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16.1. Altitude accuracy

ViewRanger can show two types of altitude.

Map altitude is generally more accurate than GPS calculated altitude.

The app does not use the barometer sensor in the device!

Map altitude

Map altitude is calculated from .VRH files installed on the device and is based on a grid.  The map altitude is displayed on the coordinate bar.  It is reasonably accurate, but can miss peaks, and is always the ground height value.

Map altitude in coordinate bar

Free map altitude files should install automatically but can be installed manually using Menu tab (in green bar), Map shop.
If the map altitude files are installed you will see an altitude reading in the coordinate bar at the top of the map screen.  You will also see a dual altitude profile, (Head & Shoulders icon, Tracks tab, tap a track to open preview, Graphs, tap the "Height" tab) with both types of altitude displayed, in the track "Height" graph.

Dual track altitude profile

GPS altitude

'GPS altitude' is calculated from the GPS location so requires an accurate GPS fix throughout the duration of the recorded track.  The accuracy is variable.

Display in app

Only the map altitude, from the .VRH files, is displayed in the coordinate bar on the map screen.
Both types of altitude are displayed in the track height graphs.  The statistics on the track summary page are based on .VRH file map height.

Track Summary

The altitude profile shown on the my.viewranger.com web site is generated by a separate web service so a small amount of difference in altitude values, when compared to the app, is normal on the Website.

Watch

The watch uses the barometer to calculate altitude gain/loss, which is different to what the phone or website uses.

GPX files

Altitude data exported in GPX files is always GPS altitude data.

Other apps

If you are comparing tracks recorded on other apps and devices remember that other apps and websites will use a different methods to calculate altitude, so a small amount of difference in height between apps and websites, is normal.  

Troubleshooting

Always obtain a GPS fix prior to recording a track to avoid zero altitude being recording into your track.
To do this, enable location, press the Compass/GPS icon on the map screen and wait for a red circle to appear at your position, before you start to record a track.

Loss of GPS fix or a GPS "spike", while recording a track, will have a detrimental effect on the accuracy of the altitude reading no matter which type of altitude you are using.

Mean sea level correction

Normally GPS altitude is shown relative the WGS84 datum, which is different to sea level. In Cambridge for example the difference is about 45 metres.
If you copy this optional GPSAltitude.DAT configuration file into the Config sub-folder of your ViewRanger folder, then ViewRanger will apply a "mean sea level" correction to the GPS altitude. This will make the GPS altitude relative to sea level. The accuracy of the reading can be pretty good, but it can take a while to settle down when the GPS first starts.
The correction will be applied to newly recorded figures, it does not apply to old or imported tracks. The resulting figure will be exported with tracks.

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