Elevation settings
QNH, GPS altitude and flight levels in Cubio apps
Altitude is not one number. A watch can show GPS altitude, pressure altitude, QNH-corrected altitude, height above ground, or a flight level. They sound similar, but they answer different cockpit questions.
The short version
- QNH is an atmospheric-pressure value reduced to mean sea level. When set on a barometric altimeter, it should make the instrument indicate the aerodrome elevation while on the ground.
- Standard means the fixed 1013.25 hPa reference. Use it for flight levels and airspace limits published as FL.
- GPS altitude comes from the satellite position fix. Garmin's reported activity altitude may instead come from the barometer, GPS, or Garmin's own calibrated altitude processing, depending on the watch and data field.
- FL means flight level: hundreds of feet of pressure
altitude on Standard, shown as values like
FL065.
Why this matters
Glider pilots use altitude for more than one job. GlideRange needs your height above a field to estimate whether you can reach it with your chosen glide ratio, glide speed, wind setting and safety margin. GlideSpace needs the right vertical reference to compare you with airspace floors and ceilings. Vario and Vario Pro need clean pressure changes for climb and sink.
If the altitude reference is wrong, the app may still draw a tidy number, but the number may answer the wrong question. A few hectopascals of QNH error can move the indicated altitude by tens of metres. Cold air and pressure variation across an area can add more error. That is not cosmetic when you are comparing yourself with airspace or thinking about terrain clearance and a marginal final glide.
QNH: altitude above mean sea level
QNH is an atmospheric-pressure value reduced to mean sea level. When QNH is set on a barometric altimeter, the instrument indicates altitude referenced approximately to mean sea level and should indicate the aerodrome elevation while on the ground. In Cubio apps, setting QNH lets the watch convert measured air pressure into an indicated MSL altitude using the same kind of reference a cockpit altimeter uses.
Use QNH when the question is "how high am I above sea level?" That is the useful reference for many airspace limits, field elevations and glide calculations. If your QNH is off by 1 hPa, the altitude error is roughly 8 metres near sea level, so a stale setting can become visible quickly.
QNH-corrected altitude is still pressure-derived indicated altitude. It is not true geometric height. Differences from the standard temperature profile, and pressure variation across the area you fly, mean that it can differ from true altitude. In unusually cold air, true altitude can be appreciably lower than indicated altitude.
Calibrate to field elevation
If you know the elevation of the field where you are standing, Cubio can work backwards from the current pressure and that known elevation to find the implied QNH. This is the same practical idea as setting a panel altimeter until it reads the airfield elevation.
This is useful before launch when you trust the field elevation more than the last pressure value you remember. It still depends on the local pressure at that moment, so calibrate on the ground before flight, not after the weather has moved on.
Standard atmosphere: 1013 hPa
Standard uses the fixed aviation reference of 1013.25 hPa. It does not try to show true height above sea level on today's weather. Instead, it gives every aircraft the same pressure reference.
That is why Standard matters for flight levels. On a low-pressure day,
an aircraft at FL065 is not necessarily 6500 feet above mean
sea level. FL065 is the pressure surface on which a
standard-pressure altimeter indicates 6500 feet. Its geometric elevation
above mean sea level is what varies.
Flight levels: FL
A flight level is pressure altitude on Standard, expressed in hundreds
of feet. FL065 means flight level 65: the pressure surface
on which a standard-pressure altimeter indicates 6500 feet. Cubio formats
flight levels with three digits, such as FL010,
FL065 and FL100.
Use FL when the airspace or procedure is published as a flight level. Do not treat FL as GPS altitude, and do not expect it to match QNH altitude on a non-standard day.
GPS altitude and Garmin-reported altitude
Raw GNSS altitude comes from the position solution. It may be referenced to an ellipsoid, while consumer devices often convert or present it as an MSL-related value using a geoid model. On Garmin watches, depending on the watch and the Garmin data field used, reported altitude may be derived from the barometer, GPS, or Garmin's own calibrated altitude processing.
That can be good enough as a fallback, especially before you have set a QNH, but it is not the same as deliberately flying a pressure altimeter on a known subscale.
GPS altitude can jump or lag. That is why Cubio's vario uses barometric pressure for climb and sink rather than GPS altitude. A small GPS altitude step can look like a large fake climb rate; pressure changes are the cleaner source for a variometer.
MSL, AGL and airspace
MSL means altitude above mean sea level. GlideSpace interprets each airspace boundary using its published reference. MSL boundaries are compared with QNH-derived indicated altitude, while FL boundaries are compared with standard-pressure altitude calculated using 1013.25 hPa.
AGL means above ground level. That is harder on a watch, because proximity may depend on terrain at the relevant boundary or area, not only directly under the aircraft. Cubio does not carry a full terrain model for evaluating AGL airspace bounds. In the current GlideSpace logic, an AGL floor or ceiling is treated as non-excluding in the vertical filter, so an otherwise relevant warning is not suppressed only because that AGL reference cannot be evaluated on the watch.
Which setting should you use?
For GlideRange and most MSL-based flying, set the current local QNH or calibrate to a known field elevation before takeoff. GlideSpace uses the published reference on each airspace boundary, so FL boundaries are compared against Standard even while MSL boundaries depend on QNH. For Vario Pro's FL readout, FL comes from barometric pressure on Standard, not from GPS altitude.
The practical habit is simple: set up the watch on the ground, check the reference against what you intend to use in flight, and remember what the app is. Cubio apps are supplementary situational-awareness tools. They are not certified flight instruments and must not be used as the sole source for altitude, terrain clearance, airspace compliance or flight-level separation.