by the recommended ground distance to be measured along the line scale.

Dived the line scale into major portions of priary and secondary sections. Allocate 0 to separate the two sections.

Sub divided the primary section into equal larger segments with respects to the amount of unit length determined.

Subdivide the secondary section into 10 smaller fractions. The length of each smaller fraction is determined by taking the amount of unit length divided by the number of smaller fractions required.

2cm = 0.2cm 10

Hence; each smaller fraction = 0.2 cm (2mm).

The segments along the primary section and the smaller fractions along the secondary section should be proved if are accurate. It is by drawing the line of the 200 – 400 from 0 or from the left of the line towards the end of primary section. Similarly, measure the same angle from last point of primary section. Divide the angle lines into similar segments to those of base line and join the points of the three lines.

To the secondary section, draw the line of 900 downwards and upwards at last parts and they should be divided into 10 smaller units.

Finish by indicating the numbers and the used RF scale or statement scale.

SCALE CONVERSION.

Scale conversion is a cartographic process of changing map scale from one form of expression into another. The cases may include the following:-

Statement scale into scale

RF scale into statement scale

Linear scale into statement scale or RF scale

Statement scale into RF scale:-

Examples:

1 cm to 0.5 km.

The RF scale from the given statement scale above is determined as follows:-

Recall 1 km = 100,000 cm

Then

If 1 km = 100,000

0.5 km = ?

0.5km x 100,000cm = 50,000

1km

Thus,the RF scale = 1:50,000

1cm to 0.75km.

The RF scale from the given statement scale above is determined as follows.

Recall 1km = 100,000cm Then

if 1km = 100,000 0.75km = ?

0.75km x 100,000cm = 75,000

1

Thus,the RF scale is 1:75,000

RF scale into statement scale Examples

(i) 1:25,000

The statement scale from above RF scale is determined as follows Recall 1km =100,000cm

Then;

if 1km = 100,000cm

?    =    25,000cm

1km x 25,000 = 25,000    = 1

100,000cm    100,000    4

Thus;the scale is stated as 1cm represent 0.25km

(ii)1:250,000

The statement scale from the above RF scale is determined as follows.

Recall 1km =100,000cm Then;

if 1km = 100,000cm

?    =    25,000cm

1km x 250,000cm = 2.5km 100,000cm

Thus; the scale is stated as stated as 1 cm represents 2.5km

Linear scale into statement and RF scale

Linear scale can be converted into statement scale or RF scale or RF scale by considering the unit leghth used to construct it. The unit leght is realized by measuring the length of a segment with a ruler.

According to the given linear scale above; the used unit length is; 2cm measure 3km.

Then;

As 2cms = 3km 1 cm = ?

1cm x 3km = 3km = 1.5 km 2cm 2

Thus; the scale is state as; 1cm represents 1.5km.From the statement scale; The RF scale becomes; 1:150,000

IMPORTANCE OF SCALE.

Scale is useful in two varied way of map reading and interpretation;and making.

In map reading and interpretation.

1. Enables the map user to understand the relationship of distance between the map and the actual ground represented.

2. It is useful in understanding the relative size of the area mapped by considering the used scale size. For instance; if the used scale size is assessed large, it implies the used scale size is assessed small, larger area represented’

3. It is very useful in making area size, distance and gradient determination of the geographical features which appear on a map and on the actual area.

4. It is useful to judge the amount of the details of an area represented by taking into consideration of the scale size used. For instance; if the used scale is assessed large, it implies, the map makes larger representation of the small area and it is possible for many details about the area to have been included on the map.

In map making.

1. Determines the size of the map to be constructed

2. Controls the amount of details to appear on the map relatively to the size used

3. It is very useful in the cartographic process of map reduction and enlargement

4. Controls the size of the convention symbols, sign and abbreviations

5. It is useful in making map projection

6. Controls or determines the size of the ground to be mapped

EXERCISE

QN. A racing motorist travels at a speed of 120km/hr covered a ground distance measured on the map by 6cms. If traveled for 15 minutes. Determine the RF and statement scales (NECTA 1984).

QN. A racing cyclist covers a distance of 45 km between town A and b in 30 minutes. If the distance measures 3 cm on the map.

Calculate the speed of the cyclist

For how long will cover 120km

Draw a linear scale to read 120 km (NECTA 2002)

QN (a) (i) Define a scale of a map

(ii) How important is a scale of a map to map user?

(b) (i) Given 1:50,000 state the scale into centimeters to a kilometers

(ii) Given – 1cm on a map represents 4 kilometers on the ground. Express this scale as representative fraction (RF Scale) (NECTA 1996)

QN. (i) Why are scale necessary in map making?

(ii) Construct a metric linear scale for 1:30,000 to measure 1.6 km (NECTA 1983)

QN. By comparing a 1;25,000 and 1;50,000 map scales, show the usefulness of scales in map reading and interpretation (NECTA 1995)

QN. (i) convert the scale of the map to a unit lenghth of one kilometer of the linear scale.

(ii) Construct the linear scale by using the RF Scale of 1:30,000 and indicate a distance of 3.6 km (EZEB 2006)

QN. A bus driver traveling along Arusha – Babati road covering a distance of 10 cms on a map in 30 minutes, if the speed of the bus was 50km/hr.

Calculate the distance covered in km

Find its statements scale

Draw a liner scale using the above statement scale (NECTA 2000)

QN. Mention two uses of the representative fraction scale (NECTA 1988)

QN (a) Differentiate between large and small map scales

(b) outline the determinant factors for map scale size (MOCK Q.)

QN. (a) Explain why scales are necessary in map making

(b) By using the unit length of 1 cm to measure 3 km; construct a linear scale to read a distance of 8.2 kilometers.

(MOCK Qn.)

QN. (a) Differentiate between small and large scale maps.

(b) Show the potential significance of scales in cartographic process. (MOCK Q.)