A few screenshots may help

AreaBuilder will create areas from the contour lines. If you set "Create donuts" to Yes it will cut out the holes for enclosed contour areas. The LineOnAreaOverlayer will allow you to add the information of the lines to the areas (use the list attribute to store attributes of multiple lines). Finally you need to copy the height of both lines (from the list) to attributes using AttributeManager or AttributeCreator.

Thanks for this @erik_jan - would it be able to populate those areas which are either deeps (with no deeper contour) or shoals (with no shoaler contour)? Does that make sense? These areas, for which there are about 500 are the tricky ones.

You can still use the create area from contour. Create donuts, keep holes.

Then you do a spatial relator where candidate and and base are the same input. Do either contain or within.

Then test related candidates. And the countourvakue difference.

I assume you have some tolerence to call sommething shoal and how deep deep is .

Related_candidates >= 0 and contour value <= contourvalue.related{} and

(contour value - contourvalue.match{}) <=sometolerence

would be no shoals. (depending if i understand the shoal thing correctly)

The technique is applicable though.

Thanks, sounds complex, I'll have a look, probably tomorrow and post my setup when/if it doesn't work. J