Hi dangalf
I did the map for a longer article about school admission distances over on our blog here.
I got central co-ordinates from the Council for where they calculate the distances from and used a mapping website to draw radius circles around them using the distances that I got from the Council (which you can see [[http://local.mumsnet.com/Talk/local_wandsworth/2085859-School-Offer-Distances-2014 here]]).
Given that the figures are as current as can be and the point of measurement is the same the council uses, the map is an accurate reflection of where you'd have had to live to have been offered a place for Sept 2014 at the first offer date.
BUT, of course the distances will shift a bit by September as the waiting lists move AND the distances can vary hugely year on year (see my blog post for details) so a 'catchment' map can only ever really be illustrative.
I know a few people who bought houses that were in catchment one year, only to find that they'd slipped out of it by the time their kids were ready to start school the next year. It is dependent on so many factors (number of siblings, new schools opening nearby, OFSTED re-grading, birth rate etc...) that it is very hard to accurately predict.
Audrey01 I'm aware that something similar has now popped up on the 'wine region sounding' website. Please note that our map appeared a good 2 weeks' before theirs did - wonder where they got the idea from 
Whilst we are on the topic of the 'Californian wine region' sounding site (and thanks very much Audrey for the new name for it) - we have had a problem with our local talk boards in a few areas being spammed with links to it in the last week or so. So just a heads up that we will delete posts that do that from now.
Sorry to come over all schoolma'am - as you were 