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Elevation


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I want to know why after a ride starting and finishing at the same spot that the ascending and descending figures don't match. Surely if you ride downhill 200m you need to climb 200m.

Observe the orange line and note how I fell down the infamous 70m cliff at Fournos in Rosebank while having coffee on Saturday.

 

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I want to know why after a ride starting and finishing at the same spot that the ascending and descending figures don't match. Surely if you ride downhill 200m you need to climb 200m.

 

The device (Polar or other similar type of cycling computer) measures the air pressure acting on the device (Barometric pressure) to determine the altitude.

This air pressure changes all the time (for example think of weather hi and low pressure fronts) and it is unlikely that the start air pressure will be the same as the finish air pressure, hence giving a different altitude reading.

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Typically, you'll start a ride early (when it's cold) and finish when the sun has risen some and the temp is a bit warmer. Now we all know that hot air rises ... but that same hot air also fools barometers, making them believe that they've been "elevated", since the rising air effectively indicates lower pressure and therefore less air stacked on top of said barometer (ie. thinks it's sitting on a mountain somewhere). The Polar software also has an altitude Error Correction function (right-click on the Curve screen and select Error Correction. Then select the Altitude tab and click on the sqare up/down arrow under the graph and make your choice from the radio buttons offered).

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