altitude recalibration on polar watch

[Kristian]

Just wondering if other people have to recalibrate their 710i/720i watches as much as me. If i dont recalibrate it before every ride it is always out, and sometimes as much as 100m. Is this normal and im just winging, or is there a problem?? Even during the course of a 2hr ride start and finish altitude values may differ buy 20m but the start and end point is the same. Has my house mysticly elevated 20m in my absence??? im not so sure

Anyone have some sort of explanation, advise etc?

 

[maarten]

How is the altitude measured?
If its with air density the explanation is simple, change of weather changes also the air density and changes your altitude reading. Other effects can be temperature(effects on air density) related and so on.

 

[Duckwah]

altitude is always measured by atmospheric pressure and so will vary from day to day depending on weather and temperature, the measurements the HRM takes are relative to the first measurement it records and could be affected if for example the weather changed during a long ride.

 

[DiabloScott]

Reminds me of an interesting urban legend about multiple techniques of measuring altitude with a barometer:

http://www.snopes.com/college/exam/barometr.htm

 

[Kristian]

thanks for the replies. the temp and alt are measured at the same time so I dont think changes in air temp, thus pressure, should through the readings out.

 

[Leon]

There is a facility in the software to make altitude corrections.
You can force the end elevation to be the same as the start (obviously only applicable for circular routes) which works quite well.

Accurate altimeters are very expensive so do not expect too much from your 710.

Leon

 

[andy_welch]

I would have thought this is pretty normal. With the HAC4 you define a home altitude so every time you press the bottons to start a session it sets the altitude to that. Obviously you have to change this if you are not starting your ride from home or just fix it in the downloaded file).

Changes in air pressure due to weather fronts can easily make a difference of 100m

Cheers,

Andy

 

[bockers]

You can revert to a preset altitude on your Polar S710 by pressing start and then scrolling through the main display so that altitude is shown and then pressing the Light button for 3-4 seconds. Very similar to the way of resetting Trip distance.

I set my altitude on the watch to my home altitude and so can set it at the start of every ride.