Adjusting altitude data on the GPS
Posted by David Albert on 06 June 2017 11:36 AM

The elevation data on a GPS device can vary depending on several factors:

  • Weather conditions

  • Sensors available on the device (e.g. a barometric altimeter increases precision)

  • Parameter settings like the source of the elevation data, the barometer’s calibration or the minimum altitude for elevation gain.

There is thus an unavoidable margin of error in the elevation measurements made by a GPS device. Moreover, different makers of GPS devices, applications or websites will calculate elevations differently, meaning the data can vary from one environment to another.


In TwoNav, the margin of error can be minimized by adjusting the following two parameters in the GPS settings as needed.


Source for elevation data

(“Settings > Calibration > Altimeter”)

The elevation information can be obtained from three different sources:

  • GPS: Takes data supplied by the GPS chip. In principle, the least reliable method.

  • CDEM: Takes data from the relief map (CDEM) that’s loaded. This can supply information with no large errors but with little sensitivity to changes.

  • Barometer (only if the device has a barometric altimeter). This is the ideal method, since it is very sensitive to elevation changes. The barometer requires calibration, which can be done in one of three ways:

    • Manually: Entering the current elevation, which must be known beforehand.

    • GPS self-calibration: The barometer will calibrate itself using averaged data taken from the GPS.

    • CDEM self-calibration: The barometer will calibrate itself using the elevation for the contour line for the current location.

The recommended method, and the one selected by default, is the barometer with GPS self-calibration.


Minimum Altitude Increase

(“Settings > Full settings > Advanced”)

This parameter defines the elevation change needed to count toward the elevation gain.

During an activity, you will usually climb and descend several times. For example, you may climb 200 m to reach a hilltop, then descend before climbing another 500 m to reach the main summit, for a total of 700 m climbed.

But during an activity, there may be small changes to the elevation reading due to an error inherent to the measurement system. For example, while walking on flat terrain, the elevation may swing between 98 and 102 m.

TwoNav will not add these small oscillations to the total elevation gain for the activity.

The parameter “Minimum altitude increase” will determine the smallest change in elevation needed to count toward the gain. Its default value is 5 m, but if you see that the device is overestimating the elevation gain during an activity, you can raise its value to 10, 15 or 20 m without significantly affecting the elevation calculation.

This parameter affects both the Ascent data field shown during an activity and the properties of the track recorded.


This setting is also available in Land. It’s important that TwoNav and Land use the same value for minimum elevation gain, since otherwise they could give different elevation data for the same track.
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