[Taxacom] Inappropriate accuracy of locality data

Bob Mesibov mesibov at southcom.com.au
Mon Nov 29 14:06:06 CST 2010

Many thanks for the on- and off-list comments.

A separate issue to the one I posted about is the best way to record or report the spatial uncertainty around a locality. No matter how you write a lat/long, it's still a point - a mathematical point with no horizontal spread at all.

In real life there's a spread. Some of it derives from the uncertainty of the GPS reading, but often there's another element: perhaps you collected 3 specimens over ca 0.25 ha, or your bottom-grabber was some distance from your boat, or you couldn't get a reading in the forest so you walked to a nearby road, got a GPS reading there and estimated an offset.

Best practice is to record a locality plus an uncertainty. In museum databases I have seen this as a field named 'Accuracy', 'Precision', 'Reliability' or 'Uncertainty'. Darwin Core calls it UncertaintyInMetres and defines it as:

"The horizontal distance (in meters) from the given decimalLatitude and decimalLongitude describing the smallest circle containing the whole of the Location. Leave the value empty if the uncertainty is unknown, cannot be estimated, or is not applicable (because there are no coordinates). Zero is not a valid value for this term."

with this comment:

"Example: "30" (reasonable lower limit of a GPS reading under good conditions if the actual precision was not recorded at the time), "71" (uncertainty for a UTM coordinate having 100 meter precision and a known spatial reference system)."

I have been recording uncertainty in my own collecting-events databases for many years, but I was only brave enough to start publishing them as part of locality data in 2009. No reviewer or editor has complained so far...

You might be using Google Earth, as I do, for estimating locations where your GPS couldn't get a good position reading. Please be aware that Google Earth imagery is not always spot-on, and can be offset by ca 20 m or more from the correct location. A workaround is to get a GPS reading from a nearby landmark which can be clearly seen in the satellite image, to 'calibrate' your Google Earth estimate of the sampling site location.
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery, and
School of Zoology, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
Ph: (03) 64371195; 61 3 64371195
Webpage: http://www.qvmag.tas.gov.au/?articleID=570

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