It is being described, with no sense of false modesty, as the biggest exercise in human history: counting and digitally archiving data on each of India's estimated 1.2 billion people. The epic task of the Indian census gets under way today, gathering information on jobs, education, income, migration and mortality, data vital to planning the development of the second most populous nation. But it will also gather fingerprints and iris scans as part of the most ambitious aspect of the project: issuing all Indians with hi-tech identity cards. Officially the 2011 census, it will be conducted by 2.1 million census takers over three years. Home Minister P. Chidambaram described the operation, conducted every 10 years, as 'the biggest exercise in the history of mankind - the only instance of a government anywhere in the world counting, identifying and issuing photo identity cards to an estimated 1.2 billion people'. The 2000 census China did likely involved more people, counting the population at 1.295 billion. The Indian tally is not expected to exceed that, but the amount of data being gathered will be unprecedented for any census. According to C. Chandramouli, the registrar general and census commissioner, India has held censuses every 10 years without a break since 1872, through the turmoil of the nationalist movement, world wars, partition of the subcontinent, post-independence riots, Sino-Indian and India-Pakistan wars, floods and earthquakes. This time, enumerators will also collect information for India's first National Population Register (NPR) - the digital database of every single Indian. 'The census is a purely statistical exercise but NPR is an individual-specific survey. The census will want to know how many people in a village are literate, not whether Sita Devi or Fatima Bibi can read and write - that's the NPR's domain,' said Varsha Joshi, a director of census operations in the national capital, Delhi. At present, Indians produce ration cards, voter IDs, passports, driving licences and income tax cards to prove their identity. But under the NPR plan, every Indian will be issued an ID card with a Unique Identification number. Biometric identifiers - fingerprints and pictures of the eye - will be stored online for instant online authentication of the identity of each Indian. Last year, Prime Minister Manmohan Singh's government brought in Infosys Technologies co-chairman Nandan Nilekani to head the Unique Identification Authority of India. Nilekani, who resigned from the Infosys board of directors and was inducted into Singh's cabinet, says the ID project 'is the first of its kind in the world with no model available to us to emulate'. In the first phase of Census 2011 starting today in West Bengal, Assam and Andaman and Nicobar Islands, enumerators will fan out with two questionnaires, one about the dwelling, another seeking the personal details of the occupants. Analysts say the national census has survived for 138 years because there is no alternative. 'Small developed countries like the Nordic nations have stopped conducting censuses and rely on birth and death registration and immigration records to calculate the population,' said Pronab Sen, India's chief statistician. 'But we can't do that because barely 60 per cent of births and deaths are registered in India. For reliable data, there is no substitute for the old-school census. 'But the data should be disseminated fast. Even in this age of computers, the complete data takes years to come. We still have not got all the information gathered in the 1991 census,' he said.