Understanding the definition of these terms will allow for a better understanding of the information shown by the vario app. This knowledge will enable paragliding pilot to make efficient use of above ground level information and units like Flight Level or QNH altitude.
The concepts of altitude, elevation and height are easily mixed up or used confusingly. Before going into altitudes and elevations lets do a recap. Within the context of paragliding:
Height is a measure of vertical distance. For instance, the height of a building refers to the vertical distance from top to bottom.
Elevation is the vertical distance or height of a location on the earth’s surface to the mean sea level acting as reference level.
Altitude is the vertical distance of a point above some reference level like the aforementioned mean sea level. Based on context that reference can be defined by something like a reference pressure like QNH or another specific ground level / elevation.
A question that frequently arises in paragliding is the use of different altitudes in standalone varios or vario apps. Pilots will notice that most variometer provide the opportunity to select between different altitudes although most of the time an auto preselected altitude will be flown. So, which altitudes are supported by the vario app?
The MSL altitude is aligned upon start-up of the vario app with the Global Navigation Satellite Systems (GNSS) reported Mean Sea Level (MSL) altitude. This is the altitude above mean sea level as reported by the GNSS and is automatically selected. This altitude is parsed from NMEA sentences and is not the WGS84 altitude that is reported as default.
$GNGGA, 091133.00, 5215.695596, N, 01030.233892, E, 1, 12, 0.8, 111.7, M, 43.2, M, ,*73
example GNGGA nmea sentence at 09:11:33 UTC with msl altitude of 111.7m
You can check the difference between WGS84 and MSL altitudes reported by the GNSS in Settings -> Status. It is important to note that some, rather cheap phones, might not be equipped with the capability to provide NMEA sentences.
The Flight Level is the barometric altitude for a reference pressure of 1013.25hPa. The flight level itself is reported in hundreds of feet. With that a flight level of FL95 refers to 9500 feet which in turn equals roughly 3000 meters. This is the altitude assuming a standard mean sea level pressure of 1013.25 hPa. Whenever the true mean sea level pressure is higher than 1013.25 hPa the flight level reported will be lower than the MSL altitude and vice versa. The idea of flight levels can be summarized as to provide the capability to fly on different or rather separated flight levels (in terms of altitude) using just the (calibrated) barometer and an agreed reference pressure.
which is a barometric altitude with the reference pressure set to a QNH pressure from a relevant or nearby METAR station. QNH can be described as the virtual mean sea level pressure. METAR is a format for reporting weather information. This is the altitude assuming the QNH pressure at mean sea level. Setting the QNH pressure as reference at mean sea level, a QNH altitude of zero meters or feet would be expected. A sample METAR could look like this:
2024/03/02 13:20
EDVE
021320Z 13008T 100V160
CAVOK 14/04 Q1005
The virtual mean sea level pressure is reported as 1005 hPa (lower than the standard 1013.25 hPa) by a station with EDVE as ICAO airport code.
which is the barometric pressure at field elevation e. g. airports. Setting this pressure, at field elevation an altitude of zero should be provided.
This is an altitude that can be set to any value by the pilot and initially is set to the ground level elevation at the current location. This can be helpful if GNSS is providing wrong altitudes and a QNH pressure is not known but the correct altitude is known.
You can select different altitudes by short clicking the altitude field. The order is mean sea level followed Flight Level followed by QNH followed by QFE followed by Manual. You can set altitudes by long clicking the altitude field for more than three seconds. Depending on the type of altitude you will be able to set a reference pressure or an altitude itself. QNH can also be set by clicking the METAR marker on the map directly.
Of course, Flight Level and GNSS Mean Sea Level Altitude cannot be set by the user.
The XC Vario app supports ground level information with respect to the underlying ground surface. Besides providing information known as height above ground level (AGL), ground level information is also used to indicate a rough orientation for the vertical distance to airspaces.
ground level information of 526m for Lake Bohinj in Slovenia
The ground level information used in our phone varios is based on high resolution 1° x 1° terrain tiles, as opposed to the widely used low resolution tiles in many commercial varios that one can buy. Each 1° x 1° terrain tile provides in total 12.967.201 heights or data points providing elevations with a resolution of roughly 20m x 30m within each tile. Tiles are available for almost all regions of the earth.
For most european countries extremely detailed lidar data exists and is partially used. Even remote places like Iceland are covered.
Importing Terrain Tiles is accomplished by simply clicking on the required region to download elevation data. Tiles for many regions of the earth are already highlighted to make the process of downloading for paragliding sites and areas more comfortable. However, download is not restricted to these regions. You can click anywhere you need to create and highlight a new terrain tile and click again to download. Only few if any paragliding pilot at all gets to fly all regions or sites. So downloading only what you need wont take more memory on your phone vario as really needed.
click on a premarked terrain tile or anywhere else to download ground level information
We do not support the import of .hgt files. In case you are looking for .hgt data, the viewfinder panoramas site might be a starting point. However, we are not affiliated with that site!
While paragliding the information on ground level heights is being used and visualized in different features within the variometer app.
1417m above ground level with vertical terrain view airspace on top visualized
One feature used in the XC Vario is the AGL in m field that indicates the expected difference in meter between your flight altitude and the ground elevation. This is the expected difference and cannot ever be 100% accurate due to two reasons.
Reason 1: the information processed from the terrain tiles is most of the times very good but not perfect due to failed or empty readings, abrupt changes in terrain and or other sources of error that are inherent to digital elevation models (DEM) and simply happen when you try to scan the earth from an altitude of 233 kilometers above the earth to generate a digital elevation model.
Reason 2: the altitude you are flying at and that is used to calculate the above ground level information is obtained from GNSS NMEA readings of the phone which varies depending on the phone, chipset and the quality of the current GNSS fix. While phones like the Pixel 6 or higher are equipped with Dual Rate GNSS chipsets others are not and may be less precise and accurate in certain conditions.
In any way, the AGL information should just be seen as a rough estimation.
Another feature utilized by the vario is the terrain view that appears once your flying approximately straight for a few seconds. In this case the terrain ahead of you is visualized together with your current flight path with respect to your expected altitude at a certain distance. The terrain view looks 5000 meters ahead and 1000m behind you.
Last but not least the above ground level information is used to calculate the distances to airspaces around you and which is only visualized if you are below the vertical threshold set in the settings.
