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Understanding Altitude Values

When using our API, you will encounter three distinct altitude values related to flight telemetry data points. This section provides an overview of what these values represent, how they are obtained, and how you can utilize them in your applications.

Altitude Values​

  • WGS84 HAE (Height Above Ellipsoid) — Altitude above the WGS84 ellipsoid, in meters

    • This is the raw altitude reported directly by the internal GNSS sensor onboard Dronetag devices.
    • It represents the height above the ellipsoid (not the geoid or Mean Sea Level). This is sometimes mistakenly interpreted as MSL, but it’s important to note that WGS84-HAE refers to the ellipsoid height, not sea level.
    • This altitude can have variable accuracy depending on GNSS signal conditions and the limitations of the earth model stored in GNSS receivers.
    • In APIs available as HAE-WGS84

  • QNE (Pressure Altitude) — Altitude calculated from barometric pressure, in meters

    • This value is derived from the onboard barometer using a standard sea-level pressure of 1013.25 hPa (QNE).
    • It is highly accurate for relative measurements, such as comparing altitudes of nearby drones, but it may not represent an accurate absolute altitude compared to the ground.
    • Often used in aviation for standard altitude readings, especially when flying at higher altitudes where atmospheric pressure decreases predictably.
    • In APIs available as PA-QNE

  • ATO (Above Takeoff Level) — Altitude above takeoff point, in meters

    • This value is calculated as the difference between the current QNE (pressure altitude) and the pressure altitude at the moment of takeoff.
    • It provides a relative height above the initial takeoff point, which can be useful for determining altitude changes during a flight.
    • ATO values are sensitive to atmospheric pressure changes over time, meaning accuracy may degrade during longer flights or significant weather changes.
    • This altitude can sometimes be perceived as AGL (Above Ground Level) when the environment remains consistent and flight has took off at ground.
    • In APIs available as ATO
Upcoming Feature

We are planning to introduce MSL (Mean Sea Level) altitudes soon, which will provide a more standardized altitude measurement based on global models.

Frequently Asked Questions​

Why isn’t there a reliable AGL (Above Ground Level) altitude?​

We do not offer AGL altitude because our devices do not have sensors capable of directly measuring the distance to the ground or terrain at all locations globally. Without consistent and accurate ground-level pressure or elevation data, calculating true AGL altitude is unreliable.

However, users can calculate their own AGL estimates if they have access to reliable ground-level pressure data from their region. Using the raw pressure values reported by the device, you could compare them to known ground-level pressure to estimate the altitude.

Why does my ATO altitude sometimes show negative values?​

The ATO field is calculated relative to the recorded pressure altitude at takeoff. There are two key factors that can cause negative values:

  1. Takeoff Pressure: If you start tracking your flight when the drone is already airborne (e.g., 10m above the ground), the takeoff pressure will reflect that height. When the drone lands, the height will then show a negative value (e.g., -10m) because it's now below the original takeoff point.

  2. Pressure Changes Over Time: Atmospheric pressure changes throughout the day due to weather conditions. If you're tracking a long flight that spans hours, the height might gradually drift because the reference pressure from takeoff no longer matches the current ground pressure. This can lead to incorrect readings, especially for long-duration flights.

Why does my commercial drone show AGL altitudes, but Dronetag does not?​

Most commercial drones do not actually show AGL altitude; instead, they use a method similar to our ATO (height above takeoff). The drone's controller records the takeoff pressure and subsequently displays the difference between the takeoff altitude and the current altitude. This is presented as the drone's height above ground, even though it's technically just height relative to the takeoff point, not true AGL.