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Herbaria united vc lookup discrepancy

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Herbaria united vc lookup discrepancy

by ohebrides2 » Tue Oct 18, 2016 9:19 pm

I sent a record from NH05236 08224 to a referee, a specimen from a few metres S of the watershed at Aonach air Chrith and therefore in vc97. He looked it up in the Herbaria United vc lookup, which shows apparently a 100m square, the centre of the square is in vc105 so this is what it claims. Using http://www.cucaera.co.uk/ gives it correctly very close to the boundary but in vc97, and entering the grid ref in MemoryMap shows it S of the watershed. I thought the Herbaria United lookup was more accurate than this. Anyone know what is going on?

paul
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Re: Herbaria united vc lookup discrepancy

by AndyAmphlett » Sat Oct 22, 2016 9:31 am

Hi Paul,

the various grid ref x vc lookup tools work in slightly different ways. The herbaria united tool works at 100m precision, so in your case, rounded the more detailed grid ref down to 100m precision. The problem is that it then allocates a single vc to that 100m grid ref. That might be based on the centroid of the 100m square or the vc with the greatest area within the 100m square. Both lead to errors, so i would not recommend using this tool.

Cucaera also resolves vcs at 100m precision, but where a grid ref is close to a vc boundary it does not display a vc number. It displays the grid ref correctly against zoomable vc boundaries and OS mapping so it is simple to work out which vc the grid ref refers to. If you use a 100m grid ref in Cucarea and it overlaps the vc boundary it displays both vcs, prompting further investigation of which is the correct one.

There is also the DDb grid reference lookup tool at http://bsbidb.org.uk/gridref.php This looks to be working at 10m precision, though it might be working at full precision. If you enter your 10 fig grid ref into the parser ie http://bsbidb.org.uk/gridref.php?ref=NH05236+08224 it gives both vcs.

So the DDb tool is the most accurate, but by using its maps display, Cucaera is as accurate, and provides more flexibility over mapping backdrop. I would not recommend the herbaria united tool as it will create errors at the boundaries of vcs.

Andy.
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