You must have seen graphs, diagrams and maps showing different types of data. For example, the thematic maps shown in Chapter 1 of book for Class XI entitled Practical Work in Geography, Part-I (NCERT, 2006) depict relief and slope, climatic conditions, distribution of rocks and minerals, soils, population, industries, general land use and cropping pattern in the Nagpur district, Maharashtra. These maps have been drawn using large volume of related data collected, compiled and processed. Have you ever thought what would have happened if the same information would have been either in tabular form or in a descriptive transcript? Perhaps, it would not have been possible from such a medium of communication to draw visual impressions which we get through these maps. Besides, it would also have been a time consuming task to draw inferences about whatever is being presented in non–graphical form. Hence, the graphs, diagrams and maps enhance our capabilities to make meaningful comparisons between the phenomena represented, save our time and present a simplified view of the characteristics represented. In the present chapter, we will discuss methods of constructing different types of graphs, diagrams and maps.
Representation of Data
The data describe the properties of the phenomena they represent. They are collected from a variety of sources (Chapter 1). The geographers, economists, resource scientists and the decision makers use a lot of data these days. Besides the tabular form, the data may also be presented in some graphic or diagrammatic form. The transformation of data through visual methods like graphs, diagrams, maps and charts is called representation of data. Such a form of the presentation of data makes it easy to understand the patterns of population growth, distribution and the density, sex ratio, age–sex composition, occupational structure, etc. within a geographical territory. There is a Chinese proverb that ‘a picture is equivalent to thousands of words’. Hence, the graphic method of the representation of data enhances our understanding, and makes the comparisons easy. Besides, such methods create an imprint on mind for a longer time.
General Rules for Drawing Gwing Graphs, Diagrams and Maps
1. Selection of a Suitable Method
Data represent various themes such as temperature, rainfall, growth and distribution of the population, production, distribution and trade of different commodities, etc. These characteristics of the data need to be suitably represented by an appropriate graphical method. For example, data related to the temperature or growth of population between different periods in time and for different countries/states may best be represented using line graphs. Similarly, bar diagrams are suited best for showing rainfall or the production of commodities. The population distribution, both human and livestock, or the distribution of the crop producing areas may suitably be represented on dot maps and the population density using choropleth maps.
2. Selection of Suitable Scale
The scale is used as measure of the data for representation over diagrams and maps. Hence, the selection of suitable scale for the given data sets should be carefully made and must take into consideration entire data that is to be represented. The scale should neither be too large nor too small.
3. Design
We know that the design is an important cartographic task (Refer ‘Essentials of Map Making’ as discussed in Chapter 1 of the Practical Work in Geography, Part-I (NCERT, 2006), a textbook of Class XI). The following components of the cartographic designs are important. Hence, these should be carefully shown on the final diagram/map.
Title
The title of the diagram/map indicates the name of the area, reference year of the data used and the caption of the diagram. These components are represented using letters and numbers of different font sizes and thickness. Besides, their placing also matters. Normally, title, subtitle and the corresponding year are shown in the centre at the top of the map/diagram.
Legend
A legend or index is an important component of any diagram/map. It explains the colours, shades, symbols and signs used in the map and diagram. It should also be carefully drawn and must correspond to the contents of the map/diagram. Besides, it also needs to be properly positioned. Normally, a legend is shown either at the lower left or lower right side of the map sheet.
Direction
The maps, being a representation of the part of the earth’s surface, need be oriented to the directions. Hence, the direction symbol, i. e. North, should also be drawn and properly placed on the final map.
Construction of Diagrams
The data possess measurable characteristics such as length, width and volume. The diagrams and the maps that are drawn to represent these data related characteristics may be grouped into the following types:
1. One-dimensional diagrams, such as line graph, poly graph, bar diagram, histogram, age, sex, pyramid, etc.;
2. Two-dimensional diagram, such as pie diagram and rectangular diagram;
3. Three-dimensional diagrams, such as cube and spherical diagrams.
It would not be possible to discuss the methods of construction of these many types of diagrams and maps primarily due to the time constraint. We will, therefore, describe the most commonly drawn diagrams and maps and the way they are constructed. These are :
• Line graphs
• Bar diagrams
• Pie diagram
• Wind rose and star diagram
• Flow Charts
Line Graph
The line graphs are usually drawn to represent the time series data related to the temperature, rainfall, population growth, birth rates and the death rates. Table 3.1 provides the data used for the construction of Fig 3.2.
Construction of a Line Graph
1. Simplify the data by converting it into round numbers, such as the growth rate of population as shown in Table 3.1 for the years 1961 and 1981 may be rounded to 2.0 and 2.2, respectively.
2. Draw X and Y-axis. Mark the time series variables (years/months) on the X axis and the data quantity/value to be plotted (growth of population in per cent or the temperature in 0C) on Y axis.
3. Choose an appropriate scale and label it on Y-axis. If the data involve anegative figure, then the selected scale should also show it as shown in Fig. 3.1.
Fig. 3.1 : Construction of a Line Graph
4. Plot the data to depict year/month-wise values according to the selected scale on Y-axis, mark the location of the plotted values by a dot and join these dots by a free hand drawn line.
Example 3.1 : Construct a line graph to represent the data as given in Table 3.1:
Table 3.1 : Growth rate of Population in India – 1901 to 2011
Year
Growth rate in percentage
1901
-
1911
0.56
1921
-0.30
1931
1.04
1941
1.33
1951
1.25
1961
1.96
1971
2.20
1981
2.22
1991
2.14
2001
1.93
2011
1.79
Fig. 3.2 : Annual Growth of Population in India 1901-2011
Activity
Find out the reasons for sudden change in population between 1911 and 1921as shown in Fig. 3.2.
Polygraph Polygraph is a line graph in which two or more than two variables are shown by an equal number of lines for an immediate comparison, such as the growth rate of different crops like rice, wheat, pulses or the birth rates, death rates and life expectancy or sex ratio in different states or countries. A different line pattern such as straight line ( ____ ), broken line (- - - ), dotted line (……) or a combination of dotted and broken line (-.-.-) or line of different colours may be used to indicate the value of different variables (Fig 3.3).
Table 3.2 : Sex-Ratio (Female per 1000 male) of Selected Sates – 1961-2011
States/UT
1961
1971
1981
1991
2001
2011
Delhi
785
801
808
827
821
866
Haryana
868
867
870
860
846
877
Uttar Pradesh
907
876
882
876
898
908
Fig. 3.3 : Sex-Ratio of Selected States 1961-2011
Bar Diagram
The bar diagrams are drawn through columns of equal width. It is also called a columnar diagram. Following rules should be observed while constructing a bar diagram:
1. The width of all the bars or columns should be similar.
2. All the bars should be placed on equal intervals/distance.
3. Bars may be shaded with colours or patterns to make them distinct and attractive.
The simple, compound or polybar diagram may be constructed to suit the data characteristics.
Simple Bar Diagram
A simple bar diagram is constructed for an immediate comparison. It is advisable to arrange the given data set in an ascending or descending order and plot the data variables accordingly. However, time series data are represented according to the sequencing of the time period.
Example 3.3 : Construct a simple bar diagram to represent the rainfall data of Thiruvananthapuram as given in Table 3.3 :
Table 3.3 : Average Monthly Rainfall of Thiruvananthapuram
Month
J
F
M
A
M
J
J
A
S
O
N
D
Rainfall in cm
2.3
2.1
3.7
10.6
20.8
35.6
22.3
14.6
13.8
27.3
20.6
7.5
Construction
Draw X and Y-axes on a graph paper. Take an interval of 5 cm and mark it on Yaxis to plot rainfall data in cm. Divide X-axis into 12 equal parts to represent 12 months. The actual rainfall values for each month will be plotted according to the selected scale as shown in Fig. 3.4.
Fig. 3.4 : Average Monthly Rainfall of Thiruvananthapuram
Line and Bar Graph
The line and bar graphs as drawn separately may also be combined to depict the data related to some of the closely associated characteristics such as the climatic data of mean monthly temperatures and rainfall. In doing so, a single diagram is drawn in which months are represented on X-axis while temperature and rainfall data are shown on Y-axis at both sides of the diagram. Example 3.4 : Construct a line graph and bar diagram to represent the average monthly rainfall and temperature data of Delhi as given in Table 3.4 :
Table 3.4 : Average monthly Temperature and Rainfall in Delhi
Month
Temp. in C
Rainfall in cm.
JAN.
14.4
2.5
FEB
16.7
1.5
MAR
23.30
1.3
APR
30.0
1.0
MAY
33.3
1.8
JUNE
33.3
7.4
JUL
30.0
19.3
AUG
29.4
17.8
SEP
28.9
11.9
OCT
25.9
1.3
NOV
19.4
0.2
DEC
15.6
1.0
Construction
1. Draw X and Y-axes of a suitable length and divide X-axis into 12 parts to show months in a year.
2. Select a suitable scale with equal intervals of 5° C or 10° C for temperature data on the Y-axis and label it at its right side.
3. Similarly, select a suitable scale with equal intervals of 5 cm or 10 cm for rainfall data on the Y-axis and label at its left side.
4. Plot temperature data using line graph and the rainfall by bar diagram as shown in Fig. 3.5.
Fig. 3.5 : Temperature and Rainfall in Delhi
Multiple Bar Diagram
Multiple bar diagrams are constructed to represent two or more than two variables for the purpose of comparison. For example, a multiple bar diagram may be constructed to show proportion of males and females in the total, rural and urban population or the share of canal, tube well and well irrigation in the total irrigated area in different states.
Example 3.5 : Construct a suitable bar diagram to show decadal literacy rate in India during 1951–2011 as given in Table 3.5 :
Construction
1. Multiple bar diagram may be chosen to represent the above data.
2. Mark time series data on X-axis and literacy rates on Y-axis as per the selected scale.
Table 3.5 : Literacy Rate in India, 1951–2011 (in %)
Year
Literacy Rate
Total population
Male
Female
1951
18.33
27.16
8.86
1961
28.3
40.4
15.35
1971
34.45
45.96
21.97
1981
43.57
56.38
29.76
1991
52.21
64.13
39.29
2001
64.84
75.85
54.16
2011
73.0
80.9
64.6
Source : Census, 2011
3. Plot the per cent of total population, male and female in closed columns (Fig 3.6)
.
Fig. 3.6 : Literacy Rate in India, 1951-2011
Table 3.6 : Gross Generation of Electricity in India (in Billion KWh)
Year
Thermal
Hydro
Nuclear
Total
2008-09
616.2
110.1
14.9
741.2
2009-10
677.1
104.1
18.6
799.8
2010-11
704.3
114.2
26.3
844.8
Source: Economic Survey, 2011-12
Construction
1. Arrange the data in ascending or descending order.
2. A single bar will depict the gross electricity generation in the given year and the generation of thermal, hydro and nuclear electricity be shown by dividing the total length of the bar as shown in Fig 3.7.
Fig. 3.7 : Gross Electricity Generation in India
Pie Diagram
Pie diagram is another graphical method of the representation of data. It is drawn to depict the total value of the given attribute using a circle. Dividing the circle into corresponding degrees of angle then represent the sub-sets of the data. Hence, it is also called Divided Circle Diagram.
The angle of each variable is calculated using the following formulae
Value of given State/Region X 360 / Total Value of All States/Regions
If data are given in percentage form, the angles are calculated using the given formulae.
Percentage of x X 360 / 100
For example, a pie diagram may be drawn to show the total population of India along with the proportion of the rural and urban population. In this case, the circle of an appropriate radius is drawn to represent the total population and its sub-divisions into rural and urban population are shown by corresponding degrees of angle.
Example 3.7: Represent the data as given in Table 3.7 (a) with a suitable diagram.
Table 3.7 (a) : India’s Export to Major Regions of the World in 2010–11
Unit/Region
% of Indian Export
Europe
20.2
Africa
6.5
America
14.8
Asia and ASEAN
56.2
Others
2.3
Total
100
Calculation of Angles
1. Arrange the data on percentages of Indian exports in an ascending order.
2. Calculate the degrees of angles for showing the given values of India’s export to major regions/ Source : Economic Survey 2011-12 countries of the world, Table 3.7 (b). It could be done by multiplying percentage with a constant of 3.6 as derived by dividing the total number of degrees in a circle by 100, i. e. 360/100.
3. Plot the data by dividing the circle into the required number of divisions to show the share of India’s export to different regions/countries (Fig. 3.8).
Table 3.7 (b) : India’s Export to Major Regions of the World in 2010-11
Countries
%
Calculation
Degree
Europe
20.2
20.2 x 3.6 = 7.2
73o
Africa
6.5
6.5 x 3.6 = 23.4
23o
America
14.8
14.8 x 3.6 = 53.28
53o
Asia and ASEAN
56.2
56.2 x 3.6 = 202.32
203o
Others
2.3
2.3 x 3.6 = 8.28
8o
Total
100
360o
Construction
1. Select a suitable radius for the circle to be drawn. A radius of 3, 4 or 5 cm may be chosen for the given data set.
2. Draw a line from the centre of the circle to the arc as a radius.
3. Measure the angles from the arc of the circle for each category of vehicles in an ascending order clock-wise, starting with smaller angle.
4. Complete the diagram by adding the title, sub-title, and the legend. The legend mark be chosen for each variable/category and highlighted by distinct shades/colours.
Precautions
1. The circle should neither be too big to fit in the space nor too small to be illegible.
2. Starting with bigger angle will lead to accumulation of error leading to the plot of the smaller angle difficult.
Fig. 3.8 : Direction of Indian Exports 2010–11
Flow Maps/Chart
Flow chart is a combination of graph and map. It is drawn to show the flow of commodities or people between the places of origin and destination. It is also called Dynamic Map. Transport map, which shows the number of passengers, vehicles, etc., is the best example of a flow chart. These charts are drawn using lines of proportional width. Many government agencies prepare flow maps to show density of the means of transportation on different routes. The flow maps/ charts are generally drawn to represent two the types of data as given below:
1. The number and frequency of the vehicles as per the direction of their movement
2. The number of the passengers and/or the quantity of goods transported.
Requirements for the Preparation of a Flow Map
1. A route map depicting the desired transport routes along with the connecting stations.
2. The data pertaining to the flow of goods, services, number of vehicles, etc., along with the point of origin and destination of the movements.
3. The selection of a scale through which the data related to the quantity of passengers and goods or the number of vehicles is to be represented.
Table 3.8 : No. of trains of selected routes of Delhi and adjoining areas
S.No.
Railway Routes
No. of Trains
1.
Old Delhi – New Delhi
50
2.
New Delhi-Nizamuddin
40
3.
Nizamuddin-Badarpur
30
4.
Nizamuddin-Sarojini Nagar
12
5.
Sarojini Nagar – Pusa Road
8
.6.
Old Delhi – Sadar Bazar
32
7.
Udyog Nagar-Tikri Kalan
6
8.
Pusa Road – Pehladpur
15
9.
Sahibabad-Mohan Nagar
18
10.
Old Delhi – Silampur
33
11.
Silampur – Nand Nagari
12
12.
Silampur-Mohan Nagar
21
13.
Old Delhi-Shalimar Bagh
16
14.
Sadar Bazar-Udyog Nagar
18
15.
Old Delhi – Pusa Road
22
16.
Pehladpur – Palam Vihar
12
Example 3.10 : Construct a flow map to represent the number of trains running in Delhi and the adjoining areas as given in the Table 3.8.
Construction
1. Take an outline map of Delhi and adjoining areas, in which railway line and the nodal stations are depicted (Fig.3.9).
2. Select a scale to represent the number of trains. Here, the maximum number is 50 and the minimum is 6. If we select a scale of 1cm = 50 trains, the maximum and minimum numbers will be represented by a strip of 10 mm and 1.2 mm thick lines, respectively, on the map.
3. Plot the thickness of each strip of route between the given rail route (Fig. 3.10).
(d) Draw a terraced scale as legend and choose distinct sign or symbol to show the nodal points (stations) within the strip
Fig. 3.9 : Map of Delhi
Fig. 3.10 : Traffic (Railway) Flow Map of Delhi
Example 3.10 : Construct a water flow map of Ganga Basin as shown in Fig. 3.11.
Fig. 3.11 : Ganga Basin
Construction
1. Take a scale as a strip of 1cm width = 50,000 cusecs of water.
2. Make the diagram as shown in Fig. 3.12.
Fig. 3.12 : Construction of a Flow Map
Thematic Maps
Graphs and diagrams serve a useful purpose in providing a comparison between the internal variations within the data of different characteristics represented. However, the use of graphs and diagrams, at times, fails to produce a regional perspective. Hence, variety of maps may also be drawn to understand the patterns of the regional distributions or the characteristics of variations over space. These maps are also known as the distribution maps.
Requirements for Making a Thematic Map
1. State/District level data about the selected theme.
2. Outline map of the study area alongwith administrative boundaries.
3. Physical map of the region. For example, physiographic map for population distribution and relief and drainage map for constructing transportation map.
Rules for Making Thematic Maps
1. The drawing of the thematic maps must be carefully planned. The final map should properly reflect the following components:
1. Name of the area
2. Title of the subject-matter
3. Source of the data and year
4. Indication of symbols, signs, colours, shades, etc.
5. Scale
2. The selection of a suitable method to be used for thematic mapping.
Classification of Thematic Maps based on Method of Construction
The thematic maps are, generally, classified into quantitative and non-quantitative maps. The quantitative maps are drawn to show the variations within the data. For example, maps depicting areas receiving more than 200 cm, 100 to 200 cm, 50 to 100 cm and less than 50 cm of rainfall are referred as quantitative maps. These maps are also called statistical maps. The non-quantitative maps, on the other hand, depict the non-measurable characteristics in the distribution ofgiven information, such as a map showing high and low rainfall-receiving areas. These maps are also called qualitative maps. It would not be possible to discuss drawing these different types of thematic maps under the constraint of time. We will, therefore, confine to discuss the methods of the construction of the following types of quantitative maps :
1. Dot maps
2. Choropleth maps
3. Isopleth maps
Dot Maps
The dot maps are drawn to show the distribution of phenomena such as population, cattle, types of crops, etc. The dots of same size as per the chosen scale are marked over the given administrative units to highlight the patterns of distributions.
Requirement
1. An administrative map of the given area showing state/district/block boundaries.
2. Statistical data on selected theme for the chosen administrative units, i.e., total population, cattle, etc.
3. Selection of a scale to determine the value of a dot.
4. Physiographic map of the region, especially relief and drainage maps.
Precaution
(a) The lines, demarcating the boundaries of various administrative units, should not be very thick and bold. (b) All dots should be of same size.
Example 3.12 : Construct a dot map to represent population data of 2001 as given in Table 3.9.
Table 3.9 : Population of India, 2001
Sl. No
States/Union Territories
Total Population
No. of dots
1.
Jammu & Kashmir
10,069,917
100
2.
Himachal Pradesh
6,077,248
60
3.
Punjab
24,289,296
243
4.
Uttarakhand
8,479,562
85
5.
Haryana
21,082,989
211
6.
Delhi
13,782,976
138
7.
Rajasthan
56,473,122
565
8.
Uttar Pradesh
166,052,859
1660
9.
Bihar
82,878,796
829
10.
Sikkim
540,493
5
11.
Arunachal Pradesh
1,091,117
11
12.
Nagaland
1,988,636
20
13.
Manipur
2,388,634
24
14.
Mizoram
891,058
89
15.
Tripura
3,191,168
32
16.
Meghalaya
2,306,069
23
17.
Assam
26,638,407
266
18.
West Bengal
80,221,171
802
19.
Jharkhand
26,909,428
269
20.
Odisha
36,706,920
367
21.
Chhattisgarh
20,795,956
208
22.
Madhya Pradesh
60,385,118
604
23.
Gujarat
50,596,992
506
24.
Maharashtra
96,752,247
968
25.
Andhra Pradesh
75,727,541
757
26.
Karnataka
52,733,958
527
27.
Goa
1,343,998
13
28.
Kerala
31,838,619
318
29.
Tamil Nadu
62,110,839
621
Fig. 3.13 : Population of India, 2001
Construction
1. Select the size and value of a dot.
2. Determine the number of dots in each state using the given scale. Forexample, number of dots in Maharashtra will be 9,67,52,247/100,000 = 967.52. It may be rounded to 968, as the fraction is more than 0.5.
3. Place the dots in each state as per the determined number in all states.
4. Consult the physiographic/relief map of India to identify mountainous,desert, and/or snow covered areas and mark lesser number of dots in such areas.
Choropleth Map
The choropleth maps are also drawn to depict the data characteristics as they are related to the administrative units. These maps are used to represent the density of population, literacy/growth rates, sex ratio, etc.
Requirement for drawing Choropleth Map
1. A map of the area depicting different administrative units.
2. Appropriate statistical data according to administrative units.
Steps to be followed
1. Arrange the data in ascending or descending order.
2. Group the data into 5 categories to represent very high, high, medium,low and very low concentrations.
3. The interval between the categories may be identified on the followingformulae i.e., Range/5 and Range = maximum value – minimum value.
4. Patterns, shades or colour to be used to depict the chosen categoriesshould be marked in an increasing or decreasing order.
Example 3.13: Construct a Choropleth map to represent the literacy rates in India in 2001 as given in Table 3.10.
Construction
1. Arrange the data in ascending order as shown above.
2. Identify the range within the data. In the present case, the states recording the lowest and highest literacy rates are Bihar (47%) and Kerala (90.9%), respectively. Hence, the range would be 91.0 – 47.0 = 44.0
3. Divide the range by 5 to get categories from very low to very high. (44.0/5 = 8.80. We can convert this value to a round number, i. e., t 9.0
4. Determine the number of the categories alongwith the range of eachcategory. Add 9.0 to the lowest value of 47.0 as so on. We will finally get following categories :
47 – 56 Very low (Bihar, Jharkhand, Arunachal Pradesh, Jammu and Kashmir)
Data on Literacy in India as arranged in Ascending order
S.No.
States / Union Territories
Literacy Rate
10.
Bihar
47.0
20.
Jharkhand
53.6
12.
Arunachal Pradesh
54.3
1.
Jammu & Kashmir
55.5
9.
Uttar Pradesh
56.3
26.
Dadra & Nagar Haveli
57.6
8.
Rajasthan
60.4
28.
Andhra Pradesh
60.5
17.
Meghalaya
62.6
21.
Odisha
63.1
18.
Assam
63.3
23.
Madhya Pradesh
63.7
22.
Chhattisgarh
64.7
13.
Nagaland
66.6
29.
Karnataka
66.6
6.
Haryana
67.9
19.
West Bengal
68.6
11.
Sikkim
68.8
24.
Gujarat
69.1
3.
Punjab
69.7
14.
Manipur
70.5
5.
Uttarakhand
71.6
16.
Tripura
73.2
33.
Tamil Nadu
73.5
2.
Himachal Pradesh
76.5
27.
Maharashtra
76.9
25.
Daman & Diu
78.2
34.
Puducherry
81.2
35.
Andaman & Nicobar Islands
81.3
7.
Delhi
81.7
4.
Chandigarh
81.9
30.
Goa
82..0
31.
Lakshadweep
86.7
15.
Mizoram
88.8
32.
Kerala
90.9
65 – 74 Medium (Nagaland, Karnataka, Haryana, West Bengal, Sikkim, Gujarat, Punjab, Manipur, Uttarakhand, Tripura,Tamil Nadu)
74 – 83 High (Himachal Pradesh, Maharashtra, Delhi, Goa)
83 – 92 Very high (Mizoram, Kerala)
5. Assign shades/pattern to each category ranging from lower to higher hues.
6. Prepare the map as shown in Fig. 3.14.
7. Complete the map with respect to the attributes of map design.
Isopleth Map
We have seen that the data related to the administrative units are represented using choropleth maps. However, the variations within the data, in many cases, may also be observed on the basis of natural boundaries. For example, variations in the degrees of slope, temperature, occurrence of rainfall, etc. possess characteristics of the continuity in the data. These geographical facts may be represented by drawing the lines of equal values on a map. All such maps are termed as Isopleth Map. The word Isopleth is derived from Iso meaning equal and pleth means lines. Thus, an imaginary line, which joins the places of equal values, is referred as Isopleth. The more frequently drawn isopleths include Isotherm (equal temperature), Isobar (equal pressure), Isohyets (equal rainfall), Isonephs (equal cloudiness), Isohels (equal sunshine), contours (equal heights), Isobaths (equal depths), Isohaline (equal salinity), etc.
Requirement
(a) Base line map depicting point location of different places.
(b) Appropriate data of temperature, pressure, rainfall, etc. over a definite period of time.
(c) Drawing instrument specially French Curve, etc.
Rules to be observed
(a) An equal interval of values be selected.
(b) Interval of 5, 10, or 20 is supposed to be ideal.
(c) The value of Isopleth should be written along the line on either side or in the middle by breaking the line.
Interpolation
Interpolation is used to insert the intermediate values between the observed values of at two stations/locations, such as temperature recorded at Chennai and Hyderabad or the spot heights of two points. Generally, drawing of isopleths joining the places of same value is also termed as interpolation.
Method of Interpolation
For interpolation, follow the following steps:
(a) Firstly, determine the minimum and maximum values given on the map.
(b) Calculate the range of value i.e. Range = maximum value – minimum value.
(c) Based on range, determine the interval in a whole number like 5, 10, 15, etc.
The exact point of drawing an Isopleth is determined by using the following formulae.
Point of Isopleth = Distance between two points in cm / Difference between the two values of corresponding points x Interval
The interval is the difference between the actual value on the map and interpolated value. For example, in an Isotherm map of two places show 28 oC and 33 oC and you want to draw 30 oC isotherm, measure the distance between the two points. Suppose, the distance is 1cm or 10 mm and the difference between 28 and 33 is 5, whereas, 30 is 2 points away from 28 and 3 points behind 33, thus, exact point of 30 will be
Thus, isotherm of 30 oC will be plotted 4mm away from 28 oC or 6mm ahead of 33 oC.
(d) Draw the isopleths of minimum value first; other isopleths may be drawn accordingly.
Fig. 3.15 : Drawing of Isopleths
Excercises
1. Choose the right answer from the four alternatives given below:
(i) Which one of the following map shows the population distribution:
(a) Choropleth maps
(b) Isopleth maps
(c) Dot maps
(d) Square root maps
(ii) Which one of the following is best suited to represent the decadal growth of population?
