Understanding Height References in Drone Operations
Why AMSL, AGL, Height above Take off and Geodetic & Barometric altitude matter for drone operations in Singapore?
If you plan to fly a drone in Singapore, understanding height references is not just a technical detail. It affects how you interpret your flight data and whether your operations may cross a regulatory threshold.
In Singapore, the Civil Aviation Authority of Singapore (CAAS) uses Above Mean Sea Level (AMSL) as the relevant height reference for certain unmanned aircraft permit thresholds. For example, the typical flight altitude restriction in Singapore is 200 feet AMSL. Flying above that altitude could put your drone within unauthorized airspace that could be reserved for manned aircraft.
However, the challenge is that many drones, RID trackers, and scanner apps do not show AMSL as telemetry. Instead, they often display heights based on the take-off point, barometric pressure, or GNSS data. These values can still be useful, but they do not all mean the same thing. That is why height data can be easy to misread.
Key terms
Before learning how to estimate AMSL from your flight data, it helps to understand a few key terms. These references may look similar at first, but each one measures height differently.
- Elevation
Elevation is the height of your take-off point above mean sea level.
This matters because your flight begins from that elevation. If you launch from a hill, rooftop, or raised platform, your aircraft may already be well above sea level before it climbs. - AMSL
AMSL means the height Above Mean Sea Level. It is the vertical distance between your aircraft and the mean sea level.
This is one of the key height references used in Singapore’s regulatory framework, including the 200 feet AMSL threshold relevant to certain permit requirements. - Height AGL
AGL means Above Ground Level. It is the vertical distance between your drone and the ground directly below it.
This is useful for obstacle clearance and flight awareness, but it is not the same as AMSL. If the terrain changes beneath your aircraft, the AGL value changes too. AGL is a common regulatory reference used in other countries. - Height above Take off Position
Height above take-off position is the height of your aircraft relative to its take-off point. Many drone and Remote ID systems do not directly compute true height above the ground beneath the aircraft and instead provide height relative to the take-off point.
This is one of the most common values shown in RID systems and most drones that do not have a visual means to determine their height AGL. Once again, this is useful for flight control, but it does not automatically refer to AMSL. - Geodetic Altitude (GPS)
GNSS-based systems typically measure height relative to the WGS-84 ellipsoid, not directly to mean sea level. The Singapore Land Authority explains that GNSS yields ellipsoid height, while a geoid model such as SGEOID09 is used to derive geoidal height, which is linked to mean sea level.
While the GNSS-based system remains fairly accurate for horizontal displacement, it does not fare that well for differentiating altitude displacement.
This is largely due to the placement of satellites, which is high up in the sky. Just imagine looking downward to the ground; it’s easier to tell horizontal displacement, but very difficult to estimate height.
Due to this reason, GPS altitude typically has a higher margin of error compared to GPS location accuracy.
GPS altitude assures that the earth is a sphere, but its actual fact, the earth’s surface is not smooth.
To convert GPS altitude into a height related to mean sea level, a geoid-based correction is required. In Singapore, this relationship is handled using the SGEOID09 geoid model. - Barometric Altitude
Barometric altitude is derived by converting air pressure into an altitude estimate. Many barometric systems use the standard pressure reference of 1013.25 hPa as part of that calculation.
This can introduce some discrepancy, because the actual air pressure at your operating location may differ from standard atmospheric conditions.
Barometric sensors are often useful for detecting relative changes in height over short periods, although they can still drift and be affected by changing weather and pressure conditions. Because of that sensitivity, many systems use changes in barometric altitude to estimate relative height from the take-off point.
How to Estimate AMSL?
For many local VLOS operations, a practical way to estimate AMSL is:
AMSL altitude = take-off elevation + height above take-off
If your drone or tracker reports height relative to take off position, you can use that value together with the known elevation of your take-off point.
This method is useful because the number shown on screen is often not AMSL by itself. A common mistake is assuming that the displayed flight height already represents your aircraft’s altitude above mean sea level.
For example, if:
- your take-off point is 60 feet AMSL
- your device shows 80 feet above take-off
your aircraft is not at 80 feet AMSL. It is at:
60 + 80 = 140 feet AMSL
This is why the reference point matters. A displayed height can be technically correct but still be the wrong reference for compliance.
For typical short-range VLOS operations, this approach is often practical because the aircraft remains close enough to the launch site for the take-off elevation reference to stay useful. Even so, it is wise to keep a buffer below any legal limit, as sensor readings and pressure-based measurements are not perfect.
A practical reference of elevation may be obtained from mapping tools such as Google Earth, but where precision matters, authoritative or surveyed elevation data is preferable.
Pre-Flight Checks for Height Awareness
Once you understand how height references work, the next step is making sure you apply them correctly before take-off. A few simple checks can help you avoid relying on the wrong number during flight.
- Know your take-off elevation
Before you fly, check the elevation of your launch point using a reliable source such as mapping data, digital elevation models, or surveyed references. - Confirm what your device is showing
Do not assume every altitude value means AMSL. Check whether your system is showing:
– relative height from take-off
– GPS-based height
– barometric height
– AGL
– or a corrected AMSL value - Leave a buffer
If a legal threshold is involved, do not fly right up to the limit. Leave a margin to account for drift, variation, or data interpretation errors. - Be extra careful on elevated launch sites
Launching from a rooftop or hill can significantly reduce your available margin because your aircraft already starts at a higher AMSL.
How AirBeep-B Helps
If you are using AirBeep-B together with the AirBridge Mobile App, you do not need to work everything out manually during flight.
AirBeep-B determines height by measuring changes in barometric pressure from the take-off point. In the AirBridge Mobile App, you can also view an additional AMSL data field, so you can interpret altitude more easily without manually converting GPS altitude yourself.
You also receive notifications if:
- your flight exceeds the configured height limit
- your aircraft enters a restricted area or no-fly zone
This gives you an additional layer of awareness to help you stay compliant and make better decisions in the field.
If you want a simpler way to view AMSL, receive altitude alerts, and stay aware of restricted areas during flight, connect your AirBeep-B with the AirBridge Mobile App. If you do not have an AirBeep-B yet, you can also explore it on our shop.
Heron AirBridge is a Singaporean aviation technology company building the digital infrastructure for safe, scalable drone operations. Offering a suite of integrated solutions, including Network and Broadcast Remote ID, a comprehensive Fleet and Flight Management System, Unmanned Traffic Management System, and Remote ID Ground Sensors.
Heron aims to connect drones, data, and regulatory oversight—enabling seamless workflows and trusted, compliant operations for enterprises and authorities. Learn more at heron-airbridge.com.
