[Manifold-l] GPS Accurracy

[Colin Driscoll]

My .02 worth. As the saying goes about the proof of the pudding....I spend
2-3 full days a week in the field with my Garmin 60cs connected (USB/NMEA)
to Manifold on my laptop. With a good quality orthorectified aerial image
under the GPS tracks and good satellite reception (high gain external
antenna) the track line is drawn where I can see I am out the vehicle
window. If I am on the left of a bush road that's where the line is. If I
drive back along a narrow road I have already been on the return line is on
top of the first- in fact I put a wish list item in a while ago to have a
blinking point at the current location because it is generally not easy to
know this on a return trip.

This is all +/- about 3meters but is cetainly reproducible here in Australia
at least- maybe it's the clear sky.

-----Original Message-----
From: manifold-l-bounces at lists.directionsmag.com
[mailto:manifold-l-bounces at lists.directionsmag.com] On Behalf Of Martin
Roseveare
Sent: Friday, 29 September 2006 7:31 PM
To: Dimitri Rotow; manifold-l at lists.directionsmag.com
Subject: Re: [Manifold-l] GPS Accurracy

I guess you're right Dimitri!

Martin Roseveare
Senior Geophysicist

Phone: +44 (0) 1989 730 564
Fax: +44 (0) 7050 369 790
Web: www.archaeophysica.co.uk

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----- Original Message -----
From: "Dimitri Rotow" <dar at manifold.net>
To: <manifold-l at lists.directionsmag.com>
Sent: Friday, September 29, 2006 1:30 AM
Subject: RE: [Manifold-l] GPS Accurracy


>
>> Looking through the NMEA web page you referenced confirms that they 
>> do differ.
>>
>> For example GSA has '3' for 3D fix, GGA has '3' for PPS fix
>
> In that link the particular value 3 for PPS fix is cited for the GGA 
> sentence is noted as "Fix quality" whilst in the GSA sentence the 
> value 3 for 3D fix is noted as "3D fix".  These are different 
> attributes, so that the same value "3" occurring in those two 
> different sentences is not assigning two different meanings to the same
thing, it is two different
> attributes occuring in two different sentences.   In this particular case,
> both attributes use integer-coded values where the one attribute uses 
> integers in the range 0 to 8 and the other uses integers in the range 1 to
> 3.   It's just happenstance that in the particular example given an 
> integer
> value of 3 is used in both cases, but it means different things for 
> the two different attributes.
>
> As far as I've been able to tell, a "quality" attribute in a sentence 
> is something that occurs in the GGA sentence only and is a value from 
> 0 to 8 inclusive.  Other attributes such as "3D fix," the various 
> "DOP"s and so on
> appear to be attributes that are different from "quality."   In the web
> crawls I've done I have not located a "quality" attribute except 
> within the GGA sentence.
>
>> and both of these are common sentences. Hence my question about which 
>> Manifold uses when in receipt of more than one sentence per message! 
>> I guess it comes down to the purpose of each type of NMEA sentence, 
>> i.e., rather than 'quality' being an absolute thing it is a purpose 
>> specific indicator only.
>>
>
> I believe "fix quality" has a specific meaning in NMEA-speak as being 
> a specific attribute that's one of those 9 values.  "Quality" in this 
> context is a specific technical term setting forth one of those 9 
> coded meanings and is not a generic English word such as a synonym for 
> circular error probability, etc.
>
> But then, I'm not an NMEA or GPS guy so the above is just my inference 
> from reading what I've been able to lay hands on via web searches...  
> perhaps someone with an expert understanding of NMEA could comment on 
> that inference.
>
> Cheers,
>
> Dimitri
>
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