NEW data released by Ordnance Survey (OS) could significantly improve how coastal erosion and flood risk are monitored across Great Britain as sea levels rise and extreme rainfall becomes more frequent.

The continuous tidelines dataset provides a clearer and more accurate picture of where high and low tides fall around the coast and along tidal rivers. The data is expected to support environmental monitoring, flood modelling, and long-term planning as the impacts of climate change accelerate.

Earlier this year, the Environment Agency updated its National Flood Risk Assessment (NaFRA), which for the first time factored in projected climate change impacts. The assessment found that around 6.3 million homes and businesses in England — roughly one in five — are currently at risk of flooding. That figure could rise to one in four properties by 2050.

The Agency has also updated its national coastal erosion risk map, which estimates that around 3,500 properties could be affected by coastal erosion by 2055.

The new OS dataset builds on existing information held in the OS National Geographic Database (OS NGD). Previously separate tidal boundary features have now been automatically merged into continuous high- and low-tide lines, creating two new feature types within the OS NGD Water theme. These can also be used as overlay layers through the OS Tiles API, allowing for clearer visual mapping and large-scale analysis.

The data is expected to support a wide range of environmental and regulatory work, including coastal erosion and flooding modelling, updating boundary features to reflect coastal change, climate change impact assessments, and identifying areas of foreshore that act as natural flood defences.

William Russell, Senior Coastal Process Scientist at the Wales Coastal Monitoring Centre, said the dataset would remove uncertainty from coastal monitoring.

“One of our core values is ‘data, not opinions’. When information is missing, we seek out or collect the data needed to support sound decision-making,” he said.
“The OS Continuous Tidal Boundaries dataset removes uncertainty from our strategic monitoring programme and gives us clearer insight into intertidal extents, delivering cost savings and efficiencies for our survey contractors.”

Ordnance Survey says the data is particularly valuable for environmental policy and regulation, including decisions around water discharge and flood management. Accurately defining the intertidal zone helps identify natural coastal defences, which play a vital role in protecting communities and ecosystems.

Intertidal information has also been enhanced within the Land theme of the OS NGD, with improved coverage of previously obscured areas to create a more complete national coastal dataset.

Lily Brown, Product Manager at Ordnance Survey, said the data would support evidence-based decision-making.

“The OS National Geographic Database contains trusted, accurate features that support everything from climate resilience and infrastructure planning to coastal management,” she said.
“By incorporating authoritative tideline data, we are enabling decisions that will help protect and monitor Britain’s evolving coastlines.”

Following the dataset’s release, Ordnance Survey analysed how ceremonial counties change in size between high and low tide. Merseyside showed the greatest variation, with a nearly 17% difference, followed by Orkney at around 7%.

The analysis also highlighted that counties considered landlocked, such as Nottinghamshire and South Yorkshire, still experience significant tidal influence due to rivers like the Trent and the Don being tidal far inland.

When combined with Unique Property Reference Numbers (UPRNs), the tideline data shows that Greater London has the highest number of homes and businesses within 15 metres of the high-tide line, at around 5,500. Cornwall ranks second with just over 3,000 properties, followed by Devon with around 2,400.

The new data has been released under the Public Sector Geospatial Agreement (PSGA), which provides emergency services and public sector organisations with access to Ordnance Survey data and services.