GPS callibration problem?

[Tortoise]

Well, , my first confession is that i'm still embarrassingly clueless re how to use a fraction of my GPS's functions, but it generally tells me where I am if I want to know, and it's been brilliantly useful at times.

I tried to select and save the bit of track to show what I wanted to ask about, but alas, hours have passed, and I have other things I need to be doing.

The problem:
While I walked up and back the same bit of road approaching Syd's track the other day, the breadcrumb trail showed a difference of up to about 50m between the outgoing and return route. ie it looks like I took a completely different track. I might have been 4 m away from my outward steps, but certainly not 50! I haven't struck this before, and it makes me nervous.

I did change batteries in between, and forgot to recallibrate the compass. I didn't bother later, as I was using my other compass, and I didn't think that would change my recorded position. I just thought if I used the compass function, it wouldn't necessarily be accurate.

Was this the problem, or could it have been something else?

Thanks for your thoughts.

[photohiker]

Not a Compass Calibration problem.

GPS accuracy can vary, especially if the reception is marginal. Was this under tree cover or in a valley?

Usually, if you go to the page on the GPS that shows the satellites, it will also give a rough indication of accuracy.

[GPSGuided]

Compass calibration has no relevance, far more likely to be due to inadequate sat signal and lock-on on one or both of legs of your walk eg. Narrow gorge, heavy overhead vegetation etc. It always help to wait for the GPS to achieve a good lock at the start.

[wander]

I have found it helps to have had the GPS running for several minutes fully locked onto the sats prior to starting to get a repeated track overlay.

One of my regular wander routes starts in a steep sided valley with limited sky vision for the 1st 2 km. So I often start the GPS on the way to the trail head and once out of the car and ready to start close the old track from the car and start the new a track for the wander leg. I delete the car track later. This seems to result in well overlaid tracks.

Also look into if your GPS has a various modes such as "regular" and "ultra track" the latter being a Garmin version of tracking using less points to reduce battery drain and save on memory space.

And check if the setting for the tracking has been altered to be set at "every 10M" or "every minute" as opposed to an auto setting such as "regular" the latter should give the more accurate track.

[GPSGuided]

The other unsaid is location of your GPSr. It needs to see the sky without obstruction and water (for which 70% of body is) will block out the signals. So unless you are in the open and the unit has good antenna and is one of the late generation unit, take it out of the pocket and hang it high on the backpack or over the shoulder.

[Happy Pirate]

The other unsaid is location of your GPSr. It needs to see the sky without obstruction and water (for which 70% of body is) will block out the signals. So unless you are in the open and the unit has good antenna and is one of the late generation unit, take it out of the pocket and hang it high on the backpack or over the shoulder.

If I want to log a track or use a GPS in heavy cover I use an external aerial. A small metal plate under my hat will allow my magnetic external aerial to snap to the top of my head. The GPS unit can stay in my pack.
Trying to locate the best signal in polite (and unaware) company can be amusing.
cheers
Steve
"Meep Meep Meep - my sensors indicate lifeforms in the approaching area."

[GPSGuided]

Do many personal GPS units these days provide for a external antenna port?

[tastrax]

Do many personal GPS units these days provide for a external antenna port?

Not a lot these days, mainly those used in boats I think - a few are listed here (most are older models)

http://www.gpsoz.com.au/External_Aerials.htm

[Happy Pirate]

Do many personal GPS units these days provide for a external antenna port?

The Garmin GPS/MAP series certainly do. Not sure about other brands.
These days you could get a decent Bluetooth GPS (hat-mounted of course) to connect to an iPhone/iPad/iPod/iPoodle.
......

Of course if you're trying to port a Garmin GPS to your iPoodle you'll probably need to re-route the input to a different port than the one prioritised beneath the TAIL extension. The main iPoodle port can bottleneck because of data issues with redundant output data taking priority in a SQUAT transfer to the GRASS protocol in a Program Observed Output (P.O.O.) issue.
Generally a wipe is undesirable but protocol dictates that a collection of redundant data from the GRASS protocol is a valid operation in the majority of circumstances.

If the Assisted Signed Solution (A.S.S.) port shows signs of data blockage it may, in extreme circumstances, be necessary to apply the Forced Integrated Non-General Ergonomic Release (F.I.N.G.E.R.) solution to assist the transfer of data.
In this circumstance a wipe on the GRASS protocol is desirable after transfer.

I'm glad to be help in these technical issues
Steve

[tastrax]

The older Garmin 60 map series certainly have external aerial connections but not all the newer 62 series (62s and 62st have the connectors but not the models with cameras )

[madmacca]

A small metal plate under my hat will allow my magnetic external aerial to snap to the top of my head.

Sheesh - I already put aluminium foil in my hat to keep the government's radio waves out. It's getting crowded under there with a metal plate as well.

[Ent]

Curious what make and model you have.

As mentioned location on the body can make a huge difference. I either carry mine in the top of the pack lid or clipped high on the shoulder strap. The Garmin Fenix worn on the wrist does give the worst signal pickup. As for accuracy you have a couple of factors. It is my understanding that Garmin's maps (and probably their data compression of tracks when saved) is limited to 2.5 metres accuracy. I have found 10 metres accuracy or better the norm with the Garmin 62s and Rino 650. As for the Fenix it is not too bad but steep valleys muck it around so often you can be fifty to hundred metres out or worst still no position lock.

My favourite test circuit is the Gorge in Launceston. Generally the track will show you jumping from one side of the river to the next. A GPS can give very good position but they are not a pin point as a lot of the blab would make you think. With a clear view of the sky just about any unit will give a good account but in steep wooded valleys the extra cost and bulk of models such as the 62S pays off.

Cheers