"Larry Lard" wrote in message
oups.com...
Stephen Osborn wrote:
John Rowland wrote:
"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.

I think you are wrong there.

Contours mark places of equal height. If two contours
touch at any one point then, de definito, they have to
touch at *all* points, so the two
contours become one contour.

This is a correct argument that two contours
_indicating the same height_ must be coincident
if they have at least one point in common;

No. There are many "saddle" points in the landscape where, say, the land is
lower to the north and south, and higher to the east and west. The contour
which marks the height of the saddle point runs away from the saddle point
in 4 directions. It would be perverse to describe the contour as crossing
itself, but the contour could meaningfully be described as touching itself
at this one point. This is as true for a map of isochrones or isobars.

--
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