lane detection not very good!

[mccririck]

I an using TomTom One and today I was going along the motorway, and accidentally went up a sliproad off the motorway and had to re-join. All the time the TomTom showed me going along the motorway as normal! This seems quite poor performance imo.

 

[dhn]

How solid was your gps signal at the time?

And what map version have you on the unit?

 

[canderson]

A couple of factors are involved in this. The first is that not all roads are located with high precision on the maps .. Navteq, Teleatlas, you name them, there are always some mismatches in geolocation. It's getting better over time, but it still occurs. There could easily be a 100' error in the vector map vs. the reality.

Add to that terrain where there is additional imprecision in the ability to establish a good set of GPS coordinates (something you see rarely there, but is a frequent situation in the mountains here in Colorado). We sometimes get a weak lock here where the EPE (Estimated Position Error or Estimated Probable Error, depending upon who you believe) is pretty terrible, and actual position is not known within more than 100 feet or more to begin with.

Because such issues were anticipated by the auto nav GPS manufacturers, the "road snap" feature was incorporated into both automotive and mapping handheld units - you can usually only turn it off on the latter. The manufacturers didn't want these units constantly indicating that you were having some kind of off-road adventure! The unit makes its best guess as to where you are, and puts you squarely in the center of that guess.

In addition, each manufacturer uses their own proprietary algorithms for making the determination that you've left the road you were on, or for that matter, any known road on the map. If you don't appear to be deviating by more than xx feet from your current projected route, or more than xx feet from the geolocated position of the road, the device will assume that you have continued as expected.

In your case, it is likely that the "slip road" was very close to the main road, and in addition, it is possible that a geolocation error caused the unit to think that you were actually improving your actual position against that main road -- or at least not putting you far enough off track to assume you were on a different one.

Stuff like this happens -- thankfully, not all too frequently, but it all hinges on the accuracy of the map vs. reality and the tightness of the algorithms used by the GPS to decide what the more likely reality is.

 

[canderson]

I'm cross posting this over here only because it more directly addresses this question:

What you're really asking is for the map producing companies like Navteq and Teleatlas to tighten up their geolocation of roads sufficiently to detect where the lanes actually are. Truth to tell, we're all lucky that we're even being placed on the right road sometimes - and as you discovered, sometimes we're not. The actual road position vs. the coordinates for the road in the vector maps just aren't good enough yet to make lane detection possible no matter how good the GPS accuracy. Although it doesn't happen often around here, I've had issues similar to yours where my 720 thinks I'm on what you call a "slip road" (a "frontage" or "service" road here). Running the Teleatlas map over a well geolocated satellite shot shows offsets like that here and there. Same happens with Navteq maps. Another thing that can throw things off are road realignments that are not picked up quickly by the mapping companies.

Here's an example of a missed geolocation not far from home (for me) of a major highway. It's not a very radical example, but it shows how this problem occurs, and why the "road snap" feature on any GPS can be fooled. This particular example uses whatever mapping service Google is currently using in this area:

brighton, co - Google Maps

With that level of map accuracy, there's not a lot the GPS can do, and you can imagine examples where a slip road might actually be closer to the expected coordinates of the main road.

 

[canderson]

Here's an example of a major highway where the geolocation is off by a good 50 feet. If there had been a slip road to the west of the western lane and you took an exit down that slip road, the TomTom would likely be even more certain that you were on the highway instead of the slip road -- your coordinates would put you right on I-25 according to the vector map. I know I've seen much worse offsets (on the order of 100' or more), I just can't recall where at the moment.

http://maps.google.com/maps?f=q&sou...952477,-104.473468&spn=0.003284,0.008256&z=18

 

[dhn]

Hmmmm......a 5 hour drive from there to Roswell, canderson.

I'm just sayin .............

 

[canderson]

And not all that far from the piece of the 46th that operates out of Holloman, for that matter. Too close to the insanity in Juarez for my taste, but convenient if that's where you're going.

 

[badbob]

Could this be why my TT 920 has been telling me to make my next turn at the point that I'm on an overpass?

 

[canderson]

Could this be why my TT 920 has been telling me to make my next turn at the point that I'm on an overpass?

That's entirely possible, especially if the overpass was rebuilt at one time and the lanes significantly realigned in the process. Give me a pointer to the overpass and your position on it when things go weird and we'll try to have a peek at what Teleatlas is doing with it.

 

[gatorguy]

Related to this, I've found that of every device I have or had, from TomTom/Teleatlas to Garmin/Navteq (and Teleatlas now but that's another story), both on-road and off-road models, and just about every major manufacturer, my Motorola Droid has so far been the most accurate in positioning. None of my other devices can match it. Consistently and repeatedly putting me within 6', based on both my displayed Estimate of Position Error and visual evidence. The visual is the key. By using Google maps with the satellite layer active, it's quite easy to zoom in and follow my vehicle (or walk) using landmarks, trees, driveways, etc for confirmation that where my Droid "finds" me is pretty dang accurate. By using a combination of raw gps, known wi-fi locations, cell location and unique cell ID, my phone has now become my most reliable indicator of my exact location. It's so accurate that it can show me in the proper lane on even a two lane country road. Moving to the left and I have visual proof that my Droid also correctly shows me in the left lane. Pull to the shoulder and again that's where my phone will show me. Stand on the edge of my driveway, and sat view shows me exactly there. Having used previous smartphones for (poor) navigation, the accuracy and fast TTF of the Droid came as a huge surprise.

 

[mccririck]

Look at this abortion:

http://maps.google.co.uk/maps?f=q&s...8985,-3.160715&spn=0.003598,0.011362&t=h&z=17

 

[canderson]

Yikes. I believe your local expression is "a dog's breakfast" - at least that's what Google's source for geolocation of those roads has made of it. It'd be a miracle if a GPSr using that map didn't get pretty thoroughly confused as you pass through that one. I see the Teleatlas name at the end of that list, too. That's most likely what your TomTom is trying to sort. Yuck.