[Taxacom] Inappropriate accuracy of locality data

Bob Mesibov mesibov at southcom.com.au
Thu Dec 2 14:46:53 CST 2010

Hi, Arthur.

Many thanks for your comments, and I hope Taxacomers interested in this topic go to your 'best georeferencing practice' for more.

However, I have to disagree with "...rounding of the lat/long - i.e. reducing the precision of the recording, but this can also lead to problems (depending on the reduction in precision) as it moves the point as lat/long is only a point representation of the left hand bottom of grid square."

Rounding off *does not* move the point. If you round off to an implied precision appropriate to the measurement, the point stays exactly where the instrument told you it was. The overaccurate measurement you read off the GPS screen was the instrument fantasising by calculating and including too many significant figures.

As I suggested in an earlier post, this may be simpler to understand by using the analogy of a balance. Suppose the sensitivity of the balance is such that it can only weigh to the nearest 0.1 kg. Then the weight measurements you get from this balance are only meaningful to the nearest 0.1 kg. If you round off 55.4392 kg to 55.4 kg, you are not 'moving the weight' from 55.4392 kg to 55.4000 kg, because the 3 figures after 55.4 are meaningless.

The meaningless extra figures you get from a GPS arise from a calculation within the gadget which preserves more figures than it should. The meaningless extra figures should be ignored.

My GIS is even worse. It calculates positions to 0.000001 m, or one micron. When I digitise map data I get six significant figures after the decimal. I hope you're not going to suggest that rounding 355177.286149 m 355177 m is *moving* the point from 355177.286149 m to 355177.000000 m?
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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