Graphical Representation of Data, Class 12 Practical Geography

What Graphical Representation of Data is about

Key concepts

General rules for graphs, diagrams and maps

NCERT section General Rules for Drawing Graphs, Diagrams and Maps

1. Choose a suitable method for the data:

• Line graphs: temperature, or population growth over time and across countries or states.
• Bar diagrams: rainfall, production of commodities.
• Dot maps: distribution of human or livestock population, or crop areas.
• Choropleth maps: population density.

2. Choose a suitable scale that covers the whole data set; neither too large nor too small.
3. Design (a key cartographic task):

• Title: name of the area, reference year, and caption; normally centred at the top, with subtitle and year.
• Legend (index): explains colours, shades, symbols and signs; normally at the lower left or lower right.
• Direction: a North arrow, properly placed.

Types of diagrams:

• One-dimensional: line graph, polygraph, bar diagram, histogram, age-sex pyramid.
• Two-dimensional: pie diagram, rectangular diagram.
• Three-dimensional: cube and spherical diagrams.

Line graph and polygraph

NCERT section Line Graph; Polygraph

Line graph: for time-series data such as temperature, rainfall, population growth, birth and death rates.

Construction:
1. Round the data (e.g. growth rates 1.96 and 2.22 for 1961 and 1981 can be taken as 2.0 and 2.2).
2. Draw the axes: time (years or months) on the X-axis, the value on the Y-axis.
3. Choose and label a Y-axis scale; if any value is negative, the scale must extend below zero (India's growth rate in 1921 was -0.30 per cent, Table 3.1).
4. Plot each value as a dot and join the dots with a free-hand line.

Polygraph: a line graph with two or more variables, each as its own line, for instant comparison: e.g. growth of rice, wheat and pulses, or birth rate, death rate and life expectancy. Use different line styles (solid, broken, dotted, dot-dash) or colours. NCERT's example compares the sex ratio of Delhi, Haryana and Uttar Pradesh, 1961-2011 (Delhi rose from 785 to 866 females per 1,000 males).

Bar diagrams: simple, line-and-bar, multiple and compound

NCERT section Bar Diagram

Bar (columnar) diagrams use columns of equal width. Rules: all bars the same width, placed at equal intervals, and shaded with colours or patterns.

• Simple bar diagram: one variable, for immediate comparison. Arrange data in ascending or descending order, but keep time-series data in time order. Example: monthly rainfall of Thiruvananthapuram, X-axis divided into 12 months, Y-axis at 5 cm intervals; the wettest month is June (35.6 cm).
• Line and bar graph: combines two related variables, typically climate: months on the X-axis, temperature as a line (scale on the right Y-axis, 5 or 10 degree C steps) and rainfall as bars (scale on the left Y-axis, 5 or 10 cm steps). Example: Delhi.
• Multiple bar diagram: two or more variables side by side for comparison, e.g. males and females in total, rural and urban population; canal, tube-well and well irrigation by state. Example: India's literacy (total, male, female) 1951-2011.
• Compound bar diagram: components of one total shown as divisions of a single bar. Example: India's gross electricity generation (thermal, hydro, nuclear) for 2008-09 to 2010-11; total rose from 741.2 to 844.8 billion kWh.

Pie diagram

NCERT section Pie Diagram

A pie (divided circle) diagram shows a total as a circle, divided into sectors whose angles represent the parts.

Angle = (value of a part / total value) x 360. If data are already percentages, angle = percentage x 3.6 (since 360 / 100 = 3.6).

NCERT example (Table 3.7): India's exports by region, 2010-11:

Region	Per cent	Calculation	Angle
Asia and ASEAN	56.2	x 3.6 = 202.32	203 deg
Europe	20.2	x 3.6 = 72.72	73 deg
America	14.8	x 3.6 = 53.28	53 deg
Africa	6.5	x 3.6 = 23.4	23 deg
Others	2.3	x 3.6 = 8.28	8 deg

(NCERT rounds the Asia angle up so the total is 360 degrees.)

Construction: pick a radius of 3, 4 or 5 cm; draw a radius line from the centre; measure angles clockwise in ascending order, starting with the smallest; add title, subtitle and legend with distinct shades.

Precautions: the circle should be neither too big nor too small; starting with the bigger angle accumulates error and makes the small angles hard to plot.

Flow maps

NCERT section Flow Maps/Chart

A flow chart or map combines a graph and a map to show the movement of goods or people between origin and destination. It is also called a dynamic map; a transport map showing passengers or vehicles is the best example. Lines are drawn with width proportional to the flow.

It usually shows (1) the number and frequency of vehicles by direction, or (2) the number of passengers or quantity of goods moved.

Requirements: a route map with connecting stations; flow data with origins and destinations; a scale for the flow.

NCERT example (Delhi trains, Table 3.8): the busiest route (Old Delhi-New Delhi) has 50 trains and the least busy (Udyog Nagar-Tikri Kalan) 6. With a scale of 1 cm = 50 trains, these become strips 10 mm and 1.2 mm thick. Draw each route's strip, add a terraced scale as legend, and mark stations with a distinct symbol.

Water flow map (Ganga basin): a strip 1 cm wide = 50,000 cusecs of water.

Thematic maps: dot and choropleth

NCERT section Thematic Maps; Dot Maps; Choropleth Map

Graphs compare data but can miss the regional picture, so thematic (distribution) maps are drawn.

Requirements: state or district data on the theme; an outline map with boundaries; a physical map (e.g. physiography for population, relief and drainage for transport).
Rules: show the area name, title, data source and year, symbols and colours, and scale; choose a suitable method.
Types: quantitative (statistical) maps show measured variation (e.g. rainfall above 200 cm, 100-200, 50-100, below 50 cm); non-quantitative (qualitative) maps show non-measured differences (high and low rainfall areas).

Dot map: shows distribution (population, cattle, crops) with same-size dots, each of fixed value.

• Needs an administrative map, statistical data, a dot value, and a physiographic map.
• Steps: choose dot size and value; compute dots per unit (with 1 dot = 1 lakh people, Maharashtra's 9,67,52,247 people in 2001 gives 967.52, rounded to 968 dots); place dots, putting fewer in mountains, deserts and snow-covered areas.
• Precautions: boundary lines not too thick; all dots the same size.

Choropleth map: shades administrative units by values such as density, literacy, growth rate or sex ratio.

• Steps: arrange data in order; make 5 classes (very high to very low); class interval = range / 5, where range = maximum - minimum; use shades in increasing or decreasing order.
• NCERT example (literacy 2001): Bihar lowest (47.0 per cent), Kerala highest (90.9 per cent). Range taken as 91.0 - 47.0 = 44.0; 44 / 5 = 8.8, rounded to 9. Classes: 47-56 very low, 56-65 low, 65-74 medium, 74-83 high, 83-92 very high (Mizoram, Kerala).

Isopleth maps and interpolation

NCERT section Isopleth Map; Interpolation

For phenomena that vary continuously over space, ignoring administrative boundaries (slope, temperature, rainfall), draw isopleths: lines joining places of equal value ('iso' = equal).

Common isopleths: isotherm (temperature), isobar (pressure), isohyet (rainfall), isoneph (cloudiness), isohel (sunshine), contour (height), isobath (depth), isohaline (salinity).

Requirements: base map with point locations; data for a definite period; drawing instruments such as a French curve.
Rules: use an equal interval, ideally 5, 10 or 20; write the value along the line on either side or in a break in the middle.

Interpolation: estimating values between two observed points; drawing isopleths is itself called interpolation.
1. Find the minimum and maximum values; range = maximum - minimum.
2. Choose a whole-number interval (5, 10, 15 ...).
3. Position of the isopleth from the lower point = (distance between points / difference in values) x (isopleth value - lower value).
4. Draw the lowest-value isopleth first.

NCERT example: two places 10 mm apart record 28 and 33 degrees C. The 30 degree isotherm is (10 / 5) x 2 = 4 mm from the 28 degree point (6 mm from the 33 degree point).

Graphical Representation of Data in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Graphical Representation of Data: key facts

Pie angle

$Angle=(part/total)x360;orpercentagex3.6$

Plot smallest angle first, clockwise.

Dots per unit

$Numberofdots=value/valueofonedot$

Round to the nearest whole number.

Choropleth interval

$Classinterval=(maximum-minimum)/5$

Five classes: very low to very high.

Flow line width

$Width=flowx(mapunitsperflowunit)$

1 cm = 50 trains: 50 -> 10 mm, 6 -> 1.2 mm.

Isopleth position

$Distancefromlowerpoint=(distancebetweenpoints/valuedifference)x(isoplethvalue-lowervalue)$

28 and 33 deg C, 10 mm apart: 30 deg C at 4 mm.

Ideal isopleth interval

$5,10or20$

Keep intervals equal.

Tricks to remember Graphical Representation of Data

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Map design essentials

  "TLD": Title at top, Legend at the bottom corner, Direction arrow.

  Use it when: Every diagram or map.

• Bar types

  "Multiple = side by side; Compound = stacked in one bar."

  Use it when: Telling bar diagrams apart.

• Thematic maps

  "Dots for distribution, Choropleth for ratios, Isopleth for continuous."

  Use it when: Choosing a map.

• Iso lines

  "Therm-heat, Bar-pressure, Hyet-rain, Neph-cloud, Hel-sun, Bath-depth, Haline-salt."

  Use it when: Isopleth names.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which map is called a dynamic map?Show answer

   The flow map.

2. 2.What constant converts percentages to pie angles?Show answer

   3.6.

3. 3.Into how many classes does NCERT divide choropleth data?Show answer

   Five.

4. 4.Which map shows population distribution?Show answer

   A dot map.

5. 5.What is an isohyet?Show answer

   A line joining places with equal rainfall.

6. 6.Where is the legend normally placed?Show answer

   At the lower left or lower right of the sheet.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you choose the right diagram for a data set?
• ☐Can you state where the title, legend and north arrow go?
• ☐Can you name one-, two- and three-dimensional diagrams?
• ☐Can you draw a line graph that includes a negative value?
• ☐Can you distinguish multiple and compound bar diagrams?
• ☐Can you draw a combined temperature-rainfall graph?
• ☐Can you calculate pie angles from percentages?
• ☐Can you set a flow-map scale?
• ☐Can you calculate dots per state and choropleth classes?
• ☐Can you interpolate the position of an isopleth?

Graphical Representation of Data: frequently asked questions