S-104 and S-111 explained – how real-time tides and currents change voyage planning
In this article
S-100 will add practical value for navigators
Navigators calculate voyage routes that ensure vessel safety and operational efficiency. The task will become easier and more accurate when S-100 is integrated into the voyage planning tools that help them determine transit routes, arrival times, and under-keel clearance.
S‑100 is the International Hydrographic Organization’s (IHO) future framework for digital navigation. Designed to move beyond static charts, it enables interoperable, layered data products such as tidal variation, currents, and safety information. This information is integrated with S-102 bathymetric surface data which effectively replaces a single chart sounding with a detailed depth grid.
“S‑100 is a big change because it treats tides and currents as live navigational information rather than background context,” said Roger Proctor, chief scientist at Tidetech.
“With gridded S‑104 water levels and S‑111 surface currents, the aim is to deliver the right resolution in a way a bridge team can interpret quickly and use seamlessly in their operations.”

What is S-104
S-104 is the new IHO S-100 standard and product specification that covers water levels for surface navigation. It allows for the delivery of a time series of water levels rather than simple astronomical high and low tide predictions, and they can be for a single point or across a grided surface.
This enables navigation systems to take advantage of Tidetech’s S-104-ready models that use high resolution hydrodynamics, local datums and validation, where available, to show the spatial variability of water levels with greater accuracy than just a single point tide table reading.
The ability of S-104 to improve under-keel clearance calculations and replace single tide prediction data points with a dynamic surface can boost vessel safety – some of the largest tankers in the world sail into port with only about a metre of water beneath them. The greater accuracy can also facilitate greater optimisation of cargo load limits and extend tidal windows for port approaches which can help ports reduce waiting time and therefore local emissions.
For navigators, it answers a simple but critical question — how high is the sea surface relative to chart datum along my route, right now and over the next few hours?
Tidetech’s team of experts can answer that question with world-leading accuracy.
Tidetech’s modelling includes adjustments made to chart datum to reflect changes in tide level and the effects of wind on water level. This can improve the accuracy of water level predictions significantly, because if the wind is blowing towards the coast, it also pushes water towards the coast and consequently raises water levels above the tidal variation. Winds also create surface waves that can increase water level rise.
Conversely, if the wind is blowing offshore at low tide, it’ll push water away and make it even shallower.
“If the tidal range is quite small, you can usually just add a weather component to the tidal components,” Proctor said.
“If you’re in areas where you have high tidal range, then you get what’s called tide surge interaction, and the effect of the weather acts to displace the time of high water away from the predicted time of high water,” he said.
Atmospheric pressure also affects water level. A high pressure system pushes the water surface down resulting in a lower than the predicted tidal level. A low pressure system releases pressure on the water, and the water level tends to be higher than the tidal prediction. A rule-of-thumb says that one millibar reduction in a mean atmospheric pressure will cause sea level to rise by one centimetre. These changes can be significant in coastal waters, causing 500mm differences from predicted tide levels.
What is S-111
S-111 is the IHO S-100 specification that covers the speed and direction of currents at or near the surface down to a depth of approximately 25 metres. It allows for the delivery of time series data in a grid format or as a series of discrete points. Each data point can reflect a given depth or be an average from the sea surface down to a given depth. Data is obtained from either in situ or remote measurement or by analytic methods or hydrodynamic modelling.
Surface currents caused by tidal flow can have a consistent vertical profile across a shallow body of water, with an expected variation of around 10 percent close to the surface and the seabed, Proctor said. However, the situation becomes complicated in proximity to rivers or in sunny weather where the surface waters heat up and separate from the lower waters down to a depth of about 30 meters. Tidetech modelling includes these time and location-specific variables when producing current data.
As the UK Hydrographic Office points out, using near-real-time information on surface currents, vessels could adjust their engine speeds to reduce unnecessary fuel burn. Equally, knowing what influence surface currents could have on vessel manoeuvres, such as the effect of water-flow abeam a vessel, can assist tug operators and vessels alike, when operating within confined waters.
Researchers from the University of Genoa, Italy, have provided an example of just how much difference this data can make to vessel transits using the case of a vessel navigating from Leghorn in Italy to Barcelona in Spain at 6 knots. Even though the computed route based on calculations using S-111 data was 17 nautical miles longer, it allowed the vessel to arrive at the destination 12 hours earlier than a transit based on the most direct route.
Practical experience with S-100
Theory is now being put into practice with several onboard S-104 and S-111 trials occurring around the world. Tidetech’s high-resolution English Channel and Solent models are active in several S-100 evaluations, including UKHO and French hydrographic office (Shom) trials that have been on-going since August 2025.
Working in support of the Australian Hydrographic Office (AHO), Tidetech has supplied high‑resolution tidal and current data for Sydney Harbour as part of Australia’s first live S‑100 bridge trial which ran from April to June 2026. The trial involved two Carnival Cruise Line vessels calling in to Sydney Harbour.
The data tested included Tidetech’s gridded S‑104 water level and S‑111 surface current datasets for the harbour, delivered at 100m resolution in 20-minute time steps. The focus was on how bridge teams interpret dynamic information, how much detail is genuinely helpful, and where clarity matters more than data density on an ECDIS display.
“This trial allowed us to get real feedback from bridge crews operating regularly in a busy port,” said Alvaro Sanchez, director national charting for the AHO.
“The bridge crews of the two vessels saw the products that will be available to them in the future, with the ability to turn layers on and off depending on how much they want or need at critical times like manoeuvring in a confined space like Sydney Harbour.
“In the case of Carnival Adventure, it passes under Sydney Harbour Bridge with sometimes as little as two metres of clearance above the vessel — what we call the air gap — so dynamic depths have the potential to improve the safety of that transit,” Sanchez said.The insights from the Sydney trials will feed directly into IHO standards and help shape how S-100 is implemented worldwide. For shipowners, ports, pilots and navigators, that means clarity on what to expect and confidence that new systems will deliver real-world benefits.
Related blog articles
S-104 and S-111 explained – how real-time tides and currents change voyage planning
Real-time water levels and surface currents are reshaping how navigators plan a voyage. Tidetech's chief scientist Roger Proctor explains why S-104 and S-111 treat tides and currents as live information rather than background context.
Tidetech named a Tasmanian Employer of Choice for 2026
Tasmanian ocean data company Tidetech has been recognised as an employer of choice by the Tasmanian government, one of a select group of organisations acknowledged this year.
Why “our data is 50 percent more accurate” does not mean anything in ocean modelling
How accurate is a hydrodynamic model? Tidetech's chief scientist Roger Proctor explains why the answer is never a single number, and what actually matters when judging model quality.















































