Total ascent

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Total ascent

Postby Mick F on Fri Nov 08, 2013 6:18 am

I have used a Garmin 705 for many years and have all the Ascent records of it. I have sold my 705 and bought a Garmin Montana. The trouble is, I find the total ascent comparisons between the two units on the exact same ride to be way out.

Ever since first using Ascent, I've had the "Filter altitude changes less than" in Preferences set to the default value of 4.7ft.

If I compare one ride with the 705 to the Montana on Ascent - the exact same 30mile ride I do regularly - the 705 reads 2,900ft fairly consistently, but the Montana is now reading 4,000ft consistently.

I can play around with the setting in Preferences to get them similar, but what setting should it be, and which is more correct, the 705 or the Montana?
By changing the setting, I'm altering both figures of course.
What do I set it to?

Any ideas?
Mick.
Mick F. Cornwall
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Re: Total ascent

Postby digbymaass on Sat Dec 28, 2013 2:17 pm

The amount of ascent is not inherently accurate on GPS. You'll never get a setting in Ascent that copes with all circumstances, and it'll often be wildly out, and usually showing more than reality. Any software with a smoothing algorithm will be the same. To get an idea of actual ascent the 'corrected' ascent value on Garmin Connect is probably as accurate as you'll ever get. It uses Nasa ground altitude data to work out the ascent.
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Re: Total ascent

Postby dw152 on Sat Dec 28, 2013 10:49 pm

digbymaass wrote:The amount of ascent is not inherently accurate on GPS.


The 705 and the Montana use barometric altimeters.
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Re: Total ascent

Postby Mick F on Wed Jan 01, 2014 4:47 pm

Yes they do, and that's what was confusing me.

The 705 uses BP, but it's not calibrated. The 705 is problematical in that you can be at widely varying altitudes when you switch on. The latest software sorted this so you could tell it the altitude before you set off. Trouble is, the weather could change or you could be on a long ride and by the time you got home, you would be at a different elevation to when you left. A couple of hundred feet maybe?

The Montana is continually calibrated, so after it settles, it's as correct as it can be. Also, the receiver is far more sensitive so the altitude assessment is better.

I think the problem I have is that the Montana samples smaller variations in altitude than the 705 did. Consequently, the "fractals" are smaller/finer.

As far as my question is concerned, I've given up worrying about it. It's like the old Chinese proverb:
Man with one clock knows the time. Man with many clocks, never knows the time.

Regards to all, and Happy New Year.
Mick.
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