Altitude adjustment

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Altitude adjustment

Postby brntn on Mon Oct 11, 2010 7:08 am

My Edge 705 often starts up at the wrong altitude (due to barometric pressure change) and takes many kilometres before it has self-adjusted to show altitudes that are approximately correct. It would be nice if there was provision for the user to:
1. Manually set the true altitude at the start of a ride
2. Define a point in the ride where the GPS altitude is finally self-calibrated properly
3. Tell Ascent to automatically interpolate an adjustment to all the intervening altitude points so that they are a closer approximation to the real elevation profile

I know there are other tools that are supposed to help with this (which I haven't yet tested) but it would be much nicer to have it all incorporated within Ascent.
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Re: Altitude adjustment

Postby oshloel on Mon Oct 11, 2010 10:09 pm

One of the potential ways to address this issue is within the 705 itself. The last 2 firmware upgrades have a feature whereby the 705 will set the altitude to a previously entered waypoint value if you press Start when you are within 100 feet or so of the waypoint. As such if you generally start your rides (or start driving to a ride) from (for instance) home, just enter your home and it's altitude as a waypoint.

I have about 6-7 of my common ride start points entered as waypoints with altitude info and my 705 does a very good job of adjusting it's starting altitude to the waypoint altitude when I start out on the ride.

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Re: Altitude adjustment

Postby axiom95 on Mon Oct 11, 2010 10:27 pm

Rick,

Do you still need to give the 705 some time to "settle/calibrate" or is it stable right away? Thanks.
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Re: Altitude adjustment

Postby Mick F on Tue Oct 12, 2010 5:21 am

If you switch on your 705 a good 20 mins before you leave, it will generally be ok, but as altitude assessment isn't too good with GPS, it can be better to set a waypoint as is suggested as the altitude can be up to 100ft wrong.

Personally, I don't bother, as the (lack of) accuracy isn't important to me. I'd rather know the altitude change and the total ascent of my ride. The absolute altitude figure doesn't matter.

If you do set a waypoint, the 705 will immediately switch to the altitude you have set, and track the changes plus or minus from that. It will be a stable as it can be.

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Re: Altitude adjustment

Postby oshloel on Tue Oct 12, 2010 6:05 pm

<Do you still need to give the 705 some time to "settle/calibrate" or is it stable right away?>

No, I don't find it's necessary to give it time to calibrate. I just punch Start and immediately go. Calibration is essentially what you have done by letting the waypoint force an initial altitude. I've generally found that the altitude reading upon returning to the start point after a 2+hour ride is within 100' or less of the initial altitude. I consider that to be well within the accuracy of a barometric altimeter; particularly, since the base barometric pressure could easily change a bit over 2 hours on a sunny day.

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Re: Altitude adjustment

Postby Mick F on Wed Oct 13, 2010 10:58 am

Ah but ..........

The BP Altimeter is only half the story.

The 705 and the 305 have BP Altimeters to assist in the altitude measurement, not to take over. The 305/705 continually check GPS altitude as well and therefore continually correct the BP altitude for air pressure differences. However, the firmware "makes it mind up" about the most valid information it receives. If it didn't, you could be hundreds of feet wrong over a long ride when a weather front comes through. I doubt a 305 or 705 would ever be hundreds of feet out.

The 205 and 605 have no BP Altimeter, but they work ok, but not as accurate.

I took my 305 on an aircraft flight to Malta a couple of years ago and tracked the flight. Very interesting! We were at 35,000ft or thereabouts but my 305 levelled out at 7,000ft because of the cabin pressure. You might say that that proves the point about BP being the one the unit uses, but it took that figure because it was the most valid. A 305 (or a 705) is for cycling with, not for climbing to 35,000ft!

It's the same with the Speed/Cadence unit. The magnet on the wheel can give speed and distance if you set the unit to read manually, but if you set it to Auto, the GPS and the magnet do it together, with the unit continually assessing wheel circumference. Wheel circumference changes with tyre pressure and tyre deformation due to loads up and down hills. This is just the same with air pressure. That changes too during the day and the unit has to track that as well. There is no way to set the altitude reading just by BP or GPS - it's continually in Auto and uses both.

Also, I conduced an experiment a few months ago. I switched on my 705 at home (250ft), pressed Start, placed it in a totally sealed jar and took it for a ride in the car. We drove up to 1000ft and parked in the carpark at the top. Within 5 mins, the 705 read 1000ft. Note that the jar was sealed, I could read the display, and the only way it could have known we were at 1000ft was by GPS. Also note that BP was still at 250ft inside the jar.


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Re: Altitude adjustment

Postby leoszilard on Sun Nov 07, 2010 5:37 am

The airliner "experiment" is hardly conclusive. You have strong belief about the subject and your "results", but that is completely different than being scientifically conclusive.

To the person asking about calibration, the initial altitude seeding that can be done with the 705, after the last update or two, is a huge help. That's not the whole story, though. Temperature has a significant impact on how a barometric pressure sensor reads. You can still see changes in altitude if there is a relatively large temperature differential between where the sensor is kept and the riding environment. This is most notably in colder weather, when it's toasty in the house. I've seen big steps in measured altitude, if, say, I stop on a ride at a store and take my bike inside. Always in these cases I stop the 705 from recording before going into the store and only restart once I've left the store and am at essentially the same place where I stopped the recording. This seems to correlate with temperature changes, and the "big steps" can sometimes be on the order of ~50-100 feet.

The ideal strategy for getting the best altitude measurements would involve seeding the start altitude in the 705 with the known elevation of the starting location AND letting the 705 equilibrate with the riding environment, i.e. outside, before the ride starts.

There are a couple relatively straight forward methods for correcting sensor or pressure based altitude drift after a ride, on a computer. It would be cool if such a tool appeared somewhere down the line in Ascent.
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