This month, the BBC publishes an online interactive map of Britain of a kind not seen before. It shows where the foreigners live. In a few clicks, users can see how many "immigrants" there are in their area, which countries they came from, and how that group is faring economically. Based on censuses and other official data, the map was commissioned by the left-of-centre Institute for Public Policy Research (IPPR), and it makes both challenging and at times uncomfortable viewing. Yet perhaps the most surprising thing is that it has not been done before now.
Until the map appeared, we could only guess how, say, Indians were doing compared with Bangladeshis. And yet the answer - those born in India comfortably outperform the British-born population, while those from Bangladesh struggle - poses important questions for government and the communities themselves.
We were probably unaware that one in four people living in London was born abroad and that in Wembley, synonymous with English pride, more than half the population is immigrant. Nor did we know that the third-largest group of immigrants are German-born people - mostly the children of British servicemen stationed on the Rhine.
Why didn't we know? The Home Office is obsessive about data, but for years its Immigration Research and Statistics Service has been focusing on asylum-seekers and "illegals", even though legal migration has been the biggest factor in our population growth. According to the IPPR research, Britain has experienced net immigration since 1994 of roughly 1.2 million people. That is equivalent to the population of Hampshire.
This is a policy area with profound implications, yet it appears that facts and figures are scarce. Some argue that they merely provide dry tinder for bigots, and it may be true that Britain's success in limiting the rise of the far right is a consequence of an unspoken compact between government and much of the media to leave well alone.
Certainly the Home Office Cohesion and Faiths Unit sees it as part of its remit to "promote a positive view of diversity", and staff are told to "encourage journalists to publish good news stories". Yet diversity is a social policy about which, as the Tory focus on immigration at the May election suggested, many in Britain are dubious. And the Home Office's own research finds that "the more ethnically diverse an area is, the less likely people are to trust others within that area". In other words, diversity appears to undermine community cohesion.
While conclusions such as these make us uncomfortable, this is no reason to ignore them. Social justice obliges us to examine the way immigrant groups thrive or barely survive. Improved understanding of how cultural diversity affects communities involves revisiting assumptions about education, housing and welfare.
Statistics can be abused. When it becomes known that only 12 per cent of Somali-born immigrants have a job, compared to a national employment rate of 73 per cent, someone will no doubt suggest that Somalis are "lazy" or a "burden". But we need these numbers, because it is shocking that Somali refugees are so poor and excluded. Ministers should be challenged over it. If we fail to engage with uncomfortable statistics, we leave a dangerous vacuum in which half-truth and racism can thrive.
In a globalised world, our national identity is being reshaped, and many find this frightening. A recent survey I commissioned at the BBC revealed a country that broadly supports the idea of multiculturalism and where racist attitudes, after a period of slight increase, appear to be in decline. Yet, asked whether they thought parts of the country didn't feel like Britain any more, a third of respondents agreed.
The lack of openness about immigration and multicultural- ism leaves many people feeling that no one is listening to their concerns and they imagine the worst. I recently received a letter from a viewer who wrote of how he wept for his grandchildren, convinced they would see England transformed into an Islamic caliphate. Immigration, he felt, was out of control and our sur- vey showing broad support for multiculturalism cannot have included people in "the shires and county towns". Instead, he suggested, we had packed our poll with Muslims.
In reply, I pointed out that the 1,000 people interviewed for the survey included 24 Muslims, reflecting their proportion in the British population. I also looked on our website to see what the immigration map revealed for the area around his village in Somerset. Barely three in 100 people in the locality were born overseas, and they were most likely to be Americans or German-born offspring of British servicemen.
The map paints a picture of Britain that may reassure "the shires and county towns". Immigration is, to a great extent, a London and south-east phenomenon, yet the trend is that the areas where pollsters find most concern about foreign arrivals are those where fewest immigrants live.
That said, continued and increasing migration into Britain seems bound to have a significant impact on our economy, our public services and community cohesion. Some effects will be positive, others negative. It is vital that we have the information with which to judge and debate.
Mark Easton is the BBC's home editor. The "Born Abroad" website can be found at www.bbc.co.uk/bornabroad