GPS is off by a 1/2 mile

never had that happen before I have a 530

ive had it happen on my c530. the ans is its not a perfect world. if u think ur gps is covering the entire us every small town and road and the maps r put together my humans. my gps does a good job lol

Yes, it's a great tool, and No, it's not perfect. I have experienced this a couple of times a week, especially in more remote or sparsely populated areas, or areas recently developed. I suspect that there are three main causes:

1. The address grid laid out by the planning commission was modified when the actual development was completed.
2. When the maps were generated, less priority and attention was placed on these areas
3. Chances of someone reporting the mistake to Navteq for correction are limited

It seems to me so far that the new 2008 maps have fewer of these anomolies. At least the ones I knew about before seem to be corrected. I'm sure I will find new ones as I use the new maps more.

Especially in the translation of addresses to GPS coordinates. Unless the address has been surveyed using GPS, what you get when you enter a location by address is a best 'guesstimate' - and in rural locations, an address can be off by a half mile - in town, you're getting to the same block at best.

If the GPS is accurate on finding intersections, then I would say its the "guestimate" as Keith mentioned. These GPS's are mainly focused on where the streets are. If the intersections are off, then you should call Garmin because something might be off on your unit.

There is so much info about this topic on this site, I can't remember where, but Hornby(I misspelled his name again) did a great job explaining it once.

But it wasn't me anyway

yeah it was!!!!! Don't make me have to go and find the thread (and then look like a fool) LOL.

I've noticed two things that make my nuvi660 take me to the "wrong place." The first I call House Number Mathematics. This is the grid thing: house number 23 is estimated to be 23% of the block up from the corner. In my area, #23 is often at 100%. It'd be pretty cool if my nuvi had lon/lat for every address in NA, but alas.... Anyway, this phenomenon can put you a long way off perfect.

The other thing is Vanity Addresses. This is where someone likes the idea of a particular number and declares to the world that this is their address. They're right. It just isn't where the property's physically located lon/lat-wise, which is what these cool gizmos believe in.

Cheers.

- Keith

i got my (2nd) replacement c330 today (screen locked up. looking back it was my fault both times) and can honestly say that, as a truck driver, this thing gets me close enough to figure it out on my own and most times gets me within sight of where i need to be. i agree with the others........how can every little detail or address be exact? just a few years ago this was "military" technology. just imagine what they have to play with now and what we'll have in the near future!

The GPS calculates it's position from satellite data and then that position is overlaid on a map for us to use. There are a couple of potential errors that get introduced.

The first, is the representation of the earth as a map projection. Map projections are viewed as a mathematical operation in which latitude and longitude are transformed into Cartesian coordinates (x,y). Map projections allows longitude and latitude coordinates to be projected from a 3-dimensional position on the earth's surface to a a plane or 2-dimensional surface (paper). Several projections and coordinates systems exist that are defined differently, with each preserving shape, area, distance, and direction differently. The WGS84 is the most common one that GPS' use.

Any map projection is not perfect because the earth is not a uniform spheroid and at some points will be more or less accurate than others. Elevations from consumer GPS' have always been problematic.

Next, the map itself may not be accurate. I have had times where I was receiving 8 or more strong satellite signals, my position lock was excellent, I was stationary for a while and the I was not shown on the map where I really was, the map simply was not accurately drawn for the location I was at.

Third, the map data can be in error. I've found frequent errors when locating specific street numbers. My house number is not where most maps think it is - nor is my best friend's house. The streets are accurately drawn on the map, the map projection is great for this area - but the street numbers are not correct. Navigating just by street number will never locate my house. Mapsource (NavTeq) thinks my street number should be 17498 instead of 17418. 17418 is shown more than 0.1 mile away and on the wrong side of the street.

Usually, POIs are much more accurate than using street numbers. When I use street numbers, I assume it will just get me close and I need to find it visually after that - once found, I can save the location as a waypoint so the next time it will bring me right to the door.

OK OK, I really should be telling you guys here this, but it is funny.

Saturday night my company held a party at a restaurant that I have never been too. I went as the GPSr routed me. But I couldn't find it. So, I parked in front of a huge plantation house and called them. I told them that I was in a shopping center full of white buildings. They asked what I was in front of and I said, "A big plantation house."

Her reply was, "Well you are here then"

They need better signage...lol.

My C330, gets me close, Most of the time, it's right on.
Sometimes, it's close, But at-least I'm in the general area.

Hahha.. this is a great story! You should post this in the travel stories. Whatever did we do without cell phones and electronic gadgets?

I live on a corner and have a house number of 101. All GPS and internet maps I've encountered show this address to be at the opposite end of the street, some .3 miles SSE. Not only that, but the addresses on the my street actually range from 101 (me) to 199 (other end) while all these maps show them ranging from 101 to 300. I expect this probably happens a lot.

I wrote Navteq and a few others informing them of this error some years ago. I actually sent the URL and a clip of the county's GIS database map that illustrated my point. While it's not been corrected in subsequent map releases, I did get an answer back from Navteq (they were the only ones to acknowledge either my email or their error) basically saying I was right and it would be corrected in future releases.

What I'm curious about is this: say a registered sexual predator moves into the house with an address that, according to Navteq (or any other map maker) identifies as being physically located where a different house is located. People understandably get very upset with these individuals, doing everything from posting signs to vandalizing the property at the (wrong) address in an attempt to drive the individual away. If it can be shown that these vigilantes' used Navteq info to locate the (wrong) house by the registered address, is Navteq liable in some way?

Just always kind of wondered about that or other liability. Same question would pertain if a rescue squad or police uses GPS to locate adresses. Anyone know?