760 - Map Offsets - Anyone else experiencing this?

No problem with my 750, either with 2010.4 or 2011.1. What's especially odd is that your nuvi should have a "snap to" feature:

http://www.gpsreview.net/snap-to-roads/#more-3504

assuming you're not in "off-road" mode so you should appear to be on the road almost all the time.

Or are you referring to something other than your vehicle's location on the map display?

Try this on your Nuvi.

Press the touchscreen over the satellite reception bars for a few seconds. It will bring up a representation of how many and which satellites you have acquired. Also therein is a statement of the accuracy of the Lat/Long

The more satellites you have acquired, the better the accuracy. This is because the GPS doesn't actually triangulate signals. It figures out how far you are from the various satellites (using timestamps) and calculates overlapping spheres.

The more overlapping spheres there are, the more accurately your position is calculated. Four seems to get one within 70-ish feet. Five seems to get me 20-40 feet, depending on which satellites. (The more distant the satellites are from each other, the less their overlapping spheres.)

Thereafter, the mapping software for road use will "snap" you to the nearest road... but that can be dicey in a tightly packed neighborhood with the bare minimum of acquired satellites.

Anyway... that's how I interpretted what I read on Wikipedia.

Yep, my vehicle does stay on the roads, it's just that when I'm driving I'm either 10-50 yds ahead or behind the intersections that I'm passing -- it all depends on which direction I'm driving.

With the Satellites screen I'm getting 4m (4 yd) accuracy.

I do English metrics so...

4m is about 13 feet... far far better than any accuracy I've seen (20ft is the best I've seen, 160 ft the worst, when I've looked) and I only occassionally have the problem you're describing.

I've got no other suggestions at this time.

In my city I find most adresses are right where my 885T says they should be, and others are off by 500-600 feet.

I checked http://www.navteq.com/ for one address, and found that Navteq is showing the address at the wrong location on the street. My Nuvi is taking me to this location so the problem is not with the accuracy of my Nuvi, but with the Navteq maps.

This was verified by approaching a particular address from both directions. No matter which direction I approached from, the Nuvi brought me to the same exact spot on the street, which was 500 feet from the correct address.

I found a couple like that in FL.They were on the wrong side of the street.I put a change into Naveteq for correction.

The discrepancy is caused by the algorithm used to determine addresses. it assumes there are 50 lots on each block and puts the coordinates at the percentage corresponding to the last 2 digits. If the number is say 40, it's 40% down the block.

Right. My understanding is that the maps do not have addresses embedded.

So anomalies introduced by city or postal numbering systems that are not standard can be problematic.

What amazes me is how it figures out some of the rural addresses.

By any means of reckoning I can figure out, I am not nn% of the way down the road. Not taking the whole road into account, not taking any "block" distance I can discern.

There must be something in the metadata that is not visually obvious along the road itself.

Thanks a-user, that kind of makes sense because the streets that I have noticed the greatest errors have several large apartment buildings, or school yards. The remainder of the street are the standard 40-50 foot lots.

However in my city, not all blocks are the same length. For example Wellington Street may only have 5 homes on one block and 40 on the next block. Yet the numbers continue from block to block without any missing numbers. We do not use the 100-block, 200-block etc. numbering systems that larger cities do, and what you appear to be describing.

If Navteq knows the number of the house at the beginning of each block, then your explanation of spacing them evenly could explain the problem.

Rick

Each city and area is different of course. They may know the starting number for a block, but I don't know what Navteq records as part of the metadata.

Generally speaking though, most cities do the hundreds block numbering, some here in this area, Washington DC is a great example, will number one block beginning at 00 and the next at 50 so your thought hold true as well.

This is an occasional problem with all my units (350, 765T, 1390T). Sometimes satellite reception is not the best and I can be off a bit. This is more likely to happen when the sky is heavily overcast or in a big city with tall skyscrapers.

Has the default zoom level changed for 2011.10? I'm still running 2010.10 on a 760, but I can create this lagging behaviour by increasing the zoom level.

I often get 3m accuracy on the 760 in Ontario. Which city/location are you seeing the lag in?

This is called "overzoom" in Garmin-speak. The term is not used on the nuvi but is common in Garmin handheld units and was present on older autonav units such as the GPS V, Street Pilot and Quest. Basically, the more you zoom into the map, the more the map becomes distorted and inaccurate. The overzoom is present at 200ft zoom level or below. Due to this, your relative position shown on the map will be increasingly more inaccurate the more you zoom in.

Overzoom is also present when using MapSource on the PC.

I also notice it on my 855. Map version 2010.40. It is only bad when you zoom in really close. As stated by previous posts.

All over the golden horseshoe (GTA to Hamilton)

And I'm not talking street addresses here, I'm talking intersections (i.e. where two roads cross ) are off.

As for zoom levels, I'm at the very first level you start seeing side streets in the "street angle" view (not the overhead view)

I'll try zooming out more and see if this changes it.

Mapping is indeed offset by varying amounts, depending on just where you are - after all, the mapping depends on the quality of the underlying survey work.

Up here in Montreal my home crossed the street with the release of the 2009 maps - it used to have me on the east side of the street, but now it has my home in the neighbour across the street's back yard based on the cookies and where they show up on the map - as I approach it kept telling me my home is on the left (until I redefined the waypoint) - and if you look at the tracklog it is obvious, since the log points are not locked to road, this in 2D.

When driving into the city on the highway it keeps thinking I am on the service road and keeps telling me to get on at each entrance we pass, even though I am in the far left lane of the highway. And THAT messes up the traffic algorythms.

On the way home it keeps thinking I've missed my exit - and the Trans-Canada Highway has been where it is for a lot longer than maps have been digitised.

So it is NOT a question of GPS accuracy (unless the entire constellation has somehow moved in unison), but rather map registration issues.

I've learned to tolerate it, but it is very annoying. However the fact that upcoming cross streets have been removed from the banner box and the secondary streets don't display when zoomed beyond and 1/3 mile on newer models are a heck of lot MORE annoying.

Maybe I'll go out and buy a TomTom and do some comparing. Their mapping comes from different sources.

From what I have been told a street is divided into sections and one Lat & Longitude is assigned for each section. So if you put in an address if it falls in that section of roadway it will give you the same point no matter the exact address. It is not exact but pretty close. I guess it would be very difficult for a map maker to assign an exact Latitude and longitude for every address. For the greatest accuracy check on a map like Google maps, or Google Earth for the Lat & Long and put that in your GPS.

Are you using 2D or 3D. On my Nuvi 750 if i use 3D the accuracy is compromised, Ex: I will pass an intersection and my GPS might show I am still approaching. If I place it in 2D which I prefer I not only see more of what is around me but the accuracy is almost spot on if not spot on.

I use the 3D View, always have, and before the 2010.40 update I never had this problem.

Oh I tried zooming out and it looks like it works -- sort of

Good enough for now, most likely the next update it'll get fixed again

Maybe it has something to do with that earthquake back in Feb, that was around the same time that I noticed the offset... who knows LOL

One thing I've noticed lately is, I'm only locking on to 7 birds at the most, not the usual 9 or 10.

This started about 2 weeks ago. I wonder why?

I think there were 10-ish earlier in the week, but I can't be sure.

http://www.navcen.uscg.gov/navinfo/OSSMap.aspx

Yeah, there were problems in the past week with PRNs 9 (7 May), 13 (11 May) and currently 22 (14 May until 15 May)

Also, SVN49 (PRN01) is out for 6 months of testing which began in February.