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VC 113 mapping discrepancies

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VC 113 mapping discrepancies

by Fred@nhm » Mon Apr 06, 2015 10:22 pm

Having just been trying to survey Isoetes histrix in Guernsey I was coming across problems locating previous DDB records most of which according to my map and GPS were in the sea! Some belated detective work shows that all our records are made using the ED-50 datum - as mapped on the old Guernsey OS map - last printed 1966 (and an expensive collectors item!). The new Guernsey map - published by the States in 2012 uses WGS-84 as its datum - and this is what I'd set my GPS to. Is there any intention to "correct" historical figures to the new so that field surveyors and those on the ground can use what is now available?
I've done back of fag-packet calculations based on my data and the DDB records for 8 different taxa/unique localities and the mean difference between the old and the new is that the new (WGS-84)figures are c. 103m west of the old and c.219m south of the old.... this seems to be in accord with what I can glean from the web but I'm sure that as long ago as 2008 the NBN were working on a fix? Is there a precise figure I can apply to convert?
What are the views of the C.I. recorders - how are they mapping things?
many thanks
Fred Rumsey
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Re: VC 113 mapping discrepancies

by admin » Mon Apr 06, 2015 11:49 pm

Hi Fred,

Thanks for highlighting the problem. If it's possible to identify the datasets referenced using the ED-50 then it should be possible to apply the correct transformation so that they can be correctly mapped together with modern UTM grid (WGS-84) records. How widespread was use of the ED-50 grid for botanical recording? My understanding had been that gridded records from Guernsey almost all used WGS-84 (though I had a niggling fear that things might be much more complicated than that...)

A further source of discrepancies is that until recently the DDb had a bug with Channel Island lat/lng datum adjustments. This hasn't had a widespread impact, but did mangle a fairly recently loaded set of records (offsetting them by ~ 100m, completely erroneously and not in accordance with either ED-50 or WGS-84) - reloading of those records is long overdue and I'll get that done asap.

best wishes,

Tom Humphrey
Database Officer, Botanical Society of Britain and Ireland (BSBI)
c/o Centre for Ecology and Hydrology,Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 8BB, UK.
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Re: VC 113 mapping discrepancies

by Fred@nhm » Tue Apr 07, 2015 10:53 am

Hi Tom,
as far as I'm aware all of the records that I've looked at for Guernsey on the DDB were done using the ED-50 grid. The new VC recorder who works at the Guernsey BRC should be ideally placed to say? Certainly all of the records from Charles David, the much-missed late recorder, used ED-50.
I suspect that the only records which might not would be recent UK visitors who would naturally adopt WGS-84 as their default.
all the very best and thanks for looking into this
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