The elevation gain is usually source of disagreement for cyclists. At the end of a tour they compare data of several cyclocomputer, GPS devices and smartphone applications and see that, while they agree about distance, there are very different altimetric data.

The GPS (global positioning system) allows the tracking by a radio signal, sent by a number of satellites orbiting the Earth, received by a specific device. The definition of the geographical coordinates needs the simultaneous signal of at least 4 satellites and for civil use devices is enough correct, with an approximation about a few meters.  

That’s not true about elevation, because it is more difficult to measure. GPS is base on the approximation that the Earth is an ellipsoid, while it is a geoid, a sort of potato. The difference between geoid and ellipsoid can be important (in Italy is about several tens of meters). Locally gps devices can make corrections to have an acceptable approximation. 

Another problem is the fact that satellites must be above the horizon to be “seen” by the device and that’s not the better situation to measure the elevation. The elevation gain is a processing of altitude data, so the error will become cumulative and the value won’t be reliable. 

GPS devices equipped with altimeter can integrate GPS data with atmospheric pressure measure, that can be a reliable value in the short therm if the altimeter is calibrated every time it come to be in a point with a known elevation.

Garmin Etrex35 calibrate continuously the pressure altimeter with gps known elevation values.  Data are combined to have reliable elevation values for our bike activity. 

It’s not easy to tell how much a device with altimeter is more accurate than one without it. From my own experience, I can say that for a hilly track, with many climbs and descents, a smartphone app, only based on GPS data, usually gives a value of elevation gain 20% bigger than the one given by a Garmin with pressure altimeter.  Obviously this is not a rule, but is usual.  

In this blog the elevation gain comes from a GPS device with altimeter. Probably, for an itinerary with 1000 m of elevation gain the smartphone app will give a value of 1200 m. I think that the first value is more reliable, but, given the objective difficulty of the measure, I will call the value “estimate elevation gain”.