On 09/03/11 13:33, Michael Collinson wrote:
Certainly both Chris' and Phillip's cautions are certainly true but
I've paid particular attention to the River Wharfe mid-reaches, which
I know very well and flows in a well-defined channel with high banks
and has not shifted markedly in the last 40 years. In places, it is
almost twice as wide as it should be.
You have put your finger on the solution here Mike - local involvement.
Aerial images and OS data used as an extra by someone who knows the area
is the best way to resolve any inconsistencies. Importing data, OS or
not, without tempering it with local knowledge or surveys is usually a
bad idea.
Chris may be right in suggesting that the highest water mark is being
mapped, but why map the 10 - 25-year flood event level rather than the
natural bank line?
I was thinking more of the high tides that occur about once a month on
say the Humber or the Yorkshire Ouse and the spates that occur on rivers
like the Swale whenever there is moderately heavy rain.
I am tempted to think that automated software has been used which like
PGS coastlines occasionally gets confused by nearby lineaments. I also
recall comparing with digitised 25:000 maps (vintage 1900 - 1960
surveying) and noticing that it correlates much more closely with Bing
than StreetView. Needs more analysis but be aware!
Analysis by a local is, to me, the gold standard.
Mike
--
Cheers, Chris
user: chillly
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