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;
however consider contours marking _different_ heights; these can
coincide on a non-empty set of points (eg along a vertical cliff)
without necessarily coinciding everywhere.

--
Larry Lard
Replies to group please