Incorrect Odometer readings - ClubTread Community

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post #1 of (permalink) Old 08-24-2008, 05:46 PM Thread Starter
Hittin' the Trails
 
Join Date: Jun 2007
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Default Incorrect Odometer readings

I have a GPSmap 60CSX. I've had it approx 2 years.

I've recently noticed that the odometer readings are wrong. They range anywhere from 1.1 to 1.15 times the real distance when driving (measured against my car odometer), but more importantly from 1.24 to 1.425 to 1.55 or even 1.73 times the actual distance on foot, when hiking (based on marked trails).

Has anybody else had this problem? If so, is there a cause and a solution? If so, would you like to share it? I don't feel like sending my unit to Raytech for repairs if not necessary.

I have software v. 3.70, so I'm up to date there.

Vincent
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post #2 of (permalink) Old 08-24-2008, 05:49 PM
Off the Beaten Path
 
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I think the problem is due to the fact that the gps doesn't always have a correct position. The position displayed "wanders" as it's calculated again and again, adding distance to your odometer readings. I never really trust the odometer on my gps.

Benoit
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post #3 of (permalink) Old 08-24-2008, 05:59 PM
Headed for the Mountains
 
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On some trails, I find the odometer regularly shows a different distance than what the markers show. The Garibaldi lake trail, for example, regularly shows up as 6km from trailhead to Taylormeadows-Lake junction (trail markers have it at 6.5km-ish). Other GPS based distances also usually show close to 6km, so it makes me wonder which is the inaccurate measurement out of the two...

p.s. I use a colorado which has a great antenna, and I trust the distance calculation to be fairly accurate

p.p.s. While none of this answers your question, it is important to note that sometimes trail markers can be wrong. When your odometer is showing something wacky, is your "track" on your map showing irregularities such as sudden spikes in directions you didn't walk, or detours you didn't take, etc?
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post #4 of (permalink) Old 08-24-2008, 06:13 PM
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I wonder if it is just the fact that GPS units are only accurate to a given point. Rather than type a long winded posting check out this link for more info.Hope this helps.
http://en.wikipedia.org/wiki/GPS
I would suspect your car would be more accurate as the military limits GPS.
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post #5 of (permalink) Old 08-24-2008, 06:36 PM
High on the Mountain Top
 
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If the GPS odometer reading increases slowly while you're standing in one spot that would give you an indication that the algorithm used to calculate the distance is simply a sum of the differences from the current position to the new one and that there is a random error in the reading that is rectified by the algorithm. (I've tried and I can't make that sentence any longer) That algorithm would be pretty easy to fix by placing a minimum on the difference needed before adding it to the sum such that the limit is larger than the noise. If you're in an area with poor signal then there could be an increase in noise above the threshold used in the GPS.

The distance calculation is also very sensitive to sampling rate such that if you are going up switchbacks quickly you could end up with a smaller distance value due to something similar to aliasing. Imagine it taking a reading in the middle of the switchback every time which would look to the GPS like you went straight up the hill.
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post #6 of (permalink) Old 08-24-2008, 06:48 PM
 
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an interesting note from a post on http://forums.mtbr.com:
Quote:
quote:If you have a good "connection" with the satellites for the entire ride the GPS should be accurate. However keep in mind the odometer on the GPS only accounts for distance on flat terrain and does not compensate for hills i.e. a^2+b^2=c^2. In other words it only computes a top down distance and will underestimate your distance when you ride up and down numerous hills.
now if he's right...

most marked trails i've been on have been measured with a hip-chain, and while not absolutely accurate, they run quite close.
if a bike comp or gps doesn't measure elevation gain/loss, that might explain the apparent inaccuracies you are experiencing.

your car odometer is more likely to be out than your gps measure on open flat ground.
check it out with the cops next time you see a speed trap. (i did, they didn't mind, and my speedo was reading under by 4.5 km/h.)
that means my wreck has even more mileage than i thought!

btw: the US military shut down "selective availability" ("intentional, slowly changing random errors of up to a hundred meters" in civilian gps receivers) in 2000. in 2007 it was stated that future gps 3 receivers would not have this SA in their operating software.
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post #7 of (permalink) Old 08-24-2008, 07:31 PM
Headed for the Mountains
 
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Quote:
quote:If you have a good "connection" with the satellites for the entire ride the GPS should be accurate. However keep in mind the odometer on the GPS only accounts for distance on flat terrain and does not compensate for hills i.e. a^2+b^2=c^2. In other words it only computes a top down distance and will underestimate your distance when you ride up and down numerous hills.
Ahh. That does make sense. Well, that goes a long way to explain why trail markers sometimes give greater distances than what GPSs give. Good to know, and I stand corrected.
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post #8 of (permalink) Old 08-25-2008, 07:46 AM
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Quote:
quote:Originally posted by The Hiker

I would suspect your car would be more accurate as the military limits GPS.
A vehice odometer isn't very accurate. Newer vehicles which count tire revolutions are consistant but the tire diameter (and distance travelled per revolution) changes based on temperature, tire pressure, load, and tire wear. If you change tire sizes then the odometer will be out. If the manufacturer offers different tire options then they may or may not change the calibration for each tire option.
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post #9 of (permalink) Old 08-25-2008, 08:49 AM
Headed for the Mountains
 
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I was reviewing the triliteration formula and think it has some errors.
(x-x)2+(y-y)2+z-z)2=((tr1+b-ti)c)2,i=1,2,3,4
Replace the x with number of apples and y with the time the train leaves Chicago and I think you come up with how big the my shoes size is relative to the State of Illinois (in kilo-pascals of course).

All kidding aside. I'm finding my GPS relatively accurate on trips +/- 10km. I wonder if your antenna is wacky and you're getting to many spikes in the data. When you download the data to your map does it look funny?
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post #10 of (permalink) Old 08-25-2008, 09:23 AM
Dru
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It's the error in the GPS. If you walk a straight line for 10km, the errors in the GPS position will make it seem you walked a jagged path. The mileage will be calculated between those jagged points, and the resultant line will have a greater length than 10km.

You can correct this by setting the sampling interval that the GPS uses to calculate mileage. If it samples every 5 minutes, the error will be greater than if it samples every 30, for instance, because there will be more zigzags in the line.
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