AHD means Australian Height Datum – the national vertical datum Australian drawings measure their RLs from, with zero at approximate mean sea level.
What AHD means
AHD is the Australian Height Datum – the national reference surface that Australian drawings measure heights from. When a drawing says RL 42.600 AHD, it means 42.600 m above that surface.
AHD was adopted in May 1971 by the National Mapping Council. Its zero was set by holding mean sea level at zero at 30 tide gauges around the mainland coast, using sea level observed between 1966 and 1968, then adjusting 97,320 km of primary two-way levelling to fit those 30 fixed points. A further 80,000 km of supplementary levelling was adjusted onto that primary network afterwards, in a separate adjustment that depends on it.
Tasmania has its own realisation, AHD (Tasmania) 1983, computed from 1972 mean sea level at the Hobart and Burnie gauges. You will see it written AHD-TAS83, and AHD71 for the mainland.
Two consequences matter on the drawing board. First, AHD zero is approximate mean sea level as measured six decades ago at those gauges – not today’s sea level, and not the geoid. Geoscience Australia is explicit that errors in the levelling network were distributed through the adjustment, so AHD carries known distortions that grow with distance across the continent. Second, a GNSS receiver gives you an ellipsoidal height, which differs from AHD by tens of metres. Converting requires a geoid model – currently AUSGeoid2020, used with GDA2020. Never write a raw GPS height as an AHD level.
How AHD appears on a drawing
AHD is almost always a note rather than a number. It appears once in the general notes or the survey title block, and every RL on the sheet inherits it.
LEVELS TO AUSTRALIAN HEIGHT DATUM (AHD)
DATUM: AHD. ORIGIN: SSM 84213, RL 22.146 (AHD)
SITE TBM: TOP OF KERB AT DRIVEWAY, RL 23.417 (AHD)
GROUND FLOOR FFL RL 24.350 AHD
FLOOD PLANNING LEVEL RL 24.200 AHD
Those five lines are one coherent set-out: the site bench mark is 22.146, the temporary bench mark the surveyor put in at the driveway is 1.271 m above it, and the ground floor is 2.204 m above the bench mark and 150 mm above the flood planning level. All of that arithmetic is only valid because every one of those numbers is on the same datum.
The note to watch for is the opposite one: LEVELS TO ASSUMED DATUM, sometimes with an arbitrary origin such as RL 10.000 or RL 100.000 at a nail in a kerb. An assumed datum is perfectly workable for internal set-out – falls, steps and slab thicknesses all still resolve – but it is not AHD, and you cannot use it to check anything an authority specifies.
Terms AHD gets confused with
- RL is the height; AHD is the datum it is measured from. They are not interchangeable, although plenty of drawings treat them as if they are.
- MSL. AHD zero is close to mean sea level but is not maintained as mean sea level, and the two have drifted apart since the 1960s.
- LAT. Australian nautical charts and most marine work use Lowest Astronomical Tide as chart datum, not AHD. On a wharf, jetty or dredging drawing, confirm which datum each level belongs to before you subtract one from another.
- PM, SSM, BM, TBM. These are the marks that carry AHD values to your site – permanent marks and state survey marks held in the official database, plus a temporary bench mark your surveyor establishes on site.
- GDA2020 and MGA are horizontal. AHD is vertical. A drawing on GDA2020 horizontally and AHD vertically is the normal combination, not a contradiction.
AU versus UK and US
AHD is Australia-only. If a UK or US drawing carries an AHD note, it is almost certainly a copied title block.
- Great Britain: Ordnance Datum Newlyn (ODN), defined by mean sea level at Newlyn, Cornwall, between 1915 and 1921. Heights are written with AOD – above Ordnance Datum – after the value. Northern Ireland is not on ODN; it uses Ordnance Datum Belfast, so “the UK” is not a single vertical datum.
- US: NAVD 88, which superseded NGVD 29. The difference between the two is not a single constant; it varies by location, so a US elevation quoted without naming its datum is genuinely ambiguous.
- NZ: NZVD2016 nationally, with older local vertical datums such as Wellington 1953 and Auckland 1946 still appearing on existing drawings.
The pattern is the point: a vertical datum is a jurisdictional decision, and jurisdictions are smaller than you assume. Check which one applies to the site rather than to the country.
Where to check on your own drawing set
- The survey plan: the datum note and the mark it was derived from. This is the authoritative statement for the project.
- Cover sheet general notes on the architectural and civil sets – they should agree with the survey. If they do not, that is an RFI, not a judgement call.
- Council and planning documents. Flood planning levels, minimum habitable floor levels and height limits are quoted in AHD. If your survey is on an assumed datum, compliance cannot be checked until it is tied in.
Other survey and level abbreviations are listed in the engineering drawing abbreviations list.
Other meanings of AHD
There is no strong competing meaning inside construction. Outside it, AHD most often refers to the American Heritage Dictionary. The practical point is that AHD is Australia-specific – a UK or US drawing carrying an AHD note is almost certainly a copied title block – and on mechanical sheets AHD is easily misread as AHU (air handling unit).