[Taxacom] Inappropriate accuracy of locality data
mesibov at southcom.com.au
Mon Nov 29 03:54:32 CST 2010
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
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