The essence of this chapter for last-minute revision: the three general rules of construction, every diagram/map’s formula and every worked example the chapter itself gives, the three thematic-map types, and the two named-but-not-covered gaps a topper should know about. The full notes, with the complete explanation, are available separately.
1 Definitions to learn by heart
Representation of data is the transformation of data through graphs, diagrams, maps and charts, making patterns of population growth, distribution, density, sex ratio and occupational structure easy to understand.
Diagrams are classified by dimension: one-dimensional (line graph, poly graph, bar diagram, histogram, age-sex pyramid), two-dimensional (pie diagram, rectangular diagram), three-dimensional (cube, spherical diagrams). This chapter explains only line graphs, bar diagrams and pie diagrams from this list, plus flow charts and three thematic map types.
An Isopleth (Iso = equal, pleth = lines) is an imaginary line joining places of equal value, on natural (not administrative) boundaries. A choropleth map, by contrast, ties its data to administrative units.
2 The three general rules of construction
| Rule | What it means |
|---|---|
| 1. Selection of a suitable method | Temperature/population growth → line graph; rainfall/production → bar diagram; population/livestock distribution → dot map; population density → choropleth map |
| 2. Selection of a suitable scale | Neither too large nor too small; must cover the entire data range |
| 3. Design | Title (name of area + year + caption, centred at top) · Legend/Index (colours/shades/symbols, lower-left or lower-right) · Direction (north symbol) |
3 Line graphs, bar diagrams: formulas and worked examples
| Type | Construction rule | Worked example | Answer |
|---|---|---|---|
| Line graph | Round the data → draw X/Y axes → choose scale (show negative values) → plot dots, join freehand | Ex. 3.1: India’s population growth rate, 1901-2011 (Table 3.1); 1961 (1.96) rounds to 2.0, 1981 (2.22) rounds to 2.2 | Fig. 3.2, includes a negative value for 1921 (−0.30) |
| Polygraph | Line graph with 2+ variables, distinguished by line pattern (straight/broken/dotted/dot-dash) or colour | Ex. 3.2: sex ratio of Delhi/Haryana/Uttar Pradesh, 1961-2011 (Table 3.2) | Fig. 3.3 |
| Simple bar diagram | Equal width, equal spacing; ascending/descending order, EXCEPT time series (kept in time order) | Ex. 3.3: Thiruvananthapuram monthly rainfall (Table 3.3) | Fig. 3.4 |
| Line and bar combined | Two Y-axes (one each side) for two differently-scaled variables | Ex. 3.4: Delhi monthly temperature (°C) + rainfall (cm) (Table 3.4) | Fig. 3.5 |
| Multiple bar diagram | Two or more variables, separate adjacent bars | Ex. 3.5: India literacy rate 1951-2011, total/male/female (Table 3.5) | Fig. 3.6 |
| Compound bar diagram | Different components of ONE variable, shown as segments of a SINGLE bar | Ex. 3.6: India’s electricity generation, thermal/hydro/nuclear (Table 3.6) | Fig. 3.7 (2010-11 total: 844.8 billion kWh) |
Multiple vs compound bar diagram is a named exercise question: multiple = separate adjacent bars for different variables; compound = segments WITHIN one bar. Do not mix these up.
4 Pie diagram: formula and the book’s own rounding quirk
Angle = (Value of category ÷ Total value) × 360°, or from percentage: Angle = Percentage × 360 ÷ 100 = Percentage × 3.6
| Region (Table 3.7, India’s exports 2010-11) | % | Calculation | Angle |
|---|---|---|---|
| Europe | 20.2 | 20.2 × 3.6 = 72.72 | 73° |
| Africa | 6.5 | 6.5 × 3.6 = 23.4 | 23° |
| America | 14.8 | 14.8 × 3.6 = 53.28 | 53° |
| Asia and ASEAN | 56.2 | 56.2 × 3.6 = 202.32 | 203°* |
| Others | 2.3 | 2.3 × 3.6 = 8.28 | 8° |
*The book’s own table prints “202.32” in its Calculation column but “203°” in its Degree column, not an error. Since the five angles must sum to exactly 360° (73+23+53+8=157, and 360−157=203), the book adjusts the largest category by +1°. Precautions: circle neither too big nor too small; measure angles ascending, SMALLEST first (starting with the biggest angle accumulates error).
5 Flow map (Dynamic Map)
A combination of graph and map, drawn with lines of proportional width to show flow of vehicles/people/goods between origin and destination. Requirements: route map with stations, flow data with origin/destination, and a scale for line thickness. Ex. 3.8 (Table 3.8: 16 Delhi-area rail routes, 6 to 50 trains) and Ex. 3.9 (Ganga basin, 1 cm = 50,000 cusecs) both use a terraced-scale legend.
6 Thematic maps: the three types compared
| Map type | Shows | Key formula/rule | Worked example |
|---|---|---|---|
| Dot map | Distribution (population, cattle, crops) via equal-size dots | Dots = Value ÷ chosen dot-value | Ex. 3.10: Maharashtra 96,752,247 ÷ 100,000 = 967.52 → 968 dots (fraction > 0.5 rounds up) |
| Choropleth map | Administrative-unit data (density, literacy, sex ratio) in 5 categories | Interval = Range ÷ 5 (Range = Max − Min) | Ex. 3.11: India literacy 2001, Range = 91.0−47.0 = 44.0, Interval = 44.0/5 = 8.8 → 9.0; bands 47-56, 56-65, 65-74, 74-83, 83-92 |
| Isopleth map | Equal-value lines (temperature, rainfall) on natural boundaries, spatially continuous data | Point of isopleth = (Distance ÷ value-difference) × Interval | 28°C/33°C example: 30°C isotherm is 4 mm from 28°C or 6 mm from 33°C |
Interval for isopleths: 5, 10, or 20 is ideal. Draw the isopleth of the MINIMUM value first. The 8 named isopleths: Isotherm (temperature), Isobar (pressure), Isohyet (rainfall), Isoneph (cloudiness), Isohel (sunshine), Contour (height), Isobath (depth), Isohaline (salinity).
The book’s printed dot-count column does not follow its own “round up if fraction > 0.5” rule for every one of its 29 rows (a handful, including Mizoram, print an inconsistent figure). For any exam calculation, use Maharashtra’s method exactly as demonstrated: it is the one the book’s own text walks through and is fully consistent.
7 What this chapter does NOT cover (a scope trap)
The chapter’s own introductory list names five diagrams: line graphs, bar diagrams, pie diagram, wind rose and star diagram, and flow charts, but it never gives the wind rose and star diagram a construction method anywhere in the chapter. If asked to name the five types, include it; if asked how to draw it, there is no book method to reproduce.
- 3 general rules: method, scale, design (title/legend/direction)
- Pie angle: %×3.6 · Choropleth interval: Range÷5 · Isopleth point: (distance÷value-diff)×interval
- India exports 2010-11 angles: Europe 73°, Africa 23°, America 53°, Asia&ASEAN 203°, Others 8° (sums to 360°)
- Maharashtra dot count: 967.52 → 968 · India literacy 2001 choropleth interval: 8.8 → 9.0, bands 47-56…83-92
- Isotherm worked example: 30°C is 4mm from 28°C, 6mm from 33°C
- 3 thematic map types: dot, choropleth, isopleth (isopleth’s 8 named types: Isotherm, Isobar, Isohyet, Isoneph, Isohel, Contour, Isobath, Isohaline)
- Wind rose and star diagram: named, never constructed, in this chapter