Representing data: purpose and general rules

Detail

What "representation of data" means

• Representation of data means turning data into pictures you can see: graphs, diagrams, maps and charts.
• Raw data comes as numbers in tables or as text.
• A visual form shows the pattern at a glance.

• People remember a picture longer than a table.

• Key condition: the method must suit the nature of the data. Data that changes over time, data spread over space, and data made of parts of a whole each need a different method.

• Thematic map (a map on one theme or subject, such as population density, rainfall or soil, drawn on a simple base map). It is different from a general map, which shows many features together (relief, rivers, roads, towns).
• The Survey of India says one aim of a general base map, such as the International Map of the World (IMW) at 1:1,000,000 (the "millionth" scale), is to give a base on which thematic maps can be drawn: population, soil, geology, vegetation, resources, administrative limits and statistical evaluation [3].

Why we represent data visually

• The Nagpur example: Class 11 (Introduction to Maps) shows thematic maps of Nagpur district (Maharashtra). Class 12 (Graphical Representation of Data) says these maps would lose their visual impression if the same facts were given as tables or text, and would take longer to read.
• NCERT puts it in one line: "a picture is equivalent to thousands of words."
• What visual forms do well:
• Comparison is easy. For example, two districts' densities look different at once.
• They show patterns of:
  • population growth (how fast numbers rise over time)
  • population density (persons per square kilometre)
  • sex ratio (in India: number of females per 1,000 males)
  • age-sex composition (how many people are in each age group, split by male and female)
  • occupational structure (share of workers in farming, industry and services)

• They stay in memory longer than columns of numbers.

• Official practice in India: the Map Division of the Office of the Registrar General of India (ORGI) makes two kinds of maps [2]:

• administrative maps (before the Census), used to run the Census
• thematic maps (after the Census), used to share Census data
• These thematic maps are made at State/UT, district, sub-district or village level, so that the data can be studied by place (spatial analysis) [2].
• They are published in the Census Atlas, Administrative Atlas, Language Atlas, Historical Atlas of India and the District Census Handbooks (DCHB) [2].
• For each Census the Division makes more than 10,000 administrative and thematic maps [2].
• GIS (Geographic Information System: computer software that stores, analyses and maps data tied to places) came into the Division in the late 1990s. It made map-making faster and the maps better [2].

General rule 1: choose a suitable method

• The data's character decides the method. Ask two questions: does the data change over time, or is it spread over space? Is it a count or a ratio?

Kind of data	Suitable method	What the method is
Temperature over months; population growth over years	Line graph	Points for each time period joined by a line. The slope shows rise or fall.
Rainfall per month; production of a crop or mineral	Bar diagram	Bars of equal width. Bar length shows the value. Good for comparing separate amounts.
Distribution of population, livestock or a crop	Dot map	Each dot stands for a fixed number (e.g. 1 dot = 1,000 persons). Dots are placed where the people, animals or crops actually are.
Population density	Choropleth map	Each area (state, district) is shaded. Darker shade = higher value. Used for ratios and averages such as density.

• Trap: a dot map shows absolute numbers (actual counts). A choropleth shows a ratio (e.g. persons per sq km). Shading raw counts in a choropleth misleads, because big districts look "high" just because they are big.
• Example of a density figure to map: India's population density was 382 persons per sq km (NCERT: Census 2011).

General rule 2: choose a suitable scale

• Scale here means the value each unit of length on a graph or diagram stands for (e.g. 1 cm = 100 mm of rainfall). On a map, scale is the ratio between map distance and ground distance.
• The scale must cover all the data. The highest and lowest values must both fit.
• It must be neither too large nor too small.
• Too large → the diagram runs off the paper.

• Too small → bars or lines are squeezed together, and differences cannot be seen.

• Example: if monthly rainfall ranges from 5 mm to 650 mm, a scale of 1 cm = 100 mm gives a bar of 6.5 cm for the wettest month. That fits a page and still shows the dry months.

General rule 3: design

• Title
• It says three things: the area, the subject, and the reference year of the data.
• Example form: "Nagpur District — Population Density, 2011".

• It is placed at the top, in the centre.

• Legend (also called the key: a small box that explains every colour, shade, symbol and sign used)

• Without it the reader cannot decode the map.

• Usually placed in the lower left or lower right corner.

• North arrow (direction indicator)

• It shows which way north lies on the map.

• It is needed on maps, not on graphs. (Details of map design and scale are covered under maps and scale.)

• How a national mapping agency applies these rules (Survey of India, Handbook of Topography, Chapter XI):

• Before printing, a map is checked to make sure all marginal items (title, legend, scale) are correct and in the right position [3].
• When states are coloured (political tints), each state gets a distinct, contrasting colour. All Union Territories carry the same tint unless they touch each other. Foreign countries are not coloured [3].
• Hypsometric tints (layer colours, one for each height zone between two main contour lines) show relief on small-scale maps instead of dense contour lines [3].
• Do not overload the map. Too many place names "confuse detail and make the map difficult to read". It is better to show a few names than too many [3].

Diagrams by dimension

• Dimension here means how many measurements of the shape carry the value.

Type	What carries the value	Examples
One-dimensional	Length or height only	line graph, polygraph, bar diagram, histogram, age-sex pyramid
Two-dimensional	Area (length × width)	pie diagram, rectangular diagram
Three-dimensional	Volume	cubes, spheres

• Polygraph: a line graph with more than one line, each line for a different variable. Example: growth rates of several cities on one graph.
• Histogram: bars drawn touching each other, used for continuous class intervals (e.g. age groups 0–4, 5–9).
• Age-sex pyramid: two sets of horizontal bars, males on one side and females on the other. Each bar is one age group. The shape tells you whether a population is young (wide base) or ageing (narrow base).
• Pie diagram: a circle divided into slices. Each slice's angle shows that part's share of the total (share × 360°).
• Rectangular diagram: a rectangle divided to show the parts of a total.
• Cubes and spheres: volume shows the value. They suit very large differences in value but are harder to read exactly.

What NCERT actually constructs

• Only the commonest diagrams are drawn step by step:
• line graphs (and polygraphs)
• bar diagrams
• the pie diagram

• flow charts (also called flow maps: lines whose width shows the amount moving along a route, such as trains or goods between places)

• Named but not built in detail:

• wind rose (a diagram showing how often, and how strongly, wind blows from each direction at a place)
• star diagram (lines radiating from a centre, each line's length showing a value, such as a month's rainfall)

Prelims Hooks

• "A picture is equivalent to thousands of words": NCERT's reason for representing data visually, using the Nagpur district (Maharashtra) thematic maps.
• Method–data pairs: line graph → temperature or population growth over time; bar diagram → rainfall or production; dot map → population, livestock or crop distribution; choropleth → population density.
• Trap: a choropleth suits ratios/densities, not absolute counts. Absolute distribution goes on a dot map.
• Title = area + subject + reference year, at the top centre. Legend = usually lower left or lower right. North arrow = on maps only.
• One-dimensional: line graph, polygraph, bar diagram, histogram, age-sex pyramid. Two-dimensional: pie, rectangular diagram. Three-dimensional: cubes, spheres. Trap: the pie diagram is 2-D, not 1-D.
• Wind rose and star diagram are only named in NCERT. Line graph, bar diagram, pie diagram and flow chart are the ones constructed.
• ORGI's Map Division makes administrative maps (pre-Census) and thematic maps (post-Census), more than 10,000 maps per Census. GIS was adopted in the late 1990s [2].
• Census thematic maps appear in the Census Atlas, Administrative Atlas, Language Atlas, Historical Atlas of India and the District Census Handbooks [2].
• In Survey of India political maps, foreign countries are not coloured, and states get contrasting tints [3].

Mains Points

• Why visual data matters for planning (GS-I/GS-III):
• Census thematic maps at district, sub-district and village level show where density, sex ratio or literacy is low [2].
• This lets planners target schemes to a place rather than spread funds evenly.

• The GIS-based atlases and District Census Handbooks carry this data down to the village level [2].

• The wrong method gives the wrong message:

• A choropleth of absolute numbers exaggerates large districts.
• A badly chosen scale hides real differences.

• The three rules (method, scale, design) together make sure a map or graph is honest and easy to read.

• Good design is part of accuracy: title, legend, north arrow and an uncluttered layout (e.g. the Survey of India's rule of few names over many [3]) decide whether a reader, such as an administrator or citizen, reads a map correctly.