Does Nuvi55 know what lane I'm in?

Not sure about the nuvi55 but my 1690 cannot. It can not even tell if I am on the feeder road or interstate that goes along side it. My phone GPS is no better, google maps and WAZE cannot discern the difference either.

If you come to a stop at an intersection look at the distance the garmin thinks you are from the intersection. Most units nowadays are pretty good but I doubt any for driving are this accurate.

I'd be really surprised if the GPS had that kind of resolution ... I'd be surprised if it's +/- 25 feet as a general rule.

If you were following a route at the time it would assume (I think) you were in the correct lane ...

Automotive GPS users see their location on a roadway but this gives an artificial sense of the unit's accuracy. Your GPS's raw data may have you a few tens of feet off the roadway but the automotive GPS devices have a built in "snap to" feature that places your vehicle on the actual roadway rather than being in the median or ditch where the raw data thinks are are.

An internet search for GPS snap-to will provide more info as will this forum thread:

http://www.poi-factory.com/node/24138

The GPS is more accurate than 25 feet.
As an example, I have created a Des Moines Schools and suburbs TourGuide for my Garmin. It gives me a warning message "Beware, you are entering a School zone".
I used Google Earth to pinpoint all schools and roads leading to them. Each school has at least 2 POIs at a set distance. All POIs are set at the curb side. I then with EPE take that point and draw a circle from that point and not have it cross the center line. This gives you the warning as you approach the school but not as you leave it. For 2 lane roads I start with a proximity of 10 feet.

I have looked at the accuracy of all of my units and the very best one of them ever got was 14 feet (my 765W, sitting still in my front yard). I just took the 765 and the 2595 out for a drive with both of them on the satellite page. the accuracy was all over the map but not ever less than 40 feet. In areas of high density power lines, the accuracy got more than 100 feet.

It would be helpful for those who think their automotive device is very accurate to actually check what their device says:

How Do I Access The Satellite Information Page On My Automotive Device?

Hopefully, many of you will take the challenge and drive around with the satellite page on and have a passenger call out the "accuracy" every 10 seconds or so. If you do, please give us some details on your results.

I don't need a second person... Mine talks at me all the time

I understand you made your comment to add interest to the post, but I wanted the member who was checking the accuracy of his device - which does not speak from the Satellite screen - to watch the road ahead and not look at the device.

John

With WAAS, Glonas and the demise of Selective Availability, consumer grade GPS accuracy has improved quite a bit over the years. An estimated position error (EPE) of 10 feet is now quite common with an open horizon.

Keep in mind that this is in relation to the earth's surface, not the map. The snap to feature compensates for this visually on the GPS screen but there is the map error relative to the earth's surface to consider.

For the most part, the maps we use in our GPSr's are fairly accurate but there is some error which, like EPE, varies widely with location.

With all these variables, it is not possible for the average GPSr to reliably and consistently determine which lane you are in.

I thought that only the nuvi 300 and 600 series had WAAS.

I've gotten so used to nearly continuous GPS reception with a modern unit sitting on my dashboard that it is easy to forget the early days when a receiver that could only handle four birds at a time frequently lost lock for a minute when a small turn or change in local obstructions meant it had to search for and require a replacement satellite signal for a lost one.

While the receivers listen to far more satellites now, and have somewhat more receiver gain (partly by listening more just past the edges) there is still plenty of commotion in real vehicle navigation in real road environments.

Combining that with map error, and the not entirely negligible primary errors, makes for a big enough error budget that a manufacturer would be begging for massive trouble in building in a feature that counted on establishing lane usage purely from position data.

So my answer to the original query is that it only knows what lane you are in if it has issued an instruction, and the position data as contrasted with the map don't make it clear that you are not complying with the instruction.

You are correct. Only 300 and 600 series Nuvis are WAAS enabled as well as some 700 series European models.

Newer Nuvi's with higher sensitivity receivers now get the same position accuracy without the use of WAAS.

When you say device accuracy, I think you really mean the computation of Dilution Of Precision (DOP). DOP shows the effects of how the current satellite geometry affects known system errors. It is not the actual current accuracy, although it is typically used to indicate the average accuracy. Sometimes you are almost perfectly located; other times your actual error exceeds the DOP value.

I meant "accuracy" as displayed on the Satellite Information screen of my nuvis. My assumption was that my nuvi was trying to alert me to how far off from the actual coordinates it was displaying to me might be.

I think that there are some Garmin devices that display both "accuracy" and "DOP" on their particular Satellite Information screen.

That is all I was referring to. I am way out of my element in discussing what DOP means for nuvis.

I took my Garmin outside and let it sit on my deck rail for 20 minutes and then took the coordinates. I then went to my computer and placed them in Google Earth. Then I took the ruler and measured and found out it was 10.5 feet off.
I then set the Garmin on the window sill and let it sit for 20 minutes and when I entered the coordinates in Google Earth, I was on the other side of my neighbors house or 62 feet away.
When looking at the coordinate screen and pushing the next button on the Garmin one will see all kinds of lines crisscrossing as you push the magnify button to 50 feet. If you leave the magnification screen up eventually you will see a circle with something in the middle moving around drawing lines as it searches for satellites.
The accuracy depends where you are.

Your experiment is an excellent example of what we deal with when using our GPSr’s. The 62 foot measurement you made is actually a combination of the GPS error (EPE) plus the error associated with Google Earth, or any map for that matter.

Determining the magnitude of each error separately in any one place is quite difficult and requires a great deal of work. Fortunately, these errors are rarely large enough to cause problems when using a consumer grade GPS for navigation.

sometimes when the GPS will direct me to remain on a highway and I get off an exit where the road is parallel to the highway and the GPS will show me on the highway.

I have had Google earth and google maps tell me what part of my house I am in, but my GPS Nuvi 55 get that close.