GPS mileage calculation inaccuracy based on altitude gained or lossed???

[Guest sdstack]

I was wondering how gain or loss in altitude changed the amount of distance traveled shown on my GPS. (eTrex Venture) I know that if I am walking up or down a significant incline that my lat & long would change less and show me traveling a shorter distance than if I would have taken the exact same steps on level ground. Do the units have a way to calculate actual distance based on coords. and changes in altitude? Is this too insignificant to worry about? Just thinking about it. But, isn't it true that if I took steps that equaled 1 mile on level ground and did the same steps on a 45 degree incline that my distance traveled would only be 1/2 mile. Maybe I am missing something. Feel free to call me dumb and paranoid!

 

Also, today I drove around most of the day and set my trip meter in the car and on the GPS and after 100 miles, my car showed 4 miles more traveled than did my GPS.

 

To some of the posts below and to future ones... I understand the math, I was only wondering if the GPS calculated for it.

 

[This message has been edited by sdstack (edited 25 October 2001).]

 

[This message has been edited by sdstack (edited 25 October 2001).]

[Guest Rootbeer]

es anyone have evidence that any of them is used in any handheld GPSr available to the public today? I strongly suspect that most simply calculate the straight-line distance from moment to moment and leave it at that, with maybe a tiny frill or two, since that's the simplest way. Fancier ways drain batteries faster, without adding a lot to accuracy.)

 

And as for altimeters, well, I don't think that most GPS receivers look at altitude when calculating distance. You can find out about yours pretty easily, though. Take it to the top of a tall cliff or bridge, reset the distance-traveled meter to zero, then watch what the distance meter says after you jump.

 

In the end, most GPS receivers aren't going to be as accurate at measuring total distance traveled as, say, a good pedometer would be. So, why do the manufacturers include that feature? Because consumers want it, and because the competition includes it. You wouldn't by a GPSr that's missing such an important feature as a total-distance meter, would you?

 

This may not have been the answer you wanted, but I hope that it's been illuminating for you. Cheers!

[Guest navdog]

Being an avid hiker, I am looking forward to the day when we will have 3D holgraphic GPSR's so that we may better visualize and appreciate all the elevation gain and loss we have encountered after a long day's hike.

[Guest Prime Suspect]

Using a GPS like the Vista, which has an accurate altimeter and includes altitude data in the track log, one could design a program to calculate the adjusted distance without too much trouble.

[Guest mrgigabyte]

quote:

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Originally posted by sdstack:

1 mile on level ground and did the same steps on a 45 degree incline that my distance traveled would only be 1/2 mile. Maybe I am missing something

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You are missing something... grade 6 math. :-)

 

1 mile X sin(45) = 0.707 mile

[Guest PneumaticDeath]

quote:

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Originally posted by sdstack:

..... But, isn't it true that if I took steps that equaled 1 mile on level ground and did the same steps on a 45 degree incline that my distance traveled would only be 1/2 mile. Maybe I am missing something.

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Well... it's a common question, and here's the common answer...

 

First.. if you were going 1 mile up a 45 degree angle, the horizonal distance would still register as .707 of a mile (sin 45 deg is sqrt(2)) However, it's doubtful that you'll manage to go up a 45 degree angle slope for very far... cars generally can handle about a 40% grade (about 23 degress) maximum, and generally any roads are kept below 20% (12-13 degrees).

 

So.. let's take a 20% slope (that means that for every 100 ft you go horizontally, you go up or down 20 ft.) So the horizonal distance is 100 ft. the vertial distance is 20ft, so the total distance is Sqrt(100^2 + 20^2) which is 101.98 ft....

 

So.. for the steepest slope you're likely to encounter, the "error" from staying 2D is less than 2%... and there at a lot bigger sources of error than that in the mesurement already.

 

-- Mitch

[Guest PneumaticDeath]

quote:

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Originally posted by sdstack:

...

Also, today I drove around most of the day and set my trip meter in the car and on the GPS and after 100 miles, my car showed 4 miles more traveled than did my GPS.

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Well... There are lots of sources for error..

 

It's possible that your GPS lost sat lock, and didn't keep track of a portion of the trip. It's also possible that since it was just taking readings periodicly, it was cutting across curves in the road that, and losing track of the actual distance traveled that way. It's also possible that your ODO is out of calibration (because you put smaller tires on the car for instance). All in all, I'd say that 4% error is pretty good considering.[ ]

 

-- mitch

[Guest Kerry]

should apply this to real estate. If everybody limited the deed area of their block as to include the ups and downs then there would be more to go around

 

Cheers, Kerry.

[Guest TeamFamilyAffair]

Hey don't be so harsh on each other! Actually the correct answer is 1.414 miles is the distance you walked.

 

(at a 45 degree angle you would have walked one mile horizontal horizontal and vertical) or a total distance of 1.414 miles

 

quote:

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Originally posted by mrgigabyte:

You are missing something... grade 6 math. :-)

 

1 mile X sin(45) = 0.707 mile

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[Guest mrgigabyte]

quote:

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Originally posted by TeamFamilyAffair:

Actually the correct answer is 1.414 miles is the

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You too are missing something. Grade 6 reading!

 

The original question was "But, isn't it true that if I took steps that equaled 1 mile on level ground and did the same steps on a 45 ..." One mile at 45 degress = .70 horizontal miles.