The Makings of Cartography: Proportional Symbol Map of Population in India

A map of cities in India with populations greater than 2 million, 2014. The map highlights India in pink and neighbouring countries in grey. City populations are represented by larger or smaller translucent pink circles.

This is a proportional symbol map of major cities in India created through ArcGIS and Adobe Illustrator. I compare two methods of proportional symbol mapping: absolute and perceptual scaling.

(Click to enlarge image)

This week I have been tasked with creating a proportional symbol map of major cities in India through ArcGIS and Adobe Illustrator. Proportional symbols visualize both location and quantity, and can be made through 3 methods:

  • Absolute scaling: the size of symbols is in direct proportion to the value of the data. For example, if the value doubles, the symbol is also 2 times larger. This is how I have scaled the symbols in the map above.
  • Perceptual scaling: the size of the symbols is slightly larger than the value it is representing. For example, if the value doubles, the symbol is a bit over 2 times larger. This method was created as a result of psychological research that showed how readers perceive maps and tend to underestimate the value represented in circle symbols.
  • Range grading: Data is separated into category ranges with symbols that fit each category.

(Click to enlarge image)

Perpetual Scaling vs. Absolute Scaling

In this lab, I created 2 maps of Indian cities with populations of over 2 million in both perpetual and absolute scaling methods. As a cartographer, it is important to always ask yourself, "what can the reader understand through this map?" and whether you are effectively communicating your message.

So which method is more effective? Here is a list of pros and cons for both.

For my own "Cities in India" map, I think absolute scaling is the better method to communicate the data set because the symbols are easy to match up to the three population categories in the legend and the map is fairly simple for readers to comprehend. As well, in the perceptual scaling method, the symbols are smaller overall and text is harder to read.

This is my last assignment before my final project and I'm excited to decide on my own map idea!

Here are some skills I picked up in summary:
  • Acquired UN World Urbanization Prospects 2014 Microsoft Excel table of urban agglomerations in India
  • Calculated the size of symbols appropriate to each data value using Excel to accurately design proportional symbols
  • Designed proportional symbol maps using Adobe Illustrator and ArcGIS of the population of major cities in India thorough absolute and perceptual scaling methods