Confused Garmin

[americancyclo]

was the course file .gpx or .tcx

what’s your basemap?

[Geoff]

@americancyclo 93239 wrote:

was the course file .gpx or .tcx

what’s your basemap?

The course file was a .gpx.
The basemap, unless I misunderstand the question, was whatever came with the Garmin. I did nothing to load any maps except to turn the unit on.

[Raymo853]

It sounds like the Garmin dataset does not have the W&OD as a record, or if it does it either thinks it is not open to bikes or has a variable favoring nearby roads instead. Or it could be the precision of your location was off for a bit then. GPS signal strength varies all the time depending on a number of factors.

[Vicegrip]

How long was it confused for? I have had my 500 and my 800 tell me I am off course when I am not. It tends to clear itself in a min or so and tends to do this after a sharp turn and or when under heavy tree cover

[Geoff]

@Vicegrip 93253 wrote:

How long was it confused for? I have had my 500 and my 800 tell me I am off course when I am not. It tends to clear itself in a min or so and tends to do this after a sharp turn and or when under heavy tree cover

How long? Not really sure, maybe as much as 5 minutes.
The second time it happened, it was sortof spazzin out. It chirped, recalculated, chirped, recalculated,… rinse, lather, repeat. I wasn’t making any turns at the time except for the slight bends in the trail. The W&OD doesn’t have any tree cover where I was. I think Raymo’s thought about GPS strength is on to something. Tree cover could be one of the factors, as you say. It could also be the number and position of the satellites at the time.

[Vicegrip]

@Geoff 93255 wrote:

How long? Not really sure, maybe as much as 5 minutes.
The second time it happened, it was sortof spazzin out. It chirped, recalculated, chirped, recalculated,… rinse, lather, repeat. I wasn’t making any turns at the time except for the slight bends in the trail. The W&OD doesn’t have any tree cover where I was. I think Raymo’s thought about GPS strength is on to something. Tree cover could be one of the factors, as you say. It could also be the number and position of the satellites at the time.

only one data point but I have never lost gps lock on the W&OD. What does the ride track look like? Any anomalies or straight lines between drop out points?

[Geoff]

@Vicegrip 93262 wrote:

only one data point but I have never lost gps lock on the W&OD. What does the ride track look like? Any anomalies or straight lines between drop out points?

That’s an interesting question. Just looking at the screen during the ride I didn’t notice anything unusual about the map displayed or even the directions, until it got confused. Though I could have overlooked something, since most of the time I was watching where I was going. I could probably look at the course map – is that what you meant? If you meant, are there any anomalies in my recorded ride, well, I didn’t record it.

[americancyclo]

can you upload the gpx file you used to navigate the course? Its possible that the GPX course, your basemap, and the GPS signal couldn’t agree on your exact location, and so it might have been trying to route you to a virtual parallel “track” that was 20 or 30 feet to the left or right of the W&OD. I’ve seen this happen with Google Maps on my Android phone, and maybe once or twice on my Garmin Edge 800.

[mstone]

Sometimes we get so used to the fact that we have these little devices that can figure out where we are anywhere in the world based on tiny radio signals sent by at least four satellites orbiting at 9 thousand mph more than 12 thousand miles away that it seems weird that a given track may be off by a few feet. In general all of the consumer navigation devices adjust for position uncertainty by choosing a known path near the calculated position and locking the plotted course to that path. If your calculated position is near more than one path you might end up locked onto the wrong one until your position is far enough away from it that the device is forced to choose a different path. 30-50 feet (horizontal) is a reasonable level of accuracy for a moving consumer GPS device. On a highway that’s lost in the noise as the highway is wider than that. On a path, it can be significant. I’ve seen some cases where a device thinks that the W&OD paved track and the W&OD gravel track are separate roads, and those are well within the GPS margin of error. The service roads you see in places are also hell on the gps systems.

[Author]

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