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Guest Kerry

Accuracy figures

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Guest Kerry

quote:

 

Accuracy is still the same...12 to 6 feet

 

Accuracy is superb without WAAS. average reading was 13 to 7 feet!

 

Accuracy was 13 feet without WAAS

 


 

These are a few accuracy statements/ comments from this site and sound very definite and said with some certainty?

 

Based on and compared to what?

 

Cheers, Kerry.

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Guest WaylandersMA

Take these statements as meaningless unless they come with long duration scatter plot diagrams of data.

There are way too many variables in GPS and WAAS to say, "accuracy now 9 ft with WAAS on so no difference" That calculation the GPS makes can also be meaningless when comparing.

 

A good way to do it: same model units side by side, same set of batteries from same blister pack. Both connected to PC recording position. One WAAS on, one off. Then you have removed satellite position variabilty, WAAS correction variability etc.

 

Everything else is seat of the pants which is always going to show you that the new model you just bought with WAAS is way better. (The law of payed for it subjectivity.)

 

SInce I only have one Legend, my seat of the pants is, "WAAS is great". My coordinates don't seem to budge when I have lots of "D"'s on the satellite power page.

 

Paul

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Guest PneumaticDeath

quote:
Originally posted by WaylandersMA:

....

SInce I only have one Legend, my seat of the pants is, "WAAS is great". My coordinates don't seem to budge when I have lots of "D"'s on the satellite power page.

 

Paul


 

My experience is just the opposite, though just as anecdotal. Since I'm in California, and both WAAS sats visible to me are close to the horizon, I rarely actually get one long enough to make much of a difference, but if I turn off WAAS that means that I have 2 more channels to receive regular satelites, which means that I hold lock in tree cover better, and usually get better readings. So turning off WAAS has actually helped the accuracy in my case. YMMV.

 

-- Mitch

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Guest mcb

looking at this accuracy quite right. I don't care if you are using the cheapest little yellow eTrex or the new megabuck Street pilot III if both units are talking to the same exact satellites they there error should be that same. They both have the same 12-channel receiver inside. Sure the SPIII may have a better antenna and a faster processor but this does not effect the accuracy. The only thing that effects the accuracy is the number and geometry of the satellites and any interference of the signal coming through the atmosphere. Now a unit with a better antenna may get more satellites in a given situation and thus makes it seem more accurate.

 

It always rubs me the wrong way when I see a post like. "My 12 is so much more accurate then my friend eTrex." Wrong, if they were both talking to exactly the same satellites then they should give nearly the same error. Most likely the Garmin 12 was getting better reception and thus was able to make a more accurate position calculation with more satellites.

 

The only time you could really say one receiver is more accurate is if you compare a modern 12-channel parallel receiver to and older 8-channel multiplexing or an older 2-channel receiver. The 12 Parallel channels will make a receiver more accurate then the 8-channel multiplexed receiver.

 

You also have to watch the way the EPE number is calculated. I think Garmin calculates a 50% confidence number on most of its units. This means that fifty percent of the time you will be within the EPE number of ft./m. of the given coordinate. Some units may calculate EPE on a different confidence level. EPE is only an estimate and you can never actually know you actual error or you would be able to correct for it and have no error. icon_smile.gif

 

So if you are comparing modern 12 channel GPS unit they there should be really no argument about which unit is more accurate. Valid arguments would be about which unit gets better receptions, or which had more useful features, but when it comes down to it almost all modern civilian GPS unit should give you nearly identical accuracy.

 

Your only way to get more accurate would be to use WAAS or DGPS, but this is a whole new can of worms. icon_biggrin.gif

 

mcb

 

[This message has been edited by mcb (edited 28 June 2001).]

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Guest mcb

looking at this accuracy quite right. I don't care if you are using the cheapest little yellow eTrex or the new megabuck Street pilot III if both units are talking to the same exact satellites they there error should be that same. They both have the same 12-channel receiver inside. Sure the SPIII may have a better antenna and a faster processor but this does not effect the accuracy. The only thing that effects the accuracy is the number and geometry of the satellites and any interference of the signal coming through the atmosphere. Now a unit with a better antenna may get more satellites in a given situation and thus makes it seem more accurate.

 

It always rubs me the wrong way when I see a post like. "My 12 is so much more accurate then my friend eTrex." Wrong, if they were both talking to exactly the same satellites then they should give nearly the same error. Most likely the Garmin 12 was getting better reception and thus was able to make a more accurate position calculation with more satellites.

 

The only time you could really say one receiver is more accurate is if you compare a modern 12-channel parallel receiver to and older 8-channel multiplexing or an older 2-channel receiver. The 12 Parallel channels will make a receiver more accurate then the 8-channel multiplexed receiver.

 

You also have to watch the way the EPE number is calculated. I think Garmin calculates a 50% confidence number on most of its units. This means that fifty percent of the time you will be within the EPE number of ft./m. of the given coordinate. Some units may calculate EPE on a different confidence level. EPE is only an estimate and you can never actually know you actual error or you would be able to correct for it and have no error. icon_smile.gif

 

So if you are comparing modern 12 channel GPS unit they there should be really no argument about which unit is more accurate. Valid arguments would be about which unit gets better receptions, or which had more useful features, but when it comes down to it almost all modern civilian GPS unit should give you nearly identical accuracy.

 

Your only way to get more accurate would be to use WAAS or DGPS, but this is a whole new can of worms. icon_biggrin.gif

 

mcb

 

[This message has been edited by mcb (edited 28 June 2001).]

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Guest WaylandersMA

quote:
Originally posted by mcb:

You also have to watch the way the EPE number is calculated. I think Garmin calculates a 50% confidence number on most of its units. This means that fifty percent of the time you will be within the EPE number of ft./m. of the given coordinate. Some units may calculate EPE on a different confidence level. EPE is only an estimate and you can never actually know you actual error ......


 

Excellent point. You shouldn't use EPE for anything but "My EPE is lower than it was 5 minutes ago so I am now more accurate". Using it as a "real" number, or even worse, comparing the number between models and vendors is a bad move. The vendors give a vague expplanation as to how it is calculated but after that it is a "proprietary".

 

I would argue that being able to pull down more satellites with a stronger signal is a measure of the units accuracy.

 

So if you have the now stanadard 12channel, then the only things left that can effect accuracy on two units in the identical lcation are:

 

antena, size, shape, patch quad etc

WAAS on or off

DGPS on off

battery strength ( to a much lesser extent, if the unit is getting close to prescribed 2.5v then not an issue

 

And also the peculiar but true situation that pnuematic brought up.

If you are on the west coast the gps demarcates two positions for the two WAAS satellits thus knocking the room for others down to 10.

If you are just barely getting the WAAS data this can give you inferior accuracy than if you leave WAAS off. (If there are more than 10 satellites visible

 

On the east coast we are in a different situation. Only one (35) WAAS bird is visible (also higher in the sky) so the GPS only gives up one channel. Since I have only once achieved lock on 10 satellites plus WAAS makes 11, I haven't had a drop off of accuracy.

 

Has anyone ever gotten a satellite constellation of 12 satellites at once?

 

Paul

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Guest Kerry

There are some good points regarding EPE?s (and similar relatives) in those comments, which was the reason behind of original question. The problem with EPE (and similar) is that they are propriety values and while the background to the calculations remain a secret comparing these types of figures across different makes/models are totally hopeless, even as a standalone value.

 

It was interesting when SA was terminated that the EPE on a particular (common recreational marine GPS) unit dropped by half but the actual accuracy improved by a factor of 7, which sort of says someone was trying to kid someone, somehow? That then might make one suspect that in that case EPE was ?factored? somehow around SA being ON but the whole thing fell over when SA was suddenly terminated. Just a guess and of course nobody will confirm or deny that assumption.

 

As for 12 channel receivers? Really are 12 channels better than 10 or 9 or 8 or even 5? as far as accuracy goes. Certainly useful in some circumstances but what real purpose in determining accuracy? Some receivers will never even see 12 satellites, some won?t/don?t even use all satellites so is this 12 channel thing a myth?

 

quote:

 

Has anyone ever gotten a satellite constellation of 12 satellites at once?

 


 

Not that a 12 channel receiver will show directly but we are certainly a little spoilt with a constellation of 28 (at times). Probably all depends on location and make/model of receiver but 13 sats "in view" is possible even 14 at times which basically reflects in a higher % of there being 12 Sats in view/available.

 

13 or 14 sats is not much good to a 12 channel receiver (apart from a little selective redunancy) which I suppose asks the question why the 12 is a "12" and not say an 11+1?

 

Cheers, Kerry.

 

[This message has been edited by Kerry (edited 28 June 2001).]

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