Adjusting Altitude Data on the GPS

Altitude data from a GPS device can vary depending on several factors:

• Weather conditions

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

• Adjustment of parameters such as the source of altitude data, barometer calibration, or minimum altitude to accumulate ascent.

Therefore, there is an inevitable margin of error in altitude measurements made by a GPS device. Additionally, different GPS brands, apps, or websites will calculate altitudes differently, so data may vary from one environment to another.

In TwoNav, the margin of error can be minimized by adjusting the following two GPS configuration parameters according to our needs.

Source for Altitude Data

• “Settings > System > Altimeter”

Altitude data can be obtained from 3 different sources:

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

• 3D Relief (Elevation Map): Takes data from the relief map (DEM) that we have loaded. This usually provides data without large errors, but with low sensitivity to changes.

• Barometer (only if the device has a barometric altimeter). This is the ideal method as it offers high precision for detecting altitude changes. The barometer must be calibrated, which can be done in 3 ways:

  • Manually: Entering the current altitude, which we must know beforehand.

  • GPS Auto-calibration: The barometer will be calibrated by taking averaged GPS data.

  • 3D Relief Auto-calibration: The barometer will be automatically calibrated by taking the altitude from the relief at our current location.

The recommended method, and the default selection, is the Barometer auto-calibrated by GPS.

Minimum Altitude Increase

• “Settings > Activity Profiles > Activity Recording”

This parameter defines the altitude change required for the ascent data to be counted.

During an outing, you usually ascend and descend several times. For example, you ascend 200m to reach a hill, descend, and then ascend another 500m to reach the main summit. In total, you have ascended 700m.

But during the outing, there can be small changes in altitude data due to the inherent measurement system error. For example, walking on flat ground at 100m, your altitude may fluctuate between 98m and 102m.

TwoNav will not add these small fluctuations to the total ascent of the outing.

The “Min. Altitude Accum.” parameter determines the minimum altitude variation to accumulate ascent. By default, it is set to 5m, but if we see that the device is overestimating the ascent of our outings, we can increase it to values like 10, 15, or 20m without significantly losing ascent calculation accuracy.

This parameter affects both the Ascent data field we see during the outing and the properties of the recorded track.

This setting is also available in Land. It is important that TwoNav and Land have the same value set for Minimum Altitude to Accumulate, otherwise they could give different ascent data for the same track.

Altitude Filtering

New setting available in “Settings > System > Altimeter”

• Altitude Filter (On/Off)
• Filter Level (Low/Medium/High).
  A low filter, or turned off, will be very sensitive to small altitude changes. This avoids underestimating Ascent.
  A high filter will smooth altitudes significantly, which can cause underestimation of ascent, but will be more resistant to measurement inaccuracies.