GPS off by 100 feet?

 

Anyone ever run into an issue where their GPS is off by a few hundred feet? I was driving around the other day and I showed up about 100-200 feet due north of where I was. Didn't matter if I had just pulled out of the parking garage, was driving downtown, was on the interstate heading out of town, when I was in less populated suburbs, I was consistently 100-200 feet south of where my GPS thought I should be.

Ir's a rare occurrence, but now it's happened enough times that I'm curious as to what's causing it!

.

Back it up, and do a hard reset. That should fix the accuracy problem.

--
nüvi 3790T | Those who make peaceful revolution impossible, will make violent revolution inevitable ~ JFK

your case not likely to be multipath

An answer to your first sentence is: yes, many times. I live very near a mountain, and I believe I somewhat often get brief signal reception, typically right after I turn on the unit, for which signal from one satellite is received "on the bounce" from a concave surface up hill which by bad luck gives just enough focussing gain for the resulting wrongly timed signal to be detectable. If used in the position calculation the time is in error by hundreds of nanoseconds, and thus my position by hundreds of feet.

But this requires rather specific unlucky reflection conditions, and seems thoroughly unlikely to explain a persistent error of the specific type you've described here.

--
personal GPS user since 1992

Can't say I've ever

...had this happen before

Happens to me on occasion as well

When it happens to me I am assuming it is a feature of the "snap-to" the road not working as well as it should. In my area there seems to be a few places where the vehicle will track the road.. then, the vehicle does some wild swings and "most" times finds the road properly. I am guessing this is where some mapping tiles (?) meet and the alignment is not quite perfect.
I keep hoping the next map update fixes it.. but so far the last 5 have not.

--
Lives in Edmonton AB A volunteer driver for Drive Happiness.ca and now (since June 20 2021) uses a DS65 to find his clients.

GPS off by 100 feet?

Never

Hard Reset

I agree with Juggernaut. Since you see this deviation apparently in any environment and not just after you turn the GPS on, it appears that the innards are out of sync. As another mentioned, it isnt likely multipath reflections that sometimes occur in the canyons of big cities with tall buildings. A hard reset is a good bet to cure this. As mentioned back it up, Garmin Express will do this for you. A hard reset often deletes your Favorites/Saved Places and Routes and track history and restores your settings to factory defaults. After the reset and restoring your data, you may still have to go through the menu tree to re-establish your preferences.

You didn't mention your model and it's not in your profile, but with most nüvis, if you press and hold over the signal strength icon in the upper left corner of the main screen for a few seconds, you should get a display of the real time constellation of visible satellites above you along with relative signal strengths of each it sees. Most models have a 12 channel receiver capable of locking onto the theoretical maximum number of birds it can see at any given time. This is common on the plains of the Midwest and the open expanses of the Southwest, but it often sees less than 12 and even 5 or 6 if dispersed properly can afford excellent accuracy. The GPS position in the car also factors in as how much of the roof "shades" the receiver. All in all, 10-20' accuracy is probably common in the best of circumstances, mabe 30-50' on a bad day. 100-200' should be a very rare occurrence IMO.

--
"There's no substitute for local knowledge" nüvi 750, nüvi 3597

Garmin Express Backup

TXRVer wrote:

... A hard reset is a good bet to cure this. As mentioned back it up, Garmin Express will do this for you. A hard reset often deletes your Favorites/Saved Places and Routes and track history and restores your settings to factory defaults. After the reset and restoring your data, you may still have to go through the menu tree to re-establish your preferences.

...

I'm just nit-picking here but wanted to point out that the "Backup" that Express does is limited to your Favorites/Saved Places.

Please do not think that Express has done a full backup of the "unit".

hard reset..

I'd clear it all and start from scratch. That'll definitely fix it.

Seen that too

There's an area NW of Phoenix where the GPS if off. Not all the time but sometimes by up to a half mile. Initially I thought it might be a map error but there has been years (you'd think they would have corrected it) and it is not constant. I've never been able to come up with a convincing explanation.

...scary to think planes use GPS to land, sometimes!

What I think

Depending on where you are (mountains, tall buildings) or possibly in your case a weak signal due to location in vehicle can cause this.

When I'm in the house and have a week signal I show my track all over the place and in every direction depending on what satellites are locked up and how often the unit is switching between them.

However; once you get good reception everything should fall in place fairly quickly and the device should stop satellite hopping.

Monitor the satellite page next time and check the signal strength.

--
garry

Well it rarely happens.

Well it rarely happens. Maybe once or twice a year. And this time I came out of a parking garage in downtown, so I get the bouncing signal. What I don't get is why it didn't self correct when I left downtown.

Not weakness but bouncing

garry1p wrote:

When I'm in the house and have a week signal I show my track all over the place and in every direction depending on what satellites are locked up and how often the unit is switching between them.

In that case it is not the weakness of the signals nor the change of satellites employed that is causing the inaccuracy. Instead it is the intermittent conclusion of signal from a satellite that is reflected instead of direct path. GPS signal processing directly generates a time-of-flight from the satellite to the receiver which goes directly into the position calculation. Traveling along a bounce path adds time, which directly creates position error.

That is what multipath is, and it is a serious source of position error.

--
personal GPS user since 1992

Any interferance

Some kind of transmitter close to GPS?