"Stephen Osborn" wrote in message
...

However the contours on an OS map (and AFAIK isobars on a weather chart)
never touch let alone cross.

They can touch, but they can't cross.

Whereas, in the example that Mike gave,
the isochrones will have to cross.

No, they won't. It's just the same as a weather map, it's just a map where
every point has a real number associated with it. Draw two isochrones
crossing each other, write various times on the isochrones and on the spaces
between them, and you'll see that it can't happen.

--
John Rowland - Spamtrapped
Transport Plans for the London Area, updated 2001
http://www.geocities.com/Athens/Acro...69/tpftla.html
A man's vehicle is a symbol of his manhood.
That's why my vehicle's the Piccadilly Line -
It's the size of a county and it comes every two and a half minutes