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Variations in GPS speeds

tacklejunkie

Asked by tacklejunkie,

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tacklejunkie Grand Master

tacklejunkie

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I have an Etrax 10 Garmin. My buddy has a Lowrance H20. We get different speed readings on our GPS. Not a big difference but different, nonetheless. So, we got into the vehicle after fishing and what the speedometer on the vehicle said was different than both gps readings. Is this typical?

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BobT Grand Master

BobT

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Update rate of your GPS may have affect on accuracy. Vehicle speedometers can vary for a number of reasons such as tire size and wear as well as mechanical design imperfections.

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h8go4s Rising Star

h8go4s

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All devices have manufacturing tolerances. The GPS speed is probably more accurate than the vehicle speedometer though. The speedo can be affected by tire size, among other things, plus I suspect the auto companies err on the side of caution to prevent complaints from people who get speeding tickets.

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CaptainMusky Grand Master

CaptainMusky

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GPS will be more accurate than your car. GPS differences have to do with "averaging" that the two units do to calculate position, heading and SOG (speed). If you have two identical units I would be concerned with the difference, but each make/model has different capabilities.

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leech~~ Grand Master

leech~~

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This is very common. If you really want to see a big difference run a GPS on a motorcycle. Every bike I have owned was off by at least 5mph slower then the GPS. Kind of su$k's to find out you are only going 65 when you thought you were hitting 70! frown

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Savage Brewer Rising Star

Savage Brewer

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How much off?

Several MPH, just a couple tenths?

If one shows 55.2 and the other shows 55.6 I wouldnt worry about it.

If one shows 55 MPH and the other shows 60 mph I would go out and figure out which one is off.

Need to remember the H20 model is 7 years old now and the eTrex 10 is the cheapest GPS Garmin makes.

You just need to figure out if the difference is something you can live with.

For geocaching, driving and finding locations the speed is not important to me. But when drifting or trolling the speed is important.

Fortunately for me I have 7 gps's and they all track speed very accurately.

Lowrance Elite-7 HDI

Garmin GPSMap 60CSx

Garmin Oregon 450

Garmin Montana 650

Garmin Nuvi 40

Garmin Nuvi 3450

Magellan Meridian Gold

All of them track very well, I would say if I could only have one GPS it would be the Oregon 450.

One thing to note is speed drift is more common when under 1 MPH.

I would say when under that I might have one showing 0.2 and the other showing 0.6 and bounce around a bit. Above 5MPH I would say all of my GPS's are within 0.2 of each other.

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delcecchi Grand Master

delcecchi

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Take your gps and go for a nice long walk. Then look at the statistics on the walk. When I do that, I often see a maximum speed of over 10 MPH. I can assure you that I am not going that fast.

Or go on a path or sidewalk and walk a couple miles and turn around and walk back. (It's good for you too) Look at the track. Typically mine does not overlay in both directions.

Remember there is a certain amount of jitter in the position readings over time.

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BobT Grand Master

BobT

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Another thing that could affect the accuracy is the GPS accuracy at the moment. What was the calculated accuracy of the two units. If one showed positionaccuracy to 6 feet and the other to 15 feet, this could certainly cause differences. Did one have WAAS enabled and the other not? Maybe one of them isn't even WAAS capable.

Your automobile speed will vary too on various terrain. When going up and down hills, the GPS is probably not taking the hills into consideration but only the change in lateral position to calculate your speed. For example, your GPS calculated speed while ascending or descending my be slower than your actual ground speed. The steeper the hill the greater the error.

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h8go4s Rising Star

h8go4s

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Pick one and ignore the other. Doesn't matter which one. The variations are in tenths of a mile per hour. It's irrelevant. If you're not catching fish, speed up or slow down. Don't over-think it.

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