How come Google got it wrong?

We've noticed that various 'Points of Interest' on Google maps in Cambridge city centre are just plain wrong. For example Christ's College appears at the junction of St Andrew's Street and Downing Street when it's actually several hundred meters north on the east side of Hobson's Street. Emmanuel College and the Museum of Archaeology and Anthropology are also in the wrong place, though not so drastically. There are others.

I think Google have Christ's College in the wrong place because they think its post code is CB2 3AR when it should be CB2 3BU (even that's still not ideal for something as big as a college since it identifies the delivery entrance rather than anything suitable for visitors, but it's better than nothing).

For Christ's this seems to be a common error (try Googling for "Christ's College CB2 3AR" to see the extent of the propagation of this error) but I suspect the fact that yell.com have got it wrong may be at the root of the problem. It might be possible to get this fixed, though it's going to take a very long time for the fix to propagate.

Emmanuel has a similar problem - Google has their post code as just 'CB2' so they have been put where 'St. Andrews Street CB2' resolves to. It's harder to track down where this error originates, and I haven't tried.

The Museum of Arch and Anth is more interesting. Google have their correct address of "Downing St, Cambridge, Cambridgeshire, CB2 3DZ", and "CB2 3DZ" resolves to their correct location. However "Downing Street, CB2 3DZ" resolves to the incorrect location shown on the map so there is something odd about the way the Google geolocation service handles this particular address (perhaps it doesn't always find "CB2 3DZ" in its database and so falls back to "Downing Street, CB2"?).

These errors are not surprising, because Google isn't a traditional map company. They don't go out and survey anything but just purchase, collect and aggregate data from various sources (TeleAtlas for the streets, their own search index for address information, etc, etc.). Some of this information is wrong, which isn't surprising - what's really surprising is that the results are as good as they are. But it does cause us a very real problem, because some people feel that we can't use anything based on Google maps for as long as it shows well known University locations in the wrong place.

Addition 2009-11-05: A colleague has pointed out that part of the airport at Cambridge is labelled "London Luton Airport".