Menu > Settings > GPS and location > Altitude manager

Altitude manager enables to fine-tune obtaining altitude values from your mobile device. It is divided into three main tabs:

• main settings
• settings of altitude offset
• settings of pressure sensor

Contains general switches of offset and pressure sensor and settings of SRTM data assistance and altitude measurement filter:

• No usage - altitude values are acquired from GPS only
• Optimize GPS altitude - uses offline elevation data (SRTM) to optimize GPS altitude values. Computed altitude has similar accuracy to an average GPS but does not contain serious errors GPS units usually make. SRTM data can be downloaded several ways.
• Replace GPS values - uses offline elevation data (SRTM) only for displaying elevation

Applies filter to altitude measurement to reduce value fuzziness. Heavier filters cause less fuzzy altitude values but slower reaction on altitude changes. Scale ranges from no filtration to an ultra heavy filter.

Altitude calculated by internal or external (Bluetooth) GPS unit is related to a different method of referencing the Earth surface than that used in maps. The difference between them is called Geoid height which is distance between Geoid and reference elipsoid.

Locus Map can define this offset by three methods:

• Manual correction - basic method used when the exact offset value is known
• Automatic correction - method used in most cases - Locus Map can compute offset of the whole world from a Geoid model. This method is accurate enough for use in mobile phones and sufficiently precise for all locations (the offset slightly varies based on location).
• Altitude NMEA correction - mobile device communicates with GPS unit over a special messages called NMEA. These messages contain values of Geoid height that Locus Map can read and apply them to correct this offset.Some devices report incorrect values so this method is unreliable.
• Enable also for Bluetooth GPS - check this option if you use external GPS device

Some devices are equipped with barometric pressure sensor. Barometric sensors measure atmospheric pressure very precisely which can be used for calculating relative altitude changes. I.e. if you know certain pressure at certain altitude you can calculate altitude changes between known and current position.

The pressure sensor must be calibrated to a known altitude value so that it can be used for real altitude value calculations. Locus Map offers several methods:

• Automatic - Locus Map uses GPS altitude or SRTM files altitude and current barometric pressure as basic values and relates all other measurements to these values. It also checks measured values in 2 hour intervals and eventually re-calibrates the pressure sensor.
• Pressure at sea level - current pressure value at a particular sea level as the reference can be obtained from the Internet. The resulting altitude calculation is also quite accurate but the base pressure changes in time so it can be a problem.
• Altitude - uses some other source of ground altitude - external maps, measuring, tourist guide posts, etc. as the reference. Locus Map then calibrates the pressure sensor according to this value.