This is my first car with navigation and I find it very useful and interesting. I wish I had it 30 years ago. However, I am curious about its accuracy. Does anyone here know the degree of accuracy relative to location? Is it supposed to calculate your location within 30, 100, 0r 500 feet? Secondly, how does it determine the elevation of the current location? Is elevation derived from satelite data, or are elevations pre-programmed into the DVD based upon location (lat and long) and related elevations from USGS or other topographic mapping?

 

accuracy depends on how many satelites are in range. You can check how many satelites by pushing, Menu, Map, and Cancel Buttons for 3 seconds all at once (i think).

The system has never been more than 1 street off. I dont think ive ever seen my navigation more than 50 feet off.

Elevation is preprogrammed into the disc at a certain location.

 

Are you sure? I thought elevation was done by the GPS....

 

Elevation is derived by using multiple satellite(at least 4) positions and the reference ellipsoid programmed into the GPS ........

you can try this out in a parking garage (above ground of course). There always are slight differences between what the standard elevation map reads compared to the GPS readings.

If you really want to see how it works.....

http://www.esri.com/news/arcuser/0703/geoid1of3.html

http://www.ja-gps.com.au/whatisgps.html#anchor5186443

 

The nav unit is measuring the time delay between all of the satellite signals. Then it mathematically calculates where you would have to be to receive those signals with those delays. Each satellite puts out a pulse at a precise time and time interval. I don't know, but it probably is once each second on the second. The satellites all send out the pulse at the same time.

If you visualize your location as being 40 feet away from a GPS satellite (up in space), then clearly that satellite's signal will arrive at the nav unit far more quickly.

I'm not sure how accurate the nav unit in the Acura is. It has my house about 40 feet down the block from where it actually is. Oddly enough, the nav unit in my Honda Odyssey had the house location where it actually is.

The GPS system itself is far more accurate than what we see in our nav unit. The military uses it to map mine fields. You put the GPS receiver on top of each mine (don't try this at home) and push the "record position" button. This creates a digital map of the entire mine field. It makes it much easier to find the mines later.

This means that GPS itself is accurate to well within one foot. If the accuracy was much larger than a foot, you could step on the mine while trying to determine its position - which can ruin your whole day.

 

Some GPS systems have error circles around your position. It's a function of how many satellites are in range and can be as good as about 10-20 feet. But that' NOT the reason why when you pull into your driveway it shows you at your neighbor's address. There was no army of men out compiling data on where your house is... They have to make assumptions. The know all of the addresses on a street, so say you live on the 3700 block, they know addresses range from 3715 to 3745 on one side of the street and that there are say... 10 houses in there. If every lot is equally sized, no problem, the GPS is going to be dead nuts... I live in a housing tract where every lot is 50 ft wide. The TL knows where I live dead on. But in my previous house one side of the street had some lots 25 ft wide and others that are 50 ft wide and still more that were 100. The software programming assumed an equal distribution of address linearly along that block... It was 3 houses off. Mapquest uses the same database and is off by the same amount. It's just something your going to have to live with.

Elevation... it's actually calculated. If you go into the diagnostic screen, you can see that if you only have 3 satellites in range, it will not give you an elevation. Remember back to Geometry in high school. drawing circles are locating points. Each satellite represents a sphere, the intersection of two spheres is a volume (3d), three spheres is a line (2d) and the intersection of 4 spheres is a point. More than 4 and you start using an algorithm to 'average' and increase accuracy.

 

I guess the morals of this entire thread is:

1. Accuracy will greatly depend on weather conditions, number of sattelites available, and even time of day (less so for this one). On one day, it might be spot on and track your car right up to the intersection its supposed to be at, but on other days, it might even be 1/2 a block off (you are at the intersection, but the cursor on the map is well behind the intersection).
2. Use GPS as a general check, not a be-all, end-all tracking device.
3. Don't rely on it to get an actual address position.. Use it to get fairly close, and then use your eyes to find the actual building/place/etc.