[NatureNS] GPS use (was walking distances on CBCs)

Date: Thu, 25 Dec 2008 20:01:53 -0400
From: Stephen Shaw <srshaw@DAL.CA>
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[mine apparently didn't go through -- sending again]

Sounds like the Accuracy versus Precision distinction.  Presumably the 
intrinsic
accuracy of the system is only ± 2 meters (a guess) at any one ideal site with
an ideal number of satellites within range.  But the hand-held instrument
happens to report a number in meters with a precision of one decimal place, to
10 cm.  When the averaging function is on, the hand-held is able to 
average out
internal noise in its circuitry, to get numbers that are known 
precisely to the
nearest 0.1 meter, but these are only accurate to within +/- 2 meters (or
whatever the quoted accuracy is).

The reverse would be more interesting (to me), if the intrinsic 
accuracy of the
overall system actually were +/- 0.1 meter but that this is not achieved
immediately because of noise somewhere out there, say in disagreements (noise)
between the satellites.  In that unlikely case, repeated determinations and
averaging of the initially imprecise numbers could end up approaching the
intrinsic accuracy.

Presumably, the GPS coordinates of certain highly specific locations on the
ground like the major surveying points indicated on maps have been published
with
high accuracy (??), so if you went and stood exactly on top of one of 
these for
a while with your unit turned on you could check which it is, +/-0.1 m or +/-2
m ?  Or, standing on flat ground in the open, you could make a series of
measurements exactly 10 m apart all along a straight line.  Surely somebody of
a curious disposition must have done this.

The reason for asking originally was that at one point I fancied using one of
these units to try to map out the stability or otherwise of insect colonies
along Bay of Fundy cliffs effortlessly, but at the time the quoted 
specs looked
a bit too coarse to do the job and I never bought one.  I vaguely remember,
though, that there was some touted extra system available on the more 
expensive
units that used additional ground-based stations as well as satellites, called
WAIS or WAITS or something like that. This was supposed to increase accuracy
further.  Is this correct and is it still available?  I guess I ought 
to go and
look at the Garmin web site -- too late for Santa this year.
Steve
*************************************
Quoting Randy Lauff <randy.lauff@gmail.com>:

> Yes, I routinely use the averaging function...I watch the tenths-of-metres
> tick down, and when that levels off, I save the waypoint. The unit reports
> tenths-of-metres, I wonder if the implication is that it can differentiate
> 10 cm (4 inches)? Somehow, I think not.
> Randy
> 2008/12/23 Phil Schappert <philjs@eastlink.ca>
>
>> At 09:37 AM 12/23/2008, you wrote:
>>
>>> Interesting - that's the unit I use and I've never had better than 2 m
>>> accuracy, rarely better than 3 (these values according to the unit itself).
>>> I'll have to try it out in the open to see what it says...the vast majority
>>> of my GPS work is in the woods.
>>>
>>
>> My guess is that anything between 4 and 6 feet would register as 2m in
>> metric, Randy. Unfortunately, there doesn't seem to be any way to get it to
>> show cm. It's likely also important that I generally spend more than a
>> couple of minutes in any one location, usually on the order of 10 to 15
>> minutes (minimum) and location accuracy often increases with time...do you
>> use the averaging function?
>>
>>
>> Phil
>>
>> --
>>
>> Dr. Phil Schappert
>>
>> 27 Clovis Ave.
>> Halifax, NS Canada, B3P 1J3
>> Home: 902-404-5679
>> Cell: 902-460-8343
>>
>> www.philschappert.com
>> www.aworldforbutterflies.com
>>
>> "Just let imagination lead, reality will follow through..."
>>                                            Michael Hedges
> _________________________________
> RF Lauff
> Way in the boonies of
> Antigonish County, NS.
>

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