# [Taxacom] Inappropriate accuracy of locality data

Derek Sikes dssikes at alaska.edu
Thu Dec 2 15:02:32 CST 2010

```Bob,

A circle with a radius of 124km centered on

64.901268N 147.528120W +/- 124,605m

is in a very different place than a circle with a radius of 124km centered
on

64N 147W +/- 124,605m

(if one wanted to round to this precision).

-Derek

On Thu, Dec 2, 2010 at 11:46 AM, Bob Mesibov <mesibov at southcom.com.au>wrote:

> 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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--

+++++++++++++++++++++++++++++++++++
Derek S. Sikes, Curator of Insects
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University of Alaska Museum
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```