Tested my speedo against a gps

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If you spend more time with the GPS, you will find that the error varies.

On my bike sometimes I'm 5 mph off, and sometimes they agree.

Most people assume the GPS to be the most accurate, and that is probably true, but it has errors too. But most likely the speedo on your bike isn't consistant.

If you really like the GPS, rethink your position on luxury. There are used ones out there that are pretty cheap.

I love having mine, and I am on my 3rd upgrade.

Street Pilot

Street Pilot 3

Street Pilot 2610.



When I got my first one, it really set me free. There are a lot of people out there that say that you can never be lost when you have a full tank of gas. I never had that much confidence. I always stuck to roads that I know. But once I got the GPS I really felt free.

I warned my wife once that there were two accessories that I couldn't be without:

#1 the GPS

#2 the CB.

The warning part was because if my GPS broke, I would buy a replacement right away. And we really can't afford it.

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That's interesting that the gps would be off day by day. I wonder what would be the cause of that? The accuracy of those things is facinating, at least in relation to location. An engineer friend of mine sets up a remote link site and then runs a super accurate gps, to within 1 or 2 cm I believe he said. I think it would be nice to have a nice color waterproof gps mounted on the bike. I've been exploring those with touch screens and numbers big enough for a gloved finger.
Here in ND, you really don't get lost on a highway unless a real heavy fog rolls in. I did lose my sense of direction while deer hunting one fall when a deep fog came in. I had to walk until I found a fence and then follow that until I found a section road. Then I had to follow the road around the section until I found my truck. Wow. I was exhausted. A gps would have bee great right then, but that was before they were common.
Now, when I take my planned bike trips out of state, I'm definately gonna want a nice gps to leave bread crumbs when I get of the main roads.

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I also have a Garmin 2730 and a Garmin Nuvi 350.

The 2730 has never been wrong, and has been extremely accurate with speed indication. It and my way old Garmin Street Pilot III track each other flawlessly.

Not that damn Nuvi. It gets lost, has to be rebooted. It lags behind my vehicle at least 2 seconds and I have missed lane changes in St. Louis with the Nuvi. That did it for me. I turned it off, and finished that trip with my older 2730.

The Nuvi's database for business addresses is also flawed. It consistantly wants to route me in the weirdest manner to get to an address. The 2730 makes much better routing decisions.

I really wish there were a way to figure out that stuff before the dollars leave my hand.

I suppose that there are forums for GPS stuff too. Just never looked for one.

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Continuously Operating Reference Stations (CORS) locations are often utilized as "substitute" GPS fixes and possess millimeter accuracy. Given enough time and the right operating conditions, most high end GPS receivers (the kind used for surveying, etc) can easily achieve cm accuracy. Military receivers utilize a second signal with shorter bit length to achieve higher accuracy, since the time of travel (satellite to earth) can be determined more accurately. Their signal is encrypted for obvious reasons.

The biggest issue to GPS positioning is the ionosphere - the solar wind causes the ionosphere to become charged, which alters the timing of the GPS signal path. Most GPS algorithms incorporate a standard ionospheric model to "correct" for this issue, but if the iono starts to deviate from the model, more error will insue. As we start to enter the 11 year sun spot cycle, more extensive solar storms = a wild(er) iono = degraded GPS. In some instances, the iono storms can become so large as to temporarily block the GPS signal. GPS will generally worse around 12-3 pm, and much better at midnight for this reason as well.

Other big problems are multi-path, where the signal bounces off things like buildings or trees before finally arriving at your receiver (lengthening the time of arrival), and the geometry of the satellites relative to your position - better to have them widely spread out than clustered together. Again, various algorithms have different ways of mitigating these isses, with varying results. In general, any decent receiver should locate you within 10 meters or less; in most cases, within 3 meters.

GPS is sampling at a rate of ~1 reading per second and determing position at point A and then at point B. The time of the two readings is also determined based upon an oscillating crystal. Velocity is therefore simple distance / time. The accuracy depends upon the quality of both the time estimate and the position estimate. If you are going straight at constant velocity, the average of many readings becomes pretty good. If you just drive around in a circle, the accuracy will be less. At stong accelerations/ decellerations the "action" is happening faster than the 1 second sampling rate and the unit will have trouble getting a good estimate. A decent GPS unit should give a pretty close result to the speedometer and give an indication that one (or the other) is WAY off, but I wouldn't utilize it to calibrate a speedometer or challenge a ticket! (Although with a good lawyer anything is possible :-/ )