How does Garmin determin arrival time?

You're making it harder than it really is

It actually updates constantly....distance remaining divided by miles per hour. So it will always be right when you get there. The trick is to see what time it thinks you'll get there at the start, and then see how accurate it was at the end.

My first GPS, the GPS III+, the ETA changed every second depending on your speed and "angle of attack" towards the waypoint. This unit did not have routing so it only knew to get to the waypoint directly. If you took a curve the ETA changed. This unit used a basic ETA= Miles to waypoint / MPH towards waypoint.

My GPSmap 60CS uses user input speeds for various roads such as interstates, highways and side streets. You can enter any speed you think you average on a particular road type. The ETA on this unit is very accurate, was within 3 minutes on a 200 mile trip, which included different road types.

My Nuvi 750 I believe, uses a fixed speed per road type and since speed limits are known for many roads, it must use those. The GPS does not know what roads have traffic lights though and that causes some routing issues. On a 500 mile trip from Cleveland - Charlotte, we "gained" 35 minutes from the initial ETA, but after gas and a bite to eat we wound up arriving pretty close to the original ETA.

far more sophisticated than most of us realize when it comes to its ability to compute and predict ETA. There's an interesting bit here ( http://www.gpspassion.com/FORUMSEN/topic.asp?TOPIC_ID=78865&... ) that discusses its adaptive ability that learns over time. It sounds like this detail is covered in their patent document.

As other have noted, the devices ability to accurately predict ETA is almost uncanny :0 Enjoy!

I've found my unit is very accurate on the highway, but it is terrible if you have any city driving as it seems to have no concept of a traffic light.

Your estimated time of arrival is probably based on average driving speeds. When you drive over the posted speed limit, you are ahead of the average. This is offset by stop lights/signs, traffic, etc.

The real key is to remember what your original arrival time is and see how close you came. When I travel to DC (particularly in the morning) I always add a large margin of error to my arrival time.

I don't believe that the estimated time of arrival is based upon distance remaining divided by miles per hour.

For example, when I first start a route and am still sitting in my driveway, the GPS has already calculated an estimated time of arrival, even though my average speed is 0 MPH. For every minute I sit in the driveway, the GPS will add a minute to the estimated time of arrival.

I don't have any knowledge of how it calculates the speed, but I believe the estimated time of arrival is simply based on your current location and the GPS's algorithms to get from your current location to the destination based on how many highway miles versus side street miles. It probably has a basic formula for average speed on highways and average speed for streets, and then makes its calculations based on the roads it's using for the route. Then it updates the estimated time of arrival every X seconds based on your current location.

Anyway, not claiming to know for sure how it calculates, but that's my theory.

Very cool with that adaptive learning link.. I always assumed it did more than just know the speed limit as, for me anyway, I try to remember it's "initial ETA" and it's usually really accurate based on when I finally do arrive. I always figured it just knew Speed Limit on each leg of the trip. Would be a simple computation based on that.

Just a guess, but I would suspect the initial estimate is based on 50 mph then adjusted from there.

You should read the whole linked thread -- interesting. Evidently 8 different type of roads are monitored and avg speed per type is adjusted based on your driving habits.

- that's the key to its accuracy.

Hi stoces,

After reading all the replies, I guess that shoots down my theory.

I thought all the GPS units were equipped with the "S.W.A.G." chipset.
That's the chip that gathers all the speed and distance information, then displays a "Scientific Wild Add Guess" arrival time, on the unit.

Regards,

Tailspin

Nüvi 670
Mac User

Good information, it cracks me up that when I leave for long trips the initial time is usually right on even when I stop for lunch. hmmmm.

Im still not convinced the nuvi utilizes adaptive technology.

But, the ETA is based on the distance remaining and the speed the unit thinks you should drive for all legs of the trip. Not the speed you are driving. I would surely like it if the unit was able to alter is memory of the speed it thinks I should drive. Or, allow for any amount of stoplight time.

Time is equal to distance divided by speed.

The GPS knows the speed, distance to the destination, and the current time. It just keeps adding to the current time until one arrives to their destination.

I look at the estimated-time-of-arival at the beginning of the trip and find it amazingly accurate. On interstates I can beat that time by a little if I go 8-10 mph above the speed limit. The only times that I find it inaccurate is when I travel on roads with traffic signals.

I conclude that it takes into account speed limits. It can't account for traffic and traffic signals, however.

I enjoy noting the ETA for a route and then going a different route. As soon as it adopts the new route and the new ETA is posted, I can be pretty confident from that of which route is faster--remembering the traffic signal adjustment.

I think that the ETA is the most valuable feature of my nuvi. During a trip I can call ahead and tell them within a few minutes when I will arrive.

A GPSr receiver must have acquired and locked onto the signal of at least three satellites to calculate a 2D position (latitude and longitude) to track movement. With four or more satellites acquired, the receiver can determine the user's 3D position (latitude, longitude and even altitude/elevation !). Once the user's position has been determined, the GPSr unit can calculate other information, such as speed over ground, course or heading over ground, bearing to target, track data ("bread-crumb trails, Garmin's used to do this for us!"), trip distance, distance and time (ETA) to destination, cross-track (XTE) error, sunrise and sunset time and more. You can literally be placed anywhere on earth with a GPSr receiver and know your precise location, and plot a course to any destination !

I agree.

The GPSr doesn't calculate the arrival time based on your average speed. To prove this, set a destination and then sit in your driveway for a few minutes. Since you're not moving, the average speed will be zero; so if the GPSr used your average speed to calculate the ETA, your ETA would be "Never" because you're not moving. Or drive down your street at 1 MPH...your ETA should be hours instead of minutes for a trip across town.

The ETA is based on your current position in relation to your destination, and then calculates the remaining part of the trip.

I can't say for sure that my theory on how it calculates is correct, but I can say that it doesn't use the average speed to calculate the ETA.