4.2 Units of Measurement: Height, Altitude and Elevation

Three Words That Are Not Synonyms

Schedule 4 gives units of measurement a priority-A code, and the first thing it asks for is the difference between height, altitude and elevation. In everyday speech these are interchangeable. In aviation they are three distinct quantities with three different datums, and confusing them puts an RPA into controlled airspace.

Two abbreviations do the work. AGL — above ground level — is a height. AMSL — above mean sea level — is an altitude; MSL and AMSL are used interchangeably in Australian practice.

The regulatory consequence is immediate. CASR 101.085 limits an RPA to 400 ft above ground level, "that is, above the point on the surface of the water or earth directly below the RPA". That is a height. A Visual Terminal Chart, by contrast, prints airspace step bases and terrain figures in feet AMSL. The two numbers cannot be compared until one is converted.

The conversion

$\text{Altitude (AMSL)} = \text{Terrain elevation (AMSL)} + \text{Height (AGL)}$

$\text{Height (AGL)} = \text{Altitude (AMSL)} - \text{Terrain elevation (AMSL)}$

Worked example. You plan to operate at the standard 400 ft AGL over terrain charted at 900 ft AMSL. Your aircraft will be at 900 + 400 = 1,300 ft AMSL. The chart shows a controlled-area step with a base of 1,200 ft AMSL over that point. Your flight is above the step base and therefore inside controlled airspace — even though you never exceeded 400 ft AGL. This is not hypothetical: it is why airspace planning for an RPA has to be done in AMSL even though the operating limit is in AGL.

Reverse example. A chart shows a CTA base of 2,500 ft AMSL over terrain of 1,100 ft AMSL. The base is 2,500 − 1,100 = 1,400 ft AGL, comfortably above a 400 ft AGL operation, so the step is not a constraint at that point.

The Units Australia Uses

Australian aviation deliberately mixes metric and imperial, assigning a unit to each quantity rather than adopting one system throughout. Schedule 4 topic 3(b) lists the quantities; the table below gives the Australian answer for each.

Conversions worth knowing cold

• 400 ft = 121.9 m. CASA rounds this to 120 m in public material, so a 120 m limit and a 400 ft limit are the same rule stated twice. Take the more restrictive of the two if a question forces a choice.
• 1 NM = 1.852 km. So 3 NM = 5.556 km, which CASA rounds to the familiar 5.5 km aerodrome buffer. Again, one rule, two statements.
• 1 m ≈ 3.28 ft. A 45 m micro-RPA ceiling near an aerodrome is roughly 148 ft.
• 1 kt ≈ 0.514 m/s. A 15-knot wind is about 7.7 m/s — useful because many drone manufacturers publish maximum wind resistance in metres per second while forecasts are in knots.
• 12 NM ≈ 22 km, the offshore limit of Australian territory quoted in the standard operating conditions.

That last conversion pair is the one to rehearse. A manufacturer specifying "maximum wind resistance 10 m/s" is quoting roughly 19.4 knots. If the forecast is 27018G26KT, the 26-knot gust is well past the aircraft's limit even though the mean of 18 knots is inside it.

Where This Shows Up in the Exam

Expect three shapes of question:

1. Definition questions — "which of these is measured from mean sea level?" The answer set separates height from altitude from elevation.
2. Conversion questions — you are given a terrain elevation and an operating height and asked for the AMSL figure, or given an AMSL airspace base and asked whether a 400 ft AGL flight is below it.
3. Unit-matching questions — you are given a value with a missing unit and asked which unit is correct for that quantity, or asked to convert a manufacturer's metric limit into the knots used by a forecast.

The habit that protects you in all three: write the datum next to every number. 400 AGL and 1,300 AMSL are unambiguous. 400 and 1,300 are not, and an unlabelled number is how an otherwise competent pilot ends up comparing a height with an altitude.