Imprecise locality records
mesibov at SOUTHCOM.COM.AU
Tue Aug 22 19:59:28 CDT 2000
A widely used procedure for dealing with uncertain locality records in
databases is to specify them as well-defined points in the main locator
fields, then to add an appropriate uncertainty to a precision field.
This uncertainty entry can be something like (plus or minus) '1 mile' or
(plus or minus) '50 miles', and details (as Peter Rauch says) can be
spelled out in the metadata.
When plotting maps from such a database using GIS, you can use the
uncertainty field to vary the size of the markers. Circular markers can
be drawn by the GIS to be the same scale size as the diameter of the
uncertainty entry: '50 miles' becomes a scale circle 100 miles in
Lat/long and UTM coordinates are rectangular, not circular. The
cartographically approved way to specify a locality is to identify the
smallest rectangular box WITHIN WHICH the specimen is known to have been
recorded, NOT the intersection of lat/long or UTM lines closest to the
locality. In my part of the world, lat/long coordinates specify the
northwest corner of the lat/long box containing a locality. In mapping
such a locality using GIS, the point I would actually map is the CENTRE
of that lat/long box.
Converting between coordinate systems unavoidably creates an additional
level of uncertainty, because different grids of boxes overlap in
complex ways. There are really only two ways to deal with the problem.
One is to reduce the scale of mapping, so that the difference in
precision between good and poor locality information isn't so obvious.
A harder but more satisfactory approach is to upgrade the precision of
the original record. The taxonomic literature is rich in instances of
locality corrections and clarifications, the result of taxonomists
chasing up information in expedition reports, field notebooks and
people's memories. Again in my own case, I was able to shrink a
centipede locality from a 10 km square to a 2 km square by showing a
good-quality topographic map to the original collector, 45 years after
the event! It's the kind of tedious, nit-picking work that should only
be done when precise locality information is taxonomically important.
You're out of luck if the collector is long dead, there were never any
field notes and the label says something like 'Brushy Creek', which
wanders for 100 miles through a landscape lacking any other named
features to which the collector could have referred. I've seen published
maps in which such localities are shown as very large question marks
over arbitrary points!
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 6437 1195; international 61 3 6437 1195
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