Freenauticalchart.net: Categorisation of the BSH provides clarity about the underlying data

freenauticalchart.net is a free online nautical chart based on BSH data that is automatically updated on a weekly basis. With tide forecast, tidal current atlas, plotting tools and GPS tracking - usable in the browser or as an app, completely without registration. The tool is causing a stir in the sailing world and among commercial providers of nautical charts. Following the publication of our article on the free Freenauticalchart.net nautical chart and the disappearance of point soundings from BSH data, YACHT spoke to Thomas Dehling, Head of the Nautical Hydrography Department at the BSH. The conversation provides important clarity about the difference between open data nautical charts and official navigation products - and a clear warning to users of the free nautical chart.

Two fundamentally different data sets

The BSH provides two different types of nautical chart data, which are fundamentally different:

1. quality-assured navigation data (for official nautical charts): This data is specially analysed for safe navigation. They are checked for up-to-dateness, accuracy and reliability in accordance with the international standards of the International Hydrographic Organisation (IHO). Particularly important: All shallow areas are guaranteed to be included, the number of depth contours is limited so that the chart remains legible. This data is only available from licence holders.

2. open data bathymetry data (freely available): This data is provided in accordance with the Geodata Access Act and serves other purposes: morphologically correct representation of the seabed for the offshore industry, storm surge forecasting, anglers - but not for navigation. "We give no guarantee that this is nautically correct," explains Dehling.

Even navigation marks are not guaranteed to be up to date

Particularly explosive: the positions of buoys, beacons and other navigation marks in the BSH's open data services are not intended for navigation either. "We cannot guarantee that changes are included," says Dehling. If a navigation mark is moved, replaced or removed, the change appears in the Notices to Mariners (NfS) and in the official electronic nautical charts - but not at the same time in the open data services.

"That would be a huge effort. That's not what it's designed for," explains the head of department. For users of the free nautical chart, this means that even if buoys and beacons are nicely displayed on the chart, the position may be out of date or the buoy may have already been moved.

Why two different data sets?

The difference lies in the evaluation of the survey data. When the BSH surveys the seabed, very detailed depth information is generated. Two different products are created from this:

For navigation: A special surface model that is guaranteed to contain all shallow areas and depth contours in such a way that they can be used safely by sailors and commercial shipping. Individual depth figures are specifically selected - especially the shallow areas.

For other purposes: A morphological model that reproduces the seabed as realistically as possible. "What anglers need or what you need for the offshore industry or for storm surge forecasting," explains Dehling. "Where are the holes? Where are the fish?" This data may contain depth contours, but not the guaranteed shallow spots.

Licence model and quality control

The BSH only passes on the quality-assured navigation data to fixed partners (licence holders). The reason: "If we put it online freely, then we can no longer guarantee that the data is correct, up-to-date and fulfils the quality requirements," says Dehling. "We don't know who can make what out of it."

What's more, the Federal Court of Auditors is demanding market prices from the BSH for the data. "This is nowhere near enough to cover the production costs, which are of course financed by taxes," explains Dehling. A general release of all nautical charts would be a political decision that would have to be taken at federal government level.

S-100: The future of electronic nautical charts

A look into the future: with the new S-100 standard for electronic nautical charts, even more detailed depth data will be possible. Users will then be able to display safety lines based on their individual draught - for example, a 3.5 metre line instead of just 3 or 4 metres. "The data must be good enough for this," emphasises Dehling. This function will initially be available for ships requiring equipment (ECDIS systems); so far, only pilots have been able to use it.

What does this mean for Freenauticalchart users?

The BSH's statements are clear: the free nautical chart Freenauticalchart.net uses data that is expressly not authorised for navigation by the BSH. Neither the depth data nor the positions of navigation marks are reliably up-to-date. The BSH does not guarantee the nautical accuracy of this data.

In practice, this means

• Trip planning, overview, training: Freenauticalchart.net remains a valuable free tool
• Backup navigation in the event of GPS failure: Basic orientation possible
• Safe navigation in critical areas: Not suitable according to BSH
• Replacement for official nautical charts: Explicitly not intended
• News for seafarers: It is essential to continue to observe

The attractive presentation and weekly updates of the open data sources should not hide the fact that the data is intended for other purposes. "The data that may look nice is not necessarily correct," summarises Dehling.

The insidious thing is that the deviations are only noticeable in direct comparison with maps created from licensed data. For example, in the form of stones that have no depth information. The most noticeable discrepancies are in the soundings, which in the Wadden Sea often deviate from the licensed data by more than half a metre.

More on the topic:

• Freenauticalchart: The open source revolution for nautical charts
• Cruise planning: online portal Skippo with free nautical charts for Scandinavia
• Apps for sailing: The 20 best apps for sailors
• Navigation: The future of nautical charts - only plotters instead of paper?
• Classic navigation - this is how it works