How SonarBuddy works out depths: transducer, tide and LAT
Your depth sounder measures the water under the transducer, at the tide of the moment. Charts show depths at a fixed low-water level. SonarBuddy converts every sounding so the two can be compared fairly. It also keeps your original reading untouched.
The three steps
- Below the transducer → below the waterline. We add your transducer's depth below the waterline from your boat profile (My boat). If that's missing, the depth is shown uncorrected and marked as such. A wrong offset shifts every depth from your boat, so it is worth measuring.
- Remove the tide. For the exact time and place of each sounding we predict the tide height with a global tide model and subtract it. That gives the depth relative to mean sea level (MSL).
- Refer to Lowest Astronomical Tide (LAT). LAT is the lowest tide predicted in an 18.6-year cycle at that spot. It's the level most modern charts use for chart datum. We work out LAT for each area from the same tide model and express the depth below it.
So: depth at LAT = sounder reading + transducer depth − (predicted tide height − LAT).
That is the number the map shows: the water you would expect at the lowest possible tide. At any real moment you will normally have more water than that.
What we compare with
| Region | Compared against | Using |
|---|---|---|
| New Zealand | LINZ chart soundings (chart datum ≈ LAT) | depth at LAT |
| Fiji | GMRT seabed model (≈ mean sea level) | depth relative to MSL |
| Queensland & GBR | Geoscience Australia 30 m model (MSL) | depth relative to MSL |
A hexagon is marked shallower than the chart/model only when boats consistently measured less water than the reference after the tide has been taken out.
How accurate is it?
- The tide model is EOT20 (DGFI-TUM), a global model built from satellite altimetry. Offshore and along open coasts it is typically good to 10–30 cm. In estuaries, narrow passages, lagoons and large-range areas (e.g. Broad Sound and Mackay in Queensland) errors can be larger, sometimes half a metre or more.
- What we checked: the model's LAT and HAT agree with official tide tables to within about 0.2 m at Lautoka and Cairns and in Nelson and the Bay of Islands. It also reproduces Mackay's roughly 7 m range. It under-reads by about 0.3 m inside Auckland's Waitematā Harbour, where the tide grows as it pushes in. We plan to add official LINZ port predictions for NZ harbours.
- Weather is not modelled. High pressure, offshore winds and storm surge can lower or raise the water by 10–30 cm or more compared with the prediction.
- Your sounder measures to the first strong echo (soft mud, weed and thermoclines can confuse it), and the transducer moves as the boat heels and pitches.
- Many boats over the same water average out individual errors. Hexagons confirmed by several boats are more trustworthy than a single passage.
Because of all this, treat every SonarBuddy depth as "check this" information, not a navigation depth. Allow a safety margin, check the actual tide for the time you'll be there, allow for your own draft and squat, and use official charts.
Following a buddy's track
Buddy passages show the depths that boat recorded, reduced to LAT. To plan your own transit, add the tide height above LAT expected when you'll be there (from your tide tables), then subtract your draft and a safety margin.
Your original data
Raw readings, the transducer offset, and the tide applied are all stored with each sounding. If you correct your transducer depth later, all your past soundings are recalculated automatically. Raw readings are also what we share with the international crowd-sourced bathymetry programme (IHO DCDB), which applies its own reductions.
Tide model: Hart-Davis et al. (2021), EOT20, SEANOE, doi.org/10.17882/79489 (CC BY 4.0).
← All help articles · Still stuck? Open Help in the app, or email mail@sonarbuddy.com.