[Talk-GB] (OT) Depressions, and projections

[Phil Endecott]

Donald Allwright wrote:
>>(1) Can you think of any depressions in Britain?  I.e. places where the contour lines on the map would wind the "wrong way"?  I ask because I recently noticed that the OS OpenData contour data is wrong around Princes Street Gardens in Edinburgh.  It looks as if the contour lines are in the right places but their heights have been mis-labelled, as if the height labels had been added automatically by an algorithm that didn't expect depressions.  The gridded data is consistent with the incorrect labelling, probably confirming that the gridded data is derived from the contours, rather than vice-versa.  The only other depressions that I can think of are the sunken areas in the Fens.  Any other suggestions?
>
>
> There are many depressions on a small scale in carboniferous limestone areas, where rivers/streams have disappeared underground or where caves have collapsed. I can think of a number but whether there are any that are large enough to show up in OS data is another question.

Yes; I am an occasional caver, as it happens.  There are a few places 
where my 1:25,000 OS maps show a depression contour around a cave 
entrance, including Gaping Ghyll, Stream Passage Pot and Hunt Pot in 
Yorkshire, and Porth Yr Ogof in Wales.  But none of those show up on 
1:50,000 OS maps or in the OpenData contours.

One interesting spot is where Manchester Airport runway was extended 
over the Bollin Valley.  The river now flows under the runway in a 
culvert, making the upstream valley a depression as far as Styal.  
Unfortunately, the runway was built after the OS OpenData contours were 
frozen.  But maybe there are some other places like this?

Any other examples?


Thanks,  Phil.