A brilliant new online interactive map produced by the International Institute for Environment and Development allows you to investigate the scale and speed of urban transformation across the globe.

At the top of the map is a slider permitting you to view cities with populations above 750,000 people from any year between 1800 and 2025 (based on current data).

The size of a dot tells you how big the city is. Hovering the cursor over any city brings up data about that city. You can also see how many cities there were in the world above that size in that year.

For example in 1800 there were only three cities with populations above 750,000. By 1900 there were 25. This increased to 105 in 1950, 473 in 2000 and 611 in 2010. The map does not project the number after 2015.

The visualisation immediately brings home where the most growth has occurred in recent years: in China and India. By contrast Europe has hardly changed in the last half-century, nor has north America's east coast, up till the recent Asian expansion site of the largest conurbations.

Interesting data is revealed for each milestone year. In 1965, Tokyo exceeded 20 million inhabitants and Rio de Janeiro and São Paulo exceeded 5 million. If rejected populations prove accurate, by 2025 Tokyo will have expanded to hold 38 million inhabitants and São Paulo will be joined by Shanghai, Mumbai, Mexico City, new York-Newark, Dhaka, Beijing and Karachi with over 20 million inhabitants.

By that time Africa will also have many more large cities and three megacities: Lagos (19 million), Kinshasa (14.5 million) and Cairo (Al-Qahirah) (14.7 million).

Viewing development in this way allows you to see the long history of cities particularly in China and India (and the short history of large cities in USA). In 2020, China has will have 27 of the world's largest cities; India 9, the USA 12, Brazil 5, Japan 2 and the UK 1. Compare this with 1950: China 11, India 6, the USA 18, Brazil 2, Japan 6 and the UK 7. But go back to 1800 and China had 23 of the world's 100 largest cities; India had 19, the USA 1, Brazil 1, Japan 4 and the UK 5.

Go play with the map and tell us what you find.