# Non-Barometric altimeter GPS accuracy



## Maadjurguer (Mar 22, 2008)

All,
I have the opportunity to get a Garmin Dakota 10 through Airline miles for free. I currently use a Garmin 705....I've liked it and it's served me well, however as I get more into multi-day rides, I need a unit which takes normal batteries so I can swap the dead ones out....hence my interest in the Dakota.

As you all know, the 705 has a barometric altimeter and the Dakota 10 does not.....unfortunately, the Dakota 20 is not offered through my airline miles...otherwise, it would be an easy decision.

My question: What is your experience with the accuracy delta between barometric altimeter GPS units and non-barometric altimeter units (ie, only GPS used to compute elevation)? Bonus points go to anyone who has direct experience with the Dakota 10 vs another Garmin with a barometric altimeter.

I already understand that vertical accuracy of a GPS without barometric can be in the woods of +-20-25m...but am looking for real-world perspectives here. The Dakota user manual is no help here since they lump the 10 and 20 together, and only quote 3m vertical accuracy...which must be based off of the 20 in ideal circumstances.

I'm not looking to map here....but am a bit of a geo nerd and don't want to take a significant step back from what I already have with the 705. My only other option is to keep the 705 and get a few of those rechargable USB thingys.....

Thanks for reading my long winded query....


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## BigLarry (Jul 30, 2004)

*It's bad - non-barometric provides only a very rough guide*

The vertical accuracy I've seen on most GPS units, including Garmin, is +/-(80' to 150') (+/- 30-50 meters) when at the same time the horizontal is in the 15-25' accuracy range. I've seen it off by over 300' on occasions when first starting a ride. Yes, the vertical error is observed by me to be ~5X worse than the stated horizontal accuracy. Actual accuracy depends less on GPS unit, and more on number of satellies in view at that moment and their position. Wide angles with big timing differences are best. This vertical inaccuracy has to do more with satellites in view and math and physics than the particular GPS unit. (Less differential timing accuracy is possible if all the satellites are on one side, like all of them are above you.) It won't get better with a particular unit, no matter how good it is.

Even worse, the indicated altitude oscillates about every 10-15 minutes. So over time, the oscillations in vertical altitude typically adds 500-1000'/hour onto the total climb calculation, even if you're standing still or riding level. When I look at people's tracks without a barometric altimeter on level ground. I can see the slow random oscillations. Basically climb data is totally worthless from a non-barometric altimeter.

If you really need accuracy, get a barometer. It's good to 1' resolution, with the only issue being a very much smaller ~10'/hour drift from air pressure changes. If you want absolute accuracy as well as relative, you need to calibrate a barometric once at the beginning of a ride.


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## luap (Dec 15, 2009)

Maadjurguer said:


> All,
> My question: What is your experience with the accuracy delta between barometric altimeter GPS units and non-barometric altimeter units (ie, only GPS used to compute elevation)?


I videoed a ride, logged barometric and GPS altitude (amongst others) overlayed the data - graphed two charts altitude over distance.
Check both slope gauges - the slope gauge calculated from GPS is all over the place whilst the barometric one seems pretty accurate.

That should pretty much show you the delta between barommetric and GPS measured altitude.


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## Harold (Dec 23, 2003)

luap...what program was that that allowed you to combine the GPS source data with the video? cool.

if you care about altitude AT ALL, get a barometric altimeter. if you don't care at all, then don't bother. That's about as simply as I can put it.


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## luap (Dec 15, 2009)

http://dashware.net/ 
It used to be over USD 200 until last month.

A great program to anaylse data. 
You can use multiple data sources and synch them all together. The data from the video was from a Sigma Rox 9 and a GPS smart phone.
Here's another quick one I did showing how you add/modify a gauge. 




I'd love to see a video showing overlay of an edge 705 vs an edge 800 as I can't make up my mind as to which one to buy- Ride a few steep downhills and uphills and in a minute or so of video you'd know which is most accurate. If any one has both of these and logs a ride + video using both at the same time I could check it out!


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## BigLarry (Jul 30, 2004)

NateHawk said:


> if you care about altitude AT ALL, get a barometric altimeter. if you don't care at all, then don't bother. That's about as simply as I can put it.


+1

That is effectively the correct answer. One sees the GPS altitude within ~100' or so and easily gets fooled into thinking it might be useful. But in practice it's not, for most all desired purposes.

luap's two curves are a great example of the difference. The top curve from a barometric altimeter is clean and represents the trail. The other non-barometric curve is filled with noise on a rough baseline.


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