Australian tide tables and nautical charts
Lowest Astronomical Tide, as chart datum. Heights are measured up from it and charted depths down from it.
The reference surface
Every tide height
is measured from
something.
Almost no one states what. Get the datum wrong and a perfectly correct number becomes a dangerously wrong answer.
01 · The problem
What a datum is for
"The tide is 1.8 metres" is meaningless on its own. Above what?
A datum is an agreed zero surface. Every height is measured from it, and as long as everyone uses the same one, numbers can be added and compared. The moment two numbers come from two different datums, adding them produces a confident, plausible, wrong answer.
Australia uses several datums at once, for good reasons, and they are not interchangeable. A tide table, a nautical chart, a survey plan and a flood study can all describe the same patch of water using different zeros.
02 · Orientation
The vertical stack
Schematic only. The vertical separations between these surfaces differ by orders of magnitude around the Australian coast, from a few tens of centimetres in parts of the south west to many metres in the Kimberley.
03 · The zero
Chart datum and Lowest Astronomical Tide
Australian nautical charts and Australian tide tables use Lowest Astronomical Tide (LAT) as chart datum. LAT is the lowest water level that can be predicted to occur from astronomical causes alone, under average meteorological conditions, considering every combination of astronomical conditions over a long period.
Choosing LAT as zero is deliberate and conservative: it means the water is almost always at or above the charted depth, so a mariner reading a chart is rarely given more water than exists.
Two things follow that people find surprising:
- Tide heights are almost always positive, because the zero sits near the bottom of the astronomical range rather than in the middle of it.
- LAT is not a floor. It is the lowest astronomically predicted level. Weather can and does push real water below it. A strong offshore wind combined with high barometric pressure can produce less water than the table says at low tide, which is the condition that puts boats on the bottom.
LAT depends on an epoch
Because LAT is derived from a long analysis period, the answer depends on which period was used. Queensland's port datum, for example, is stated as Lowest Astronomical Tide on the 1992 determination, with a tidal datum epoch of 2010 to 2029. The Australian Hydrographic Office states a different determination period for the national tables. These are small differences, but they are a real reason two official-looking sources can disagree slightly about the same place, and they are a reason to record the epoch rather than just the datum name.
04 · The middle
Mean sea level
Mean sea level (MSL) is the average level of the sea surface at a place over a long period, long enough to average out tides, weather and seasonal effects.
It sits near the middle of the tidal range rather than at the bottom, so a height referenced to MSL and a height referenced to chart datum will differ by roughly half the tidal range. In the Kimberley that difference is metres.
MSL is the natural reference for oceanography and for global models, and this is where it becomes directly relevant to this site. Atmosphr's ocean conditions panel shows a modelled sea level that is referenced to global mean sea level, not to any local tide datum, which is why the panel says so explicitly and why that number must never be read as a tide height. More on that distinction.
05 · The land
The Australian Height Datum, and why a tide height is not an AHD height
The Australian Height Datum (AHD) is the national vertical datum for land surveying. It was established by adjusting the levelling network to mean sea level as observed at a set of tide gauges around the Australian coast in the late 1960s.
Everything on land is quoted in AHD: survey plans, floor levels, road levels, flood study levels, bridge soffits, planning controls. If an Australian document gives you a level in metres and it concerns land, it is almost certainly AHD.
AHD was defined from mean sea level, so it is close to MSL. It is not identical to it, and the difference is not constant around the coast. More importantly for this page:
AHD is not chart datum, and a tide height is not an AHD height.
They are different zeros, separated at any given port by roughly the distance from LAT up to mean sea level, which is to say by a substantial fraction of the local tidal range.
06 · The dangerous part
The conversion trap
This is where datum confusion stops being academic.
Suppose a bridge, a jetty deck, a boat ramp lip or a floor level is quoted at 2.4 metres AHD, and the tide table says the tide will reach 2.6 metres. It is extremely tempting to conclude the water will be 200 millimetres over it. That conclusion is wrong, and it is wrong in the unsafe direction as often as not, because the two numbers are measured from different zeros.
You cannot compare a tide height to a survey level, a bridge clearance or a design level without the local offset between chart datum and AHD. That offset is a published, place-specific value. It is not something to estimate.
Where to get it:
- State maritime or survey authorities publish tidal plane tables that list the relationship between the tidal planes and the datums for each port. Maritime Safety Queensland publishes tidal planes for Queensland ports.
- Geoscience Australia and state survey authorities publish datum relationship information for the national datums.
- For anything with a consequence, get the figure from the authority for that port rather than from any third party, including this one.
The general rule worth carrying: if you are about to add or compare two heights, first say out loud what each is measured from. If you cannot answer for both, you do not yet have enough information to do the arithmetic.
07 · In practice
Which datum each source uses
Maritime Safety Queensland
Queensland Port Datum, stated as Lowest Astronomical Tide on the 1992 determination, with a tidal datum epoch of 2010 to 2029.
Survey plans, flood studies, planning documents
Australian Height Datum. Close to mean sea level, and therefore well above chart datum at most ports.
Global ocean models, including Atmosphr's ocean panel
Global mean sea level. Not a local tide datum, and not convertible to one without a local offset. Treat as a modelled water level, never as a tide prediction.
What Atmosphr publishes against
At present, nothing: we do not publish tide predictions. If that changes, every location page will state its datum, its epoch and its source station on the page itself, and will link here. We are writing that commitment down now so it is harder to quietly break later.