Mapping error or a routing algorithm

i had something very similiar last dec heading south on I 65. It was somewhere southern tenn.

I have an anomaly with I-696 in Michigan. There is a short stretch between Coolidge and I-75 (east bound only) that my unit thinks I'm on the service drive. It keeps re-calculating and tell me to take the next ramp left. I had the same issue, in the same spot with my iQue 3600 (through multiple map updates). Satellite reception is fine and accuracy looks fine. I just ignore it.

This anomaly has happened to me on several occasions. It's happened on ramps, and driving on the highway. I think its mapping but you will never get a correct answer from Garmin or Navteq.

Have you tried it yourself in "simulation mode"?

I do not have my 765T close by but I was thinking that maybe this has something to do with external factors such as local satellite reception, electrical interference, etc., which wouldn't show up in "simulation mode".

It is just how the two work together...

The GPS estimate of position will be impacted by atmospheric interference, relative satellite positions (called geometry), signal obstructions, reflected signals (called multipath), etc. Some GPSr will report the expected accuracy of the estimated position, or Figure Of Merit (FOM).

Now, if there is a road on the map nearby and the GPS estimated solution places you on that nearby road instead of the road that you are actually on, the routing algorithm can trigger the need to recalculate the route.

This can be made worse by any mapping errors.

Some GPSr attempt to protect from this situation my keeping you on your current road. This makes sense when there is a road that parallels your current road and the estimated solution drifts onto the other road. The thought process is that you probably didn't actually drive off one road and on to the other. This can lead to some additional problems. If you are on the highway and take an exit that is not on the route (say to stop for gas), for a short time while on the offramp, the GPSr will still place you on the highway until it is clear that you actually did take the offramp.

In all of these cases, a simulation mode test of the route will not produce the anomaly, as it will force the simulated track to follow the route.

charlesd45 I have traveled that route so many times i have lost count. My Nuvi 255 does not recalculate at that location. However, there are other locations that my Nuvi has done it at. Usually happens when there is evidence that the road has been rearranged -- but not always.

Charles, you mentioned that it happen with two different maps but do not indicate how old the maps are.

As an example I own a Nuvi260 with the original map from 2006, there are two places in Tampa where I get the recalculating, there maybe more just in my area.

One is on I-4 Eastbound around 50th St, back in 2006 the exit to 50th St was a ramp on the right on I-4, now it is a ramp on the left of I-4, the other location is by the airport eastbound or westbound, majority of the ramps have been re-drawn and re-located since 2006.

So the question is, has Pedro on South of the Border redone the roads to confuse the ICE Border Patrol and your GPSr?

I have the latest map update.When new map comes out I update right then.Travel all around FL and only location so far that has caused the problem.I think Pedro has some kind of jammer there so you will take the exit to South of the Boarder .

After re-reading the description of the problem I realized my example is not valid because it only recalculates if you take the new ramps on I-4 and in what you describe it does it without taking the exit ramp to Pedro and continuing on south I-95, as far as I know that section of I-95 has not changed, the ramps to Pedro may have but are irrelevant since you are not exiting.

We need someone who drives that route to confirm the anomaly; I haven’t been in that area since 1984 when I moved to Tampa.

Correct I don't take the ramp.Just keep driving on I-95.No lost of signal or any thing.It finally figures out where we are and voice message says drive so many miles on I-95.