Up and Down hill accuracy speed and distance reporting

[Guest robamy]

I was riding my bike up a hill with my eTrex Vista the other day and

had a thought. Does my GPS actually account for the hills when it

comes to reporting my speed and distance? If my guess is correct it

seems like my speed and distance traveled will show incorrect results

while I am going up or down hill. Would somebody with expertise please

tell me if the speed and distance are accurate while I am traveling up

or down hill.

[Guest bigRahn]

I've played with in my car a bit, and I noticed it always seems pretty dang accurate. I'm using a e-trex.

 

Giving it some thought, I wouldn't expect elevation changes to make a d.fference. I could be wrong, but I assume the speed algorythm is pretty simple--something along the lines of taking a location reading every x (seconds/millesecs, whatever). Determine the distance in between the last point and the current, then divide by the time. (A pythagorean approach...there's probably a better way to do it.)

[Guest k2dave]

That's a good question because gps's are not to good at altutide measurement. In a car or bike I don't think it makes a lot of dif. though. I used the gps to measure my skiing speed flat out - (btw 64.5 mph) I wonder if it took into account the slope - probabally about 30%.

[Guest bob_renner]

The change in vertical over the typical terrain makes a very little difference in the distance travelled. Take the example of the 30% ski slope. The horizontal distance is 1, the vertical is .3 and the inclined distance is 1.044. Only slightly more than 4%. For a steep interstate of 8% the inclined distance is 1.003 (0.3%).

 

Bob

[Guest Nemesis]

quote:

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Originally posted by bob_renner:

The change in vertical over the typical terrain makes a very little difference in the distance travelled

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That is true, but for those with a GPSR with a built-in barometric altimiter (e.g., the eTrex Summit), change in elevation can be accuratly determined. It is a pity that this information is wasted and not used to calculate accurate speeds (maybe Garmin will consider adding the option of using it or not in their next firmware upgrades for the appropriate units).

 

Cheers,

Donovan.

[Guest Kerry]

All based on the XY values only. Theorically drop a GPS (vertically straight down ignoring some other issues) from a height and it's certainly travelling at what ever speed things fall at but the GPS speed will be zero.

 

Cheers, Kerry.

[Guest bob_renner]

But my etrex Legend (without an altimeter) will display "Verticle Speed - Your rate of altitude gain/loss over time".

 

Bob

[Guest Geoffrey]

1) You could tie a GPS to a small parachute, with the antenna pointing up. Then drop from a safe spot(a couple hundred feet above the ground).

 

2) You could wear the GPS on your body while parachuting, then check the Trip Computer Page on the GPS to see what happens??

 

Try this with the Etrex Vista, and the Garmin 3plus. The 3plus does not do well with altitudes, and it does not record the altitudes in it's breadcrumb trail.

[Guest Kerry]

quote:

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Originally posted by bob_renner:

But my etrex Legend (without an altimeter) will display "Verticle Speed - Your rate of altitude gain/loss over time".

 

Bob

---

 

Bob, yes so true. I suppose I should have qualified that with "Horizontal" as it relates to the context of the original speed and distance query which height plays no part in.

 

I suppose in the reverse context the change in altitude is not similarly dependent on the XY at that stage.

 

As different to vertical gain/loss (+/-) that one thing a GPS can't do with speed in the horizontal, that's go backwards.

 

Cheers, Kerry.

[Guest Peter Scholtz]

Here's a previous discussion: How fast were you going?

 

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Peter Scholtz

www.biometrics.co.za

[Guest outdrcpl]

I don't know about all this technical mumbo jumbo. To get an answer to the original question ". Does my GPS actually account for the hills when it comes to reporting my speed and distance?"

I took my mountain bike out last night with both a Garmin Legend and a seperate speedometer attached. I rode the local hills which vary from slight inclines to very steep. My speeds averaged between 15-20 mph hitting 25-30 on a few hills.

The entire time my Legend read within .5mph of the speedometer, even on the hills. This tells me that the speed displayed on your garmin is still accurate even on hills. Now, of course this only tested the accuracy when on hills, I have no experience with how accurate it would be with vertical acceleration (I don't usually jump out of airplanes or jump off cliffs to find caches)

 

I forgot to reset the trip odometer in both the speedometer and the legend so I'm not sure how the hills affected the distance readings. I'll try that next time out.

[Guest jeremy]

I don't believe it does take into consideration your altitude when showing your speed. Going up and down hills (in most situations) would most likely not affect your overall speed taking your downward movement into effect. The .5 mph difference could easily mask that.

 

The reason I believe it isn't taken into affect is the innacuracies of the GPS units' ability to determine your location above or below sea level. Newer units do have altimeters but the OS is basically the same across all the models. Of course I could be wrong. Just making logical assumptions. And yes, I know what assuming does...

 

Jeremy

[Guest jeremy]

I don't believe it does take into consideration your altitude when showing your speed. Going up and down hills (in most situations) would most likely not affect your overall speed taking your downward movement into effect. The .5 mph difference could easily mask that.

 

The reason I believe it isn't taken into affect is the innacuracies of the GPS units' ability to determine your location above or below sea level. Newer units do have altimeters but the OS is basically the same across all the models. Of course I could be wrong. Just making logical assumptions. And yes, I know what assuming does...

 

Jeremy