Choropleth map

Topic: Graphical Representation of Data and Thematic Maps · NCERT: Class 12, Ch 3 "Graphical Representation of Data"; Class 12, Ch 4 "Spatial Information Technology"

Meaning

A choropleth map is a thematic map in which each administrative unit (state, district, tehsil or country) is shaded in graded tones of one colour, from light to dark. Each shade shows the average or ratio value for that unit, such as population density or literacy rate. It is the most common thematic map in Census atlases, district handbooks and school textbooks, so it is the basic tool for showing how a variable changes from one region to the next.

Explanation

What it shows and what you need

• The name comes from Greek: choros = place, plethos = value.
• It shows ratios or averages, never raw totals. Typical variables:
• Population density (persons per sq km)
• Literacy rate (%)
• Population growth rate (%)

• Sex ratio (females per 1,000 males)

• The boundaries are fixed in advance. The data is fitted into units that already exist. The map does not draw new boundaries.

• You need two things:
• A base map of the administrative units (for example, states and UTs of India).
• One value for each unit, for the same variable and the same year (for example, literacy %, Census 2001).

How to make one (NCERT method: equal intervals)

1. Arrange the data in ascending or descending order.
2. Find the range: range = maximum value − minimum value.
3. Find the class interval: range ÷ 5. This gives five classes.
4. Name the classes: very low, low, medium, high and very high.
5. Shade from light (very low) to dark (very high). Use shades of one colour or one pattern, so that darker always means "more".
6. Add a legend that shows each class and its shade. Also add a title, scale, north arrow and source.

NCERT worked example: literacy, Census 2001

• Lowest: Bihar, 47.0%. Highest: Kerala, 90.9%.
• Range = 90.9 − 47.0 ≈ 44. Interval = 44 ÷ 5 ≈ 9.

Class	Range (%)	Shade
Very low	47–56	Lightest
Low	56–65	Light
Medium	65–74	Middle
High	74–83	Dark
Very high	83–92	Darkest

• Very high: Kerala (90.9%), Mizoram (88.8%) and Lakshadweep (86.7%). Lakshadweep is easy to miss because NCERT's class lists leave out several UTs.

The same steps with Census 2011 data

• Kerala was highest at 94% and Bihar was lowest at 61.8% [3].
• Range = 94.0 − 61.8 = 32.2. Interval ≈ 6.4.
• The class limits move, but the ranking does not:
• Kerala is still at the top and Bihar is still at the bottom.
• The whole country moved up, so "medium" in 2011 is a different number from "medium" in 2001.
• Always read the legend.

The golden rule: ratios, not totals

• Why raw totals give a wrong picture:
• A raw total grows with the size of the unit.
• Big states like Uttar Pradesh or Rajasthan would look "high" just because they are large or have many people.

• The map would then show size, not the real intensity of the variable.

• Absolute counts belong on dot maps or proportional symbol maps (circles or spheres).

Limits of choropleth maps

• One value for the whole unit.
• The whole unit gets one shade, so the map suggests the value is the same everywhere inside it.
• In reality, a district has crowded towns and empty forests.

• Cartographers call this the assumption that the phenomenon is evenly spread across the enumeration unit (the area for which data is collected, such as a census district) [2].

• Sudden jumps at boundaries.

• Values seem to change suddenly at a state or district line.

• Real change is usually gradual. This is called the artificial transition at the boundaries of mapping units [2].

• Large units with few people catch the eye.

• Big, thinly peopled units fill most of the map, so readers give them too much weight.

• Small, crowded units almost disappear.

• The way you make classes changes the story.

Method	How classes are made	Effect
Equal interval (NCERT)	Range ÷ number of classes	Easy to read. Many units may fall into one class if the values are bunched together.
Quantiles	Same number of units in each class	Every shade gets used. Very different values may share a class.
Natural breaks (Jenks)	Breaks placed at natural gaps in the data	Fits the data well. Classes change from map to map, so maps are hard to compare.

• Two-colour shading can mislead. An election map may shade a whole state in one party's colour when that party won only 50.1% of the vote. This hides the other 49.9% of voters [5].

Where it occurs

• India, Census mapping: state-wise and district-wise maps of literacy, sex ratio, population density and growth rate in Census atlases, district handbooks and NCERT textbooks.
• Literacy, Census 2011: India 72.98%, Kerala 94% (highest), Bihar 61.8% (lowest) [3].

• Literacy rose from 18.32% in 1951 to 72.98% in 2011 [3].

• Gender gap in literacy (Census 2011), a good choropleth variable: the gap was smallest in Meghalaya (3.1 percentage points), below 5 points in Kerala and Mizoram, and largest in Rajasthan (27.1 points) [3].

• Big, empty units that look too important on Indian maps: Ladakh, Rajasthan and Arunachal Pradesh.
• Small, crowded units that almost disappear: Delhi, Chandigarh and Lakshadweep.
• On world maps: Russia, Canada and Australia fill most of the map even though they have few people.
• Places where a plain choropleth spreads people onto empty land (so a dasymetric map works better):
• the Thar Desert (western Rajasthan)
• the forested Western Ghats and north-eastern hills
• the Rann of Kutch (Gujarat)
• the Sundarbans mangroves (West Bengal)

Don't confuse with

• Dot map / proportional symbol map: these show absolute counts (total population, total output) with dots or sized circles. A choropleth shows only ratios or averages by shading.
• Dasymetric map (Greek dasys = dense, metron = measure): this map redraws the zones using ancillary data (extra data that helps place the values), such as land use and land cover (LULC). Forests, deserts and water are removed, and people are placed only on settled and farmed land [2]. A choropleth keeps the fixed administrative boundaries.
• Cartogram (also called a value-by-area map): here the size of each unit is drawn in proportion to a variable, so shapes are bent on purpose [6]. A choropleth keeps the true shape and area and changes only the shade. On a population cartogram, India and China swell, while Russia and Australia shrink to thin strips [4].
• Isopleth map (settled textbook geography): it joins places of equal value with lines (isotherms, isobars) that do not follow administrative boundaries. A choropleth shades whole administrative units.

Prelims Hooks

• A choropleth map shows ratios or averages (density, literacy %, sex ratio) for administrative units, not absolute counts.
• NCERT steps: arrange data → range = max − min → interval = range ÷ 5 → five classes → shade light (very low) to dark (very high).
• NCERT literacy map (Census 2001): Bihar 47.0% to Kerala 90.9%. Range 44, interval ≈9. "Very high" (83–92) = Kerala, Mizoram (88.8%), Lakshadweep (86.7%).
• Census 2011 literacy: Kerala 94% (highest), Bihar 61.8% (lowest). Smallest gender gap: Meghalaya. Largest: Rajasthan [3].
• Trap: "A choropleth map shows how values change inside an administrative unit." False. It assumes one uniform value for each unit [2].
• Trap: equal interval, quantile and natural-breaks classes made from the same data can produce maps that look very different.

Mains Points

• Map choice affects policy (GS-I):
• A state-level literacy choropleth (Kerala 94% vs Bihar 61.8%, Census 2011) [3] hides gaps inside each state, such as female literacy (Bihar 51.5%) and rural–urban differences.

• District-level or dasymetric maps are needed to send schemes to the right blocks.

• Disaster exposure and planning (GS-III):

• Whole-district averages spread people evenly, even over forest, desert and water.
• Dasymetric maps built on LULC data place people where they actually live [2].

• This gives a better count of who is exposed to floods (Brahmaputra and Kosi plains), cyclones (Odisha and Andhra coasts) or heatwaves.

• Honest data communication:

• Area-based world choropleths give too much weight to thinly peopled giants (Russia, Canada, Australia). Population cartograms show the real human weight of South Asia and China [4]. This matters in debates on climate justice and per-capita emissions.
• Winner-takes-all election shading hides close results (50.1% vs 49.9%) [5]. Finer classes or cartograms give a fairer picture.

Related concepts

• Dasymetric map
• Cartogram

Read more

• Choropleth maps, dasymetric maps and cartograms