Representation of Geographical Data - Study Notes

12.1 Introduction

Economists, statisticians and geographers use statistical facts and data in their fields. Climatic data like temperature, pressure and rainfall may be represented by isotherms, isobars and isohyets respectively. Besides the tabular form, population, climatic and socio-economic data can be shown as graphs, diagrams, charts and maps. Geographical data must be scientifically visualised for appropriate cartographic communication, to explore the nature of data, their variations over time and space, and to identify and classify real world objects.

• Statistics: numerical data or facts collected in a systematic manner to serve some purpose and presented in a tabulated form.
• Statistical diagrams: diagrams representing statistical data, using bars, rectangles, squares, circles, cubes, spheres, curves or lines.

Purposes of statistical diagrams

1. They enable us to compare figures at a glance.
2. They cast an easily memorable impression on the mind.
3. They enable us to analyse the data and draw inference easily.

12.2 Classification of statistical diagrams

In one-dimensional diagrams, quantities are represented by the line, the length of bars or the length of radiating lines. The lesson discusses only line graphs, bar diagrams and pie diagrams.

12.2.1 Line graph

Line graphs are usually drawn to represent time series data related to temperature, rainfall, population, birth rates and death rates.

Uses of line graphs

• Climatic data: temperature, pressure, humidity.
• Population data: growth of population, birth rate, death rate.
• Economic data: crop yield, mineral extraction, production of industrial goods over a period of time.

Simple line graph

A line or curve drawn by joining points plotted with the help of two co-ordinates, a vertical axis and a horizontal scale. It represents the successive changes in the value of one variable over a certain period of time.

Steps to construct a line graph

1. Draw a horizontal line for the x-axis and a vertical line for the y-axis. Their meeting point at right angles is the starting point, normally zero (0).
2. Add label and scale for both axes.
3. Plot the data points.
4. Draw a line through the plotted points.
5. Add a title.

Example 12.1: India's literacy rate rose from 5.35% in 1901 to 74.04% in 2011 (Census 2011). Scale: x-axis 1 cm = 10 years, y-axis 1 cm = 10% literacy.

Polygraph

A line graph in which two or more variables are shown by an equal number of lines for immediate comparison, for example growth rates of rice, wheat and pulses, or birth rate, death rate and life expectancy. Different variables are shown by a straight line, broken line (---), dotted line (.....), dot-and-dash line (.-.-.) or lines of different colours.

Example 12.2: urban population percentage of Tamil Nadu, Kerala and Bihar, 1971 to 2011. Tamil Nadu rose from 30.26% to 48.45%, Kerala from 16.24% to 47.72%.

12.2.2 Bar diagram

Quantities are represented by bars whose length is proportional to the quantities. Such diagrams are called bar diagrams, columnar diagrams or bar-graphs. Compared with other diagrams, the bar diagram gives a more accurate idea of size. Bars may be drawn on maps to show regional variations and may be vertical or horizontal.

• The width of all bars should be similar.
• Bars should be placed at intervals of equal distance.
• Bars may be shaded with colours or patterns.

Kinds of bar diagrams

1. Simple bar diagram
2. Comparative or multiple bar diagram
3. Compound or sub-divided bar diagram
4. Percentage bar diagram

Simple bar diagram

A series of bars, each showing the total value of only one variable, such as total population of states, annual production of wheat, rice or cotton, or monthly rainfall. It is drawn for immediate comparison; arrange the data in ascending or descending order. Years go on the x-axis, vertical bars are erected with heights proportional to the values, and all bars are filled with the same colour.

Example 12.3: major rice producing countries in 2012 (FAO): China 197221, India 120620 and Indonesia 66412 thousand tonnes lead; Pakistan is last with 7235.

Multiple bar diagram

Represents two or more variables for comparison, such as males and females in the total population, rural and urban population, or the share of canal, tube well and well irrigation. Example 12.4: India's population 1951 to 2011, from 361,088,090 to 1,210,193,422.

12.2.3 Pie diagram

A circular diagram: a circle of 360 degrees is divided into sectors whose angles at the centre are proportional to the magnitudes of the components.

1. Draw a circle using a particular radius.
2. Convert the proportional values of the components into degrees.
3. Angle value = (value of component / total value) x 360.