GPS off by 36 feet

[DougPaul]

In other words, if you're crossing the knife edge watch your step, go slow and you will never need to even look at the GPS.

OTOH if you are on Washington or on the Tableland in a white out and have your GPS (and you are getting reception) you would have a useful tool. Sort of like map and compass in one little handheld. Until the batteries stop working ...

PB is correct here.

Just to add a scenario:

For the scenario (say on a perfectly fogged in plateau) where you want to mark a spot (say, your tent), go out, and then return:
* you have a (for instance) 10 meter accuracy upon recording your initial point (eg setting a waypoint)
* you have a (for instance) 10 meter accuracy upon measuring your location upon return.
* The combined accuacy is 14 meters. (square root of the sum of the squares)

So you would have a net accuracy of 14 meters in refinding a previously marked location.

Doug

 

[Kevin Rooney]

... WAAS is an FAA system designed to improve accuracy for flying aircraft. IMO, it is of little benefit to a hiker. 10 meter accuracy is quite adequate. I generally leave it turned off. Doug

I agree most emphatically - depending upon the model of the GPS, it can also be a huge drain on the batteries as well.

(Welcome back, Doug. Glad to hear you're back home.)

Kevin

 

[CaptCaper]

WAAS is an FAA system designed to improve accuracy for flying aircraft.

It can, under certain conditions, actually decrease accuracy. The current WAAS satellite constellation is currently being changed and reception on the ground is poor in the NE.

IMO, it is of little benefit to a hiker. 10 meter accuracy is quite adequate. I generally leave it turned off.

Doug

Sorry

I disagree again... I get excellent accuracy in the Whites etc. and have so for years.. With the WAAS on...Period.. It's just the way waas works.. the gps will carry waas corrections for a while then when locked in it again it will update the correction and on and on..

I know because I have the tracks saved and reuse them in the field..

I wouldn't turn it off..If you do you'll lose more accuracy..

Plenty of battery life in the newer models so that's not a factor at all in my book.

 

[jjmcgo]

Honing in

Thanks, DougPaul. Good to see you back on the beat.
I'm thinking that if I'm sitting at the stop sign at Vermillion Drive and Trenton Rd. and my GPS says I'm 36 feet from the intersection of those streets, well, where is the actual intersection?
Not likely at the stop sign but at the middle of Trenton Rd., which is probably 16 feet from the stop sign. Subtract 16 from 36 that's 20 feet and within the plus-minus accuracy range of 10 meters.
I'm going to look for someone locally with a survey-quality GPS who knows a survey-verified location.
This is interesting. Thanks.

 

[Kevin Rooney]

Plenty of battery life in the newer models so that's not a factor at all in my book.

That's good to know. I have a Garmin Vista that's a few years old, and if I turn on WAAS, I've got to replace my rechargeable NiMH every couple of hours or so. For the types of navigation I use my GPS for, I don't need the level of precision WAAS is designed to provide.

 

[Paradox]

But Ocifur, the little thingee on my GPS says I'm in the middle of the road and not on the sidewalk!

 

[DougPaul]

I wouldn't turn it off..If you do you'll lose more accuracy..

Obviously, you are welcome to your opinion. Nowhere is it ordained that we must agree.

Plenty of battery life in the newer models so that's not a factor at all in my book.

WAAS does draw a significant extra amount in some models (eg eTrex Vista) and no additional current in at least one newer model (60csx).

Doug

 

[DougPaul]

I'm thinking that if I'm sitting at the stop sign at Vermillion Drive and Trenton Rd. and my GPS says I'm 36 feet from the intersection of those streets, well, where is the actual intersection?
Not likely at the stop sign but at the middle of Trenton Rd., which is probably 16 feet from the stop sign. Subtract 16 from 36 that's 20 feet and within the plus-minus accuracy range of 10 meters.
I'm going to look for someone locally with a survey-quality GPS who knows a survey-verified location.
This is interesting. Thanks.

Yes, that is an issue--what part of the intersection are they using as a reference? (And, of course, an intersection is actually a zone, not a point.) Furthermore, are they using the same reference point for all routes of entry into the intersection?

How did they determine the location? Each intersection might have a different error.

It is possible that you are just expecting too much from the device/system.

Doug

 

[hikingfish]

PB is correct here.

Just to add a scenario:

For the scenario (say on a perfectly fogged in plateau) where you want to mark a spot (say, your tent), go out, and then return:
* you have a (for instance) 10 meter accuracy upon recording your initial point (eg setting a waypoint)
* you have a (for instance) 10 meter accuracy upon measuring your location upon return.
* The combined accuacy is 14 meters. (square root of the sum of the squares)

So you would have a net accuracy of 14 meters in refinding a previously marked location.

Doug

I'm curious to know if using the "average waypoint location" on a GPS (mine's a Garmin) would reduce this? It takes a bunch of readings in succession and calculates the average location.

Also, concerning another post: some GPS' have built in protection to detect if WAAS given location is worst than the "normal" GPS location and if this is the case, it reverts to using the normal GPS location even though WAAS is enabled. Read it somewhere in my Garmin manual!

Fish

 

[DougPaul]

I'm curious to know if using the "average waypoint location" on a GPS (mine's a Garmin) would reduce this? It takes a bunch of readings in succession and calculates the average location.

It will help some. There are both short-term (seconds to minutes) and long-term (hours) random errors. The averaging in the GPS can help to reduce the short-term random errors.

Also, concerning another post: some GPS' have built in protection to detect if WAAS given location is worst than the "normal" GPS location and if this is the case, it reverts to using the normal GPS location even though WAAS is enabled. Read it somewhere in my Garmin manual!

I have seen discussion of this on sci.geo.satellite-nav. Not every one seems confident that it works very well. Note that if you are recording a track, it will jump back and forth between with and without. (The use of WAAS is not recorded in the track, so you will have no idea where the transitions are.)

Also the GPS cannot not know whether with or without is actually more accurate--it can only guess based upon the estimated accuracy (itself a shakey estimate).

Doug