[Taxacom] Inappropriate accuracy of locality data

[Robert Guralnick]

  Many use the term "uncertainty", and it is also codified also in
Darwin Core, to refer to both precision and accuracy errors with
geographical coordinates.
Best, Rob
http://sites.google.com/site/robgur

On Mon, Nov 29, 2010 at 10:40 AM, Herbert Jacobson <jakejudy at hotmail.com> wrote:
>
> No, accuracy is the right word. You can get a lot of precision with GPS but not necessarily accuracy.
>
> Herb
>
>> Date: Mon, 29 Nov 2010 10:00:51 -0600
>> From: fautin at ku.edu
>> To: mesibov at southcom.com.au
>> CC: taxacom at mailman.nhm.ku.edu
>> Subject: Re: [Taxacom] Inappropriate accuracy of locality data
>>
>> Or is it precision?
>>
>> Daphne G. Fautin
>> Professor, Ecology and Evolutionary Biology
>> Curator, Natural History Museum and Biodiversity Research Center
>> Haworth Hall
>> University of Kansas
>> 1200 Sunnyside Avenue
>> Lawrence, Kansas 66045-7534 USA
>>
>> telephone 1-785-864-3062
>> fax 1-785-864-5321
>> evo user name fautin
>> website www.nhm.ku.edu/~inverts
>>
>> direct to database of hexacorals, including sea anemones
>> newest version released 1 December 2009
>> ***http://hercules.kgs.ku.edu/Hexacoral/Anemone2***
>>
>>
>> On Mon, 29 Nov 2010, Bob Mesibov wrote:
>>
>> > As a reviewer and editor of scientific papers I often see GPS location
>> and elevation data with inappropriate accuracy. This post is a brief backgrounder on the issue for interested authors, reviewers and editors on this list. For more information, please visit any of the many websites that deal with GPS accuracy.
>> >
>> > A typical 'consumer' handheld GPS unit gives a location with an accuracy
>> of plus or minus 10-20 m under typical field conditions. In hilly or densely forested country the accuracy may not be this good. The very popular Garmin E-Trex has 15 m RMS accuracy, i.e. about 2/3rds of the time the GPS location will be within 15 m of the real position - under favourable conditions.
>> >
>> > Neverthless, the GPS unit will calculate its position from the available satellite signals and will display as many significant figures as you ask it to. For example, it might display the latitude as 30d 14m 19.88s N. One second of latitude represents about 30m on the ground, so that last '8' in '19.88' represents 30 cm. This is spurious accuracy. The GPS unit may also display an instrumental accuracy, e.g. '3 m', much greater than the cartographic accuracy. (Yes, I know I am using the word 'accuracy' in a loose way. I want to keep this post short. And I use d, m and s rather than the usual symbols in order to avoid message coding problems in this email.)
>> >
>> > For this reason, locations from a typical GPS unit should be rounded off:
>> > - to the nearest second in degree-minute-second format (30d 14m 20sN)
>> > - to four decimal places in decimal degree format (30.2389d N)
>> > - to two decimal places in decimal minute format (30d 14.33m N)
>> >
>> > There are also problems with GPS elevations if the GPS is not equipped with an altimeter. Elevations calculated from satellite signals are elevations above a mathematical model of the Earth's surface, not above sea level. The difference can be tens of metres. In addition, elevation accuracy is tied to accuracy in horizontal position. If your GPS says it's at 1287 m, record the elevation as 'ca 1300 m'.
>> >
>> > Please do not use geospatial data from your GPS 'as is' on the grounds that 'those were the figures on the screen [or in the GPS memory] and who am I to change them?' You are not changing them if you round them off. It's rather like a (hypothetical) digital balance which says a medical patient weighs 55.4392 kg, even though the balance is only accurate plus or minus 0.1 kg, not 0.1 g. The patient's weight should be recorded as 55.4 kg, not 55.4392 kg. No medical journal would accept a paper with that latter figure, and it surprises me that some journal editors accept overaccurate geospatial data.
>> > --
>> > 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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