Elevation Correction from Garmin Data

[paull]

Hi Folks,

Just getting into aerolab but seeing that on most rides the end
elevation <> start elevation even though I always start and finish in
the same place. Typically it could be 25m out.

I'm guessing this is because garmin 500 uses relative air pressure to
guess elevation ?

I know garmin connect has an elevation correction, which I presume
corrects against map elevations from gps points. but to take advantage
of that for golden cheetah I'd have to upload the ride to garmin,
apply elevation correction, export, then import to golden cheetah -
that would be a lot of admin on each ride.

Is there any way of correcting elevation either on the source fit file
or within golden cheetah itself ?

Or indeed does it actually make any difference to the cda reported ?
I'm not clear on the maths inside aerolab so perhaps the difference
cancels out, but logically I'd expect it to report slightly less cda
if my elevation reports -25m at the end.

thanks
Paul

[Norm]

Paul hi,
your understanding of the garmin altitude is correct but there is
more to it than that.
When you put the correction in you should be at a known height, taken
from say a TOPO map and you can then input the correct height. What
this does is give the garmin an understanding of a certain air
pressure at that location equals a particular height. You can then
start riding and the altitudes are going to be reasonably accurate at
least for the initial part of your ride. However air pressure is
constantly changing, I am sure you have seen Highs and Lows moving
through on your weather maps, and if you return to the same point in
an hour or 3's time the altitude will be different. Not because of
any inaccuracies but due to the changing air pressure.

I personally don't use the "correction" feature of my garmin as I am
not that interested in the absolute height but only the relative
height. For example I am at 90m now and in 1km time I am at 120m or a
3% climb :-) and using relative air pressure to determine altitude
shifts over a short period of time is fairly accurate as air pressure
is a long term period like hour plus not the minute to minute
calculations that we use/need/want. Unless of course there is a
severe weather event happening ie cyclone and who would out on their
bike then :-)

I guess what I am saying is the correction is a bulk shift and only
really accurate for that location at that time due to changing air
pressure so any feature to apply corrections so you return to the same
height at the same location needs the known air pressure at a given
time at a given location. Which is the way aircraft do it but I am
not sure it is needed for bikes.

cheers

Norman

[Robert Chung]

On Monday, February 20, 2012 12:54:12 AM UTC-8, paull wrote:

Just getting into aerolab but seeing that on most rides the end
elevation <> start elevation even though I always start and finish in
the same place. Typically it could be 25m out.

I'm guessing this is because garmin 500 uses relative air pressure to
guess elevation ?

Or indeed does it actually make any difference to the cda reported ?

The elevation reported by your Garmin 500 has no effect on the estimated CdA -- it's shown so you can compare the "virtual" and "true" elevations (though in this case the "true" elevation profile isn't going to be quite true). 

[AndyF]

On Feb 20, 3:54 am, paull <ieuanlamb...@googlemail.com> wrote:
> Hi Folks,
>
> Just getting into aerolab but seeing that on most rides the end
> elevation <> start elevation even though I always start and finish in
> the same place. Typically it could be 25m out.

You know, Aerolab is a really crude tool. The GC guys are working
hard to clean up the crap I coded into it, so I'm sure it'll get
better very soon. :-)

It's generally meant to be used for short courses with specific
protocols. For example, some of the more successful virtual elevation
technicians have used loops, out-and-backs, or "half-pipe" courses
with great success. For a really good example, have a look at Joe
Bank's blog, here:
http://josephabank.blogspot.com/2011/09/better-aero-field-testing.html

I usually just go to the data editor tab and delete the altitude
column so that it doesn't get in the way of Aerolab's elevation
scale. This is probably a GC sacrilege, but I didn't put enough
smarts into Aerolab to have it scale the vertical axis properly.

Hope this helps...

Andy Froncioni
Alphamantis Technologies Inc

[paull]

Thanks Everyone.

I did 30 miles today and when I looked at the file the elevation
difference of my house is 127ft according to Garmin Connect.

In Aerolab its showing green line (elevation) as 34m (111 ft). And VE
is showing -17m but of course that moves up and down as you change
cda.

Interesting that elevations in garmin connect and gc are different.

I'm beginning to wonder if this is a fault with the garmin 500 as
thats too big a difference for todays relatively static weather ?
Still as Rob says if it makes no difference to the CDA calculation
then it doesn't matter, thats all I want it for. However I'm a bit
confused.. if you can delete the altitude column, how do you get
elevation ? clearly if you were getting elevation from a map the
start elevation would equal the end elevation!?????

Andy aerolab is bloody wonderful !!! Looking forward to using it with
my TT bike to get my position sorted - and thanks for the blog link

Cheers
Paul

[Tom Weichmann]

Paul,

What can happen sometimes is that you take your bike out of your heated house into the (relavitely) colder (or hotter) outside and it takes time for the temperature of your device to match the outside conditions due to its thermal mass.  I see this all of the time with the thermometer output, probably related to the altimeter as well.  Also, I almost always get change in altitude while riding my trainer in my apartment and I think it is related to the few degrees of temperature change.  It always shows that I'm going downhill.  I've never really paid much attention to it, but I know I've seen as much as 100ft in a 2 hour ride.

Tom

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[paull]

Thanks Tom, Thats an easy one. I'll leave it on the bike overnight
before my next ride, its kept in an unheated building. Will post back
here if that solves it.. thanks