The census currently underway in India isn’t the world’s largest -- that distinction goes to China -- but it is likely to be the most comprehensive population count ever undertaken.

The Indian government has hired 2.5 million census workers and given them the mammoth task of visiting every settlement in the country, large and small, to count and document India's population of more than a billion -- a sixth of the world’s population.

But they aren't just counting and compiling names, which is a big enough job. They are going much further to try and get a true picture of where India and its people stand today.

Using satellite photos to help find more than 600,000 villages in the country, census workers plan to find everybody in India -- even if they are living in a mud hut, or sleep on the street. Every person also will have their fingerprints taken and be photographed.

The data will be used to give every citizen biometric identity cards, and will also include a person’s income, religion and education.

But census workers won’t be asking what caste a person is in. Officials decided not to ask about caste because they thought the subject too controversial, and one that could weaken the results of the census. Instead, there will be a separate caste census held later this year.

The census in India dates back to 1872. But they have been conducted far earlier than that in other areas of the world.

Egypt is believed to have held censuses more than 5,000 years ago during Pharaonic times, and they are also mentioned in the Bible.

Governments have often used the information in the past to control their people and also as a useful tool to tax them.

The Romans became the most enthusiastic proponents of financing the state in that way.

India's census, though, is designed to help identify the people and areas most in need of help. While compulsory, the census results aren’t open to inspection, and aren't admissible as evidence in court.

Despite India's strong economic growth, its effect across the country has been very patchy, leaving many millions untouched by the new prosperity. Indian officials hope that by finding out if people have access to schools, for instance, they will be able to improve the numbers of people who will be able to learn to read and write in the future.

India is likely to be the most populous country in the world by 2025 if its current population growth continues to outpace that in China.

The hope is that by that time, its current pace of economic development and modernization will have allowed many of its people to have improved their lives -- with the help of the census.