Garmin GPSMAP 60 CSx - Accuracy Expectations

[outdoors4me]

Not trying to be picky here because I'm using my GPS for off trail hiking. If I get to within 30 feet of my destination, I'm pretty happy.

 

But I'm curious about this....

 

I can waypoint where I start my hike from, go out and have my fun and navigate back to that spot.

 

I guess I thought that a waypoint made by my unit should be able to be tracked back to within a small margin of error.

 

I've tested this several times and when standing on the exact waypoint, I could be off 30 or more feet. As stated above, no big deal for what I'm doing. I just thought they were more accurate now.

 

I do have WAAS enabled. Any comments?

 

Thanks guys!

 

Beck

Edited April 21, 2008 by outdoors4me

[+Prime Suspect]

Not trying to be picky here because I'm using my GPS for off trail hiking. If I get to within 30 feet of my destination, I'm pretty happy.

 

But I'm curious about this....

 

I can waypoint where I start my hike from, go out and have my fun and navigate back to that spot.

 

I guess I thought that a waypoint made by my unit should be able to be tracked back to within a small margin of error.

 

I've tested this several times and when standing on the exact waypoint, I could be off 30 or more feet. As stated above, no big deal for what I'm doing. I just thought they were more accurate now.

 

I do have WAAS enabled. Any comments?

 

Thanks guys!

 

Beck

Just because you have WAAS enabled, doesn't mean you're getting the full benefit of WAAS correction. This is especially true under tree cover. What's was the error reading when you did this? And did you remember that you're actually combining the error present when you made the waypoint, with the error present when you're navigating back to it?

[outdoors4me]

Not trying to be picky here because I'm using my GPS for off trail hiking. If I get to within 30 feet of my destination, I'm pretty happy.

 

But I'm curious about this....

 

I can waypoint where I start my hike from, go out and have my fun and navigate back to that spot.

 

I guess I thought that a waypoint made by my unit should be able to be tracked back to within a small margin of error.

 

I've tested this several times and when standing on the exact waypoint, I could be off 30 or more feet. As stated above, no big deal for what I'm doing. I just thought they were more accurate now.

 

I do have WAAS enabled. Any comments?

 

Thanks guys!

 

Beck

Just because you have WAAS enabled, doesn't mean you're getting the full benefit of WAAS correction. This is especially true under tree cover. What's was the error reading when you did this? And did you remember that you're actually combining the error present when you made the waypoint, with the error present when you're navigating back to it?

 

Hmmm....not sure what you mean by error reading. I'll look this up in the manual, although the manual is the worst part of this GPS.

 

And I'm not sure why, but I like your signature.

 

Beck

[+hogrod]

Hmmm....not sure what you mean by error reading. I'll look this up in the manual, although the manual is the worst part of this GPS.

 

He is talking about the position error of the gps, which is always displayed at the top of the satellite page. you can also set this as one of your data fields.

 

I would also check and make sure you don't have the unit set to battery saver mode, in this mode the unit isn't very accurate because its not updating as often as normal. To check this setting goto: Main menu, setup, system, now in the top drop-down make sure the unit is set to normal, not battery saver.

[CenTexDodger]

Not trying to be picky here because I'm using my GPS for off trail hiking. If I get to within 30 feet of my destination, I'm pretty happy.

 

But I'm curious about this....

 

I can waypoint where I start my hike from, go out and have my fun and navigate back to that spot.

 

I guess I thought that a waypoint made by my unit should be able to be tracked back to within a small margin of error.

 

I've tested this several times and when standing on the exact waypoint, I could be off 30 or more feet. As stated above, no big deal for what I'm doing. I just thought they were more accurate now.

 

I do have WAAS enabled. Any comments?

 

Thanks guys!

 

Beck

Just because you have WAAS enabled, doesn't mean you're getting the full benefit of WAAS correction. This is especially true under tree cover. What's was the error reading when you did this? And did you remember that you're actually combining the error present when you made the waypoint, with the error present when you're navigating back to it?

 

Hmmm....not sure what you mean by error reading. I'll look this up in the manual, although the manual is the worst part of this GPS.

 

And I'm not sure why, but I like your signature.

 

Beck

 

One of the data fields is accuracy, but it is more correctly an estimated position error. On my 60SCx it is usually 10-12 ft with a good view of the sky. That means that the actual position you are standing at is within 10-12 feet of the position stated on the GPS. You could be standing on the actual spot, but the GPS is only certain that you are within that specified distance. If you are under trees it will could be worse than 10-12 feet. If you are off 15 in the morning, and 15 feet the other way in the afternoon, that would account for 30 feet of error.

[outdoors4me]

Hey guys,

 

Good info. I now know where my error code is. I'm reading plus 15 feet. Battery saver is set to normal. WAAS enabled.

 

Sounds like I've got everthing set to optimum accuracy.

 

So what I get is what I get - that's cool. And I've learned some new settings.

 

Thanks again.

 

Beck

[+julianh]

Hey guys,

 

Good info. I now know where my error code is. I'm reading plus 15 feet. Battery saver is set to normal. WAAS enabled.

 

Sounds like I've got everthing set to optimum accuracy.

 

So what I get is what I get - that's cool. And I've learned some new settings.

 

Thanks again.

 

Beck

Don't treat the displayed EPE (Estimate of Precision Error) as Gospel-truth - fact is that it is just the unit's estimate of precision, and it means you have a certain probability of being within the indicated distance. Trouble, is Garmin don't say what the probability is - 50? 80%? 90%?

 

My best guess is that the displayed EPE on a Garmin is probably the 50% figure, meaning that there is a roughly equal probability that the actual error is greater than or less than the indicated EPE. (And if your actual error is greater than the indicated EPE, there is actually no upper limit on how big the error really is, although in most practical circumstances, an effective upper limit of about 2 to 3 is reasonable.) Thus, if your unit says the EPE is 5 metres say, there is a roughly 50% chance that you are actually within 5 metres of the indicated position, but there is also a 50% chance that you are more than 5 metres away.

 

To be 95% confident that your measurement is within a circle of a fixed radius, you should multiply Garmin's EPE value by two. That is, if the indicated EPE is 5 metres, then there is a roughly 95% probability that you are within 10 metres of the indicated position.

 

Hope this helps!

[+Prime Suspect]

Don't treat the displayed EPE (Estimate of Precision Error) as Gospel-truth - fact is that it is just the unit's estimate of precision, and it means you have a certain probability of being within the indicated distance. Trouble, is Garmin don't say what the probability is - 50? 80%? 90%?

It's 95%, according to Garmin documentation. But I cheated. I actually looked at the manual.

[+julianh]

Don't treat the displayed EPE (Estimate of Precision Error) as Gospel-truth - fact is that it is just the unit's estimate of precision, and it means you have a certain probability of being within the indicated distance. Trouble, is Garmin don't say what the probability is - 50? 80%? 90%?

It's 95%, according to Garmin documentation. But I cheated. I actually looked at the manual.

Are you sure you are reading the manual correctly?

 

For my Summit HC, the Specifications say the accuracy is <10 metres 95% typical with WAAS off, and 3 metres 95% typical with WAAS on. This is presumably based upon actual field testing and theoretical considerations, etc.

 

However, this is not necessarily the same thing as the Accuracy figure which the unit will display in real-time. While the unit can self-interrogate the quality of signals it is receiving, and look at the disposition of the satellites to determine whether a good fix is theoretically possible or not, it has no way of knowing for sure what the real-time accuracy is. The figure it displays is just an estimate, and it is generally accepted to indicate that there is an EQUAL probability that the error is GREATER or LESS THAN the indicated EPE.

 

http://gpsinformation.net/main/errors.htm

 

Hope this heps!

[Hertzog]

Don't treat the displayed EPE (Estimate of Precision Error) as Gospel-truth - fact is that it is just the unit's estimate of precision, and it means you have a certain probability of being within the indicated distance. Trouble, is Garmin don't say what the probability is - 50? 80%? 90%?

It's 95%, according to Garmin documentation. But I cheated. I actually looked at the manual.

Although as previously noted Garmin won't tell, I believe that their intent is for the EPE to be more like a 50% confidence number; the 95% in the manual is in regards to Garmin's stated accuracy specification. In my opinion (purely gut feel as I don't have any hard measurements to back it up - wish Garmin would make the EPE readily collectable data), the EPE is proabably better than a 50% number, maybe 60 or 70%; but I generally treat it as a 50% number to be conservative.

[outdoors4me]

OK, all this info I'm getting here is really helping. I remembered the info about the EPE number as I started my walk today and had a bit of a light go on.

 

Previously, I marked my beginning spot as soon as the "Acquiring Satellites" function had finished. Today I noticed that the EPE is kind of high right when this function finishes, then settles to a much lower number. I waited this time until the number bottomed out, in this case 15. And when I got back to the spot, I was about 18 feet off.

 

That's much closer than it has been.

 

And I know there are a lot of variables in determining accuracy. I'm just happy that I understand what they are and I know basically what range I can expect and why.

 

That's good enough for me. I only wish the manual didn't suck.

 

Thanks!

 

Beck

Edited April 22, 2008 by outdoors4me

[OGBO]

Don't treat the displayed EPE (Estimate of Precision Error) as Gospel-truth - fact is that it is just the unit's estimate of precision, and it means you have a certain probability of being within the indicated distance. Trouble, is Garmin don't say what the probability is - 50? 80%? 90%?

It's 95%, according to Garmin documentation. But I cheated. I actually looked at the manual.

Are you sure you are reading the manual correctly?

 

For my Summit HC, the Specifications say the accuracy is <10 metres 95% typical with WAAS off, and 3 metres 95% typical with WAAS on. This is presumably based upon actual field testing and theoretical considerations, etc.

 

However, this is not necessarily the same thing as the Accuracy figure which the unit will display in real-time. While the unit can self-interrogate the quality of signals it is receiving, and look at the disposition of the satellites to determine whether a good fix is theoretically possible or not, it has no way of knowing for sure what the real-time accuracy is. The figure it displays is just an estimate, and it is generally accepted to indicate that there is an EQUAL probability that the error is GREATER or LESS THAN the indicated EPE.

 

http://gpsinformation.net/main/errors.htm

 

Hope this heps!

 

Used to be that GPSRs (except Garmin) displayed the DOP numbers. This is Dilution of Precision, which is the extra error added (actually multiplied) by the constellation being used. Basic idea is if the satellites being used are all clustered in a small part of the sky, you will have a bigger error budget than if they are well-spread. DOP is different for horizontal position than for vertical (altitude). A lot of units even used to display HDOP and VDOP as well as DOP (the total). If you take all the sources of error (clock errors, ephemeris errors, atmospheric errors, etc) into account the horizontal error budget under ideal conditions is just what the manual says - 10 m without WAAS, 3 m with WAAS, 2 sigma (i.e., 95%). This was all too complicated for most people (still available on survey units), so the consumer units dropped the DOP display and started showing "error", "position error", "EPE" (= estimated position error) or some such term. Problem is, each manufacturer has its own version of "error". Not only that, but the displayed error circle on the map page and the EPE number do not always agree (my various Garmins are especially inconsistent, but several of my Magellans also show discrepancies). I have asked the tech people via phone, email, at trade shows, and in person, and never get more than a mumbled promise to look into it. Garmin's response to "what does your EPE really mean?" has always been "trade secret, can't tell you".

 

Soooo, the basic rule is - treat the EPE number and the circle on the map display as very rough qualitative (not quantitative) guesstimates of the 50% error circle and recognize that the actual location could be spot on, shifted systematically (due to multipath in canyon situations, esp urban canyons, and the EPE calculation does not indicate this), off by 2 or 3 times the EPE, or anywhere in between. It is just telling you that the present indicated position is "good", "bad", or "sort of ok".

 

I see times where I can walk up to a position (like a geocache) and zero out right on top of the container, and other days going to the same location that the indicated zero is 50 meters away, both for fully in the open and for challenging canyon/canopy situations, with the EPE saying it is 3m. But most of the time with the 60CSx (since that is the unit in question), even in challenging situations, it gets me within 3 meters. Then there are those days when the constellation is terrible and the terrain/vegetation is horrible and I want to just put the unit away and go by instinct (or Google Earth or USGS map). You just have to use the unit a lot and get a feel for how it behaves on different days.

 

Oh, yeah, it gets worse when the batteries are getting low, and Garmin just keeps displaying a tiny error circle, oblivious to its inaccuracies.

Edited May 1, 2008 by OGBO

[+JSWilson64]

Awesome description of error/accuracy, OGBO.

 

And don't forget the (in)accuracy of the map and whether you're "locked on roads," come into play, at least when it comes to drawing the "blue circle."

 

It's a wonder we can find anything at all!

[+GPSlug]

Used to be that GPSRs (except Garmin) displayed the DOP numbers. This is Dilution of Precision, which is the extra error added (actually multiplied) by the constellation being used. Basic idea is if the satellites being used are all clustered in a small part of the sky, you will have a bigger error budget than if they are well-spread. DOP is different for horizontal position than for vertical (altitude). A lot of units even used to display HDOP and VDOP as well as DOP (the total). If you take all the sources of error (clock errors, ephemeris errors, atmospheric errors, etc) into account the horizontal error budget under ideal conditions is just what the manual says - 10 m without WAAS, 3 m with WAAS, 2 sigma (i.e., 95%). This was all too complicated for most people (still available on survey units), so the consumer units dropped the DOP display and started showing "error", "position error", "EPE" (= estimated position error) or some such term. Problem is, each manufacturer has its own version of "error". Not only that, but the displayed error circle on the map page and the EPE number do not always agree (my various Garmins are especially inconsistent, but several of my Magellans also show discrepancies). I have asked the tech people via phone, email, at trade shows, and in person, and never get more than a mumbled promise to look into it. Garmin's response to "what does your EPE really mean?" has always been "trade secret, can't tell you".

I don't find the DOP, in itself, very useful. It may give you a clue to one reason why your error is bad, but not how big the error is. It's basically just what you would get as a 1 sigma error out of your position calculation if you assumed all of the ranges had a unit error. Some people still like to see the DOP, but it really doesn't contain nearly as much information as, say, an RMS position error estimate which takes into account both the geometry and estimated range errors.

 

The company line might be the EPE is a "a secret", but I don't think there's too much mystery in what it is. There may be some proprietary way they estimate the input range errors, but the EPE will still just be the 50% or 68% error calculated from the variance coming out of the positioning filter.

 

How good the EPE will be all comes down to how good the estimated range errors are. I agree that the EPE can miss some unmodeled errors, but it still tells you much more than DOP.

 

So, I use the EPE as useful under reasonably good conditions, and treat it as a 50% value. But don't trust it in a high multipath environment, especially where there could be pure reflected signals with the direct signal blocked.

 

I certainly encourage the use of WAAS, if for nothing else than that the EPE has a better estimate of the iono error than it otherwise would. Some people complain that they sometimes see the EPE goes up when the enable WAAS. But even if the EPE is bigger, it's probably better.

Edited May 1, 2008 by GPSlug

[OGBO]

I don't find the DOP, in itself, very useful. It may give you a clue to one reason why your error is bad, but not how big the error is. It's basically just what you would get as a 1 sigma error out of your position calculation if you assumed all of the ranges had a unit error. Some people still like to see the DOP, but it really doesn't contain nearly as much information as, say, an RMS position error estimate which takes into account both the geometry and estimated range errors.

 

The company line might be the EPE is a "a secret", but I don't think there's too much mystery in what it is. There may be some proprietary way they estimate the input range errors, but the EPE will still just be the 50% or 68% error calculated from the variance coming out of the positioning filter.

 

How good the EPE will be all comes down to how good the estimated range errors are. I agree that the EPE can miss some unmodeled errors, but it still tells you much more than DOP....

 

I certainly encourage the use of WAAS, if for nothing else than that the EPE has a better estimate of the iono error than it otherwise would. Some people complain that they sometimes see the EPE goes up when the enable WAAS. But even if the EPE is bigger, it's probably better.

Ummm, GPSlug, DOP is a multiplier that gives you the geometry effect of the satellite configuration as seen by the receiver. One thing it is telling you is whether you need to come back another time when the geometry is better - which is of little concern to the cacher or when driving your car (can't do anything about it other than try again later). It is of importance when mapping or other survey activities. It is NOT the 1 sigma error for unit range errors. But that discussion is beyond the scope of not only the thread, but GC Forum discussions generally. You are correct that DOP is only one component of the total error budget for a given location and time. Whether it is useful or not depends on your application and whether you know how to use it properly.

 

The problem with Garmin refusing to say what their version of EPE really means is, first, you can't really judge what kind of error you have other than good/bad/indifferent. Which is ok for 90% of the use of the typical consumer unit (yeah, yeah, if I need to know, I should - and do - use a survey-grade unit). Too many users seem to believe that their target is always going to be within the EPE distance of where the GPSR says they are located (go back to the original OP's question for an example). Too few people understand probability well enough to understand what "1 sigma" or "2 sigma" means, or even what "50% probability" means.

 

WAAS has 2 major components - the satellite errors (clock, ephemeris, and some other errors in the individual SVs) and the atmospheric errors (limited in usability to within about 500 miles of the reporting stations, which are only in the US, meaning that they are not applicable in much of Canada, nor in the rest of the world, though for those GPSRs that also pick up the EGNOS SVs, the correction messages are applicable in areas covered by the EGNOS stations). The satellite-specific corrections are applicable anywhere the GPSR is located, while the atmospheric corrections are only applicable within the areas covered by the observation stations. One reason errors with WAAS "on" happen is when outside the area where the atmospheric corrections are applicable. Garmin has acknowledged this and has suggested turning WAAS off when outside the areas where the reporting stations (WAAS and/or EGNOS) are located. Using the atmospheric corrections outside the area of applicability can indeed increase the error circle, which is the reason for Garmin's advice. Then again, as long as Garmin doesn't say what their version of EPE really means, that's a moot point. Just stick with it saying good/bad/indifferent and forget about it being quantitative. Just remember in any case that the position indicated is not exact, but only within 3 to 30 meters, most of the time being within 5 or 10 meters, but not always.

Edited May 1, 2008 by OGBO

[Grasscatcher]

Try this experiment for yourself.

Next time you go on a hike, have your unit on for a few minutes before you get to where you are going to start. In your car,or while you're getting your gear together,etc. Then when you save your starting point , "average" until the epe is under 20 ft or so.

GPS units don't "develop their thought processes " instantaneously or immediately when first turned on. You will sometimes, but not always, get inconsistent results then, so if it's important ....save more than one and compare.

Do the above and very likely, you won't have any complaints about your unit's accuracy.

[+julianh]

WAAS has 2 major components - the satellite errors (clock, ephemeris, and some other errors in the individual SVs) and the atmospheric errors (limited in usability to within about 500 miles of the reporting stations, which are only in the US, meaning that they are not applicable in much of Canada, nor in the rest of the world, though for those GPSRs that also pick up the EGNOS SVs, the correction messages are applicable in areas covered by the EGNOS stations). The satellite-specific corrections are applicable anywhere the GPSR is located, while the atmospheric corrections are only applicable within the areas covered by the observation stations. One reason errors with WAAS "on" happen is when outside the area where the atmospheric corrections are applicable. Garmin has acknowledged this and has suggested turning WAAS off when outside the areas where the reporting stations (WAAS and/or EGNOS) are located. Using the atmospheric corrections outside the area of applicability can indeed increase the error circle, which is the reason for Garmin's advice. Then again, as long as Garmin doesn't say what their version of EPE really means, that's a moot point. Just stick with it saying good/bad/indifferent and forget about it being quantitative. Just remember in any case that the position indicated is not exact, but only within 3 to 30 meters, most of the time being within 5 or 10 meters, but not always.

OGBO,

 

It certainly used to be the case with Garmins that you should turn WAAS off when you are outside the range of the ground-stations - presumably because applying the atmospheric corrections when outside the valid range of the ground-stations can introduce errors that are greater than the other satellite errors which WAAS can address globally.

 

However, my experience with a current Summit HC (with current software) in Australia leads me to think that Garmin MAY have addressed the issue so that if you are out of the valid ground-station "footprint", the unit may not be applying the atmospheric corrections, but will still apply the satellite corrections, leading to some small improvement in overall accuracy. Certainly, my old B&W Vista often gave worse accuracy with WAAS on than when off, whereas my Summit HC seems to give at least as good accuracy with WAAS on, possibly slightly better. (We're talking 2 or 3 metres here, so either way, I'm pretty happy!)

 

Sadly, this is all conjecture (based on my own observations, and those of other users), because the actual algorithm is a Garmin trade secret. However, I am prepared to suggest that on late-model Garmin, leaving WAAS on when outside the ground-station coverage will PROBABLY not degrade unit accuracy (most of the time!)

[+Klemmer]

Interesting error discussion (see what you started, outdoors4me?). But one thing you all forget is the marketing input into the EPE. Been there, done that. No company wants to show a worse EPE that their competitors under similar conditions. I'm convince there is a "fudge factor" input from marketing in the equation. Don't you think?

[+Smokiewolf]

Does averaging a waypoint increase the accuracy of a location?

[Grasscatcher]

Does averaging a waypoint increase the accuracy of a location?

 

 

Yes

[+GPSlug]

Ummm, GPSlug, DOP is a multiplier that gives you the geometry effect of the satellite configuration as seen by the receiver. One thing it is telling you is whether you need to come back another time when the geometry is better - which is of little concern to the cacher or when driving your car (can't do anything about it other than try again later). It is of importance when mapping or other survey activities. It is NOT the 1 sigma error for unit range errors.

But that's exactly how it's defined. It's derived from the position covariance matrix calculation by replacing the range covariance matrix with the identity. The GDOP is just the trace of that result. That's how it indicates the geometry-induced portion of the position error. I can't really show the matrix equations in this forum, but it's in the literature (e.g. B.W. Parkinson, "GPS Error Analysis", ch. 11 of Global Positioning System: Theory and Applications ed. by Parkinson and Spilker).

 

But that discussion is beyond the scope of not only the thread, but GC Forum discussions generally. You are correct that DOP is only one component of the total error budget for a given location and time. Whether it is useful or not depends on your application and whether you know how to use it properly.

Fair enough. It just sounded like you were indicating that the EPE didn't include the geometry. I could see you had a good background in this, so I wanted to make sure to back up what I was saying.

 

The problem with Garmin refusing to say what their version of EPE really means is, first, you can't really judge what kind of error you have other than good/bad/indifferent. Which is ok for 90% of the use of the typical consumer unit (yeah, yeah, if I need to know, I should - and do - use a survey-grade unit). Too many users seem to believe that their target is always going to be within the EPE distance of where the GPSR says they are located (go back to the original OP's question for an example). Too few people understand probability well enough to understand what "1 sigma" or "2 sigma" means, or even what "50% probability" means.

The problem with a green/yellow/red indicator are that one application's green is another application's red. Even among users of the same handheld, the threshold for green is going to be much bigger for a hiker who just wants to get back to the road than for someone looking for a pill bottle in a field of rocks. And you still need to base that threshold on some quantitative estimate of accuracy, like the EPE, so you don't get rid of the problem of needing to make a good estimate. The EPE, as questionable as it is, still gives you the choice to decide if it's good enough for your situation. Maybe they just should have bigger disclaimers on how much you can trust it. Edited May 2, 2008 by GPSlug

[OGBO]

.... Maybe they just should have bigger disclaimers on how much you can trust it.

 

You really believe people read disclaimers???!!!! "Read the manual? Read the manual? What manual?" or "We don' need no stinkin' manual".

[+Night Stalker]

There is another thing to consider here. The OP says he has WAAS enabled, but unless the D's show up on his satellite screen he isn't getting the benefits of WAAS. It is entirely possible that he was never logged on long enough prior to marking his position for WAAS to download the almanac.

 

My experience with the 60CX has shown me that the EPE displayed on the screen is a lot more conservative than the EPE shown on my older 60CS. I suspect that they were given enough grief on the old unit that they decided to err on the conservative side. I have found that the new unit is consistently closer then the EPE displayed.

[OGBO]

Does averaging a waypoint increase the accuracy of a location?

 

As Grasscatcher said, in a word, yes.

 

Problem is that the manufacturers (as usual) implement averaging differently (and change with generation of unit, too). Garmin has an explicit averaging in some of its units. Magellan has "automatic" averaging in some of its units (pause at a location for more than a few seconds and it starts averaging - some older units allowed turning "auto" on or off).

 

Main thing is to be sure you average long enough. One minute is really too short (60 samples on the Garmin units that display the count). 15 minutes is generally reasonable. It seems best if you let the unit sit for a minute or two before starting the averaging, and certainly set it down at the spot you want marked (I've seen a few people wandering around while "averaging" - seems intuitively obvious to let the unit sit in a fixed location, but still, some folks ....)

[OGBO]

Does averaging a waypoint increase the accuracy of a location?

 

As Grasscatcher said, in a word, yes.

 

Problem is that the manufacturers (as usual) implement averaging differently (and change with generation of unit, too). Garmin has an explicit averaging in some of its units. Magellan has "automatic" averaging in some of its units (pause at a location for more than a few seconds and it starts averaging - some older units allowed turning "auto" on or off).

 

Main thing is to be sure you average long enough. One minute is really too short (60 samples on the Garmin units that display the count). 15 minutes is generally reasonable. It seems best if you let the unit sit for a minute or two before starting the averaging, and certainly set it down at the spot you want marked (I've seen a few people wandering around while "averaging" - seems intuitively obvious to let the unit sit in a fixed location, but still, some folks ....)

[bmirak]

Does averaging a waypoint increase the accuracy of a location?

 

 

Yes

 

Generally, yes. If your first reading happens to be very accurate, averaging with other readings could actually degrade your accuracy! Overall, however, you will generally get more accurate waypoints by consistantly averaging.

[outdoors4me]

Wow, I guess I really know how to stir up the pot-o-poop huh?

 

Beck

 

[OGBO]

...But that's exactly how it's defined. It's derived from the position covariance matrix calculation by replacing the range covariance matrix with the identity. The GDOP is just the trace of that result. That's how it indicates the geometry-induced portion of the position error. I can't really show the matrix equations in this forum, but it's in the literature (e.g. B.W. Parkinson, "GPS Error Analysis", ch. 11 of Global Positioning System: Theory and Applications ed. by Parkinson and Spilker).

...

 

That depends on your point of view. As one trained first in astrodynamics and having spent time arranging constellations, geometry dominates all . So my goal was reducing GDOP first and foremost. It was only later that I had to worry about the folks who kept putting in outdated ephemerides, building less than perfect clocks, and other practicalities. And then I had to re-learn general relativity (20 year gap from coursework). It's like looking at orbits from the force standpoint vs the energy standpoint - same result in the end.

[Grasscatcher]

Wow, I guess I really know how to stir up the pot-o-poop huh?

 

Beck

 

 

Yeah, and really that's all it is. They are just trying to impress each other and ????? when it really doesn't amount to a hill of beans.

 

Take your GPS and use it, experiment with it, play with it, change the settings and see what results you get, learn from it , but most of all, enjoy it!

[+LifeOnEdge!]

There is another thing to consider here. The OP says he has WAAS enabled, but unless the D's show up on his satellite screen he isn't getting the benefits of WAAS. It is entirely possible that he was never logged on long enough prior to marking his position for WAAS to download the almanac.

 

My experience with the 60CX has shown me that the EPE displayed on the screen is a lot more conservative than the EPE shown on my older 60CS. I suspect that they were given enough grief on the old unit that they decided to err on the conservative side. I have found that the new unit is consistently closer then the EPE displayed.

 

I would have to agree whole-heartedly with this. There is currently a fight going on about the Garmin Colorado not having an averaging feature when you mark a waypoint. Personally, I fell that the averaging on the GPSmap 60 units, when you actually have a good, settled signal, is useless. I rarely saw the final coordinates differ by more than .001' (minutes of arc) and usually in just one of the two numbers. Now the "community" thinks that Garmin "needs to get their act together" and give the Colorado an average function. I (secretly) feel that this is probably needed ONLY due to the fact that most GPS users don't wait long enough to let their units actually function optimally before they start taking waypoints OR looking for caches.

 

Imagine the frustration when a cache owner used the old little yellow eTrex in a rush and then published those numbers and expected others to come look for their caches. I gave up on one central Texas couple's caches for this very reason. Oh the fun we had after finding one of their caches over 700 feet from ground zero!

 

Original Poster: you've learned that you need to get a good signal and possibly even a lock before you mark your waypoint. Doing so will give you an accurate point to return to.

[+Night Stalker]

As a matter of fact I have searched for caches hidden that way as well as ones set with the Magellan SporTrack Pro by a user that was to impatient to wait for it to settle down. Both were a lot more challenging then they needed to be. The worst was 1/2 mile off. The only reason I found it was that a previous finder left corrected coordinates in his log. How he found this cache still amazes me.

[2 JABs]

I realize I'm waking up an older post here but I have to agree, this is by far and away the worst GPS unit I have ever owned. It will never get you back to the same place twice. My 4 year old car GPS is more accurate and reliable by leaps and bounds over this unit. I cannot see what all hype with reviewers on this unit is. It is over priced, compliated and just not at all user friendly let alone accurate.