NLLAT2018
Chart Datum (CD) is the plane of reference to which all charted depths and drying heights are related. In the Netherlands Territorial Sea and Exclusive Economic Zone (EEZ), the Approximate Lowest Astronomical Tide (ALAT) is adopted as CD, to show the least depth of water found in any place under “normal” meteorological conditions.
On land all heights and depths are related to the NAP (land survey datum), which is almost identical to the geoid. This reference surface has been extended to the EEZ by the NEVREF project of the Technical University of Delft. The NLLAT2018 reference surface provides at sea the geoid – ellipsoid separation (1), the LAT – ellipsoid separation (2) and the geoid – LAT separation (3). For the internal waters (Westerschelde, Oosterschelde and Wadden Sea), the Mean Sea Level – LAT separation established in 2006 has been merged. The accuracy for LAT is better than 10cm, the accuracy of the geoid is better than 3 cm (at sea).
As of 1 January 2026, the designation of the reference plane in nautical products of the Dutch part of the North Sea, the Waddenzee, and the Eastern and Western Scheldt is changed. From that moment on Lowest Astronomical Tide (LAT) will be called Approximate Lowest Astronomical Tide (ALAT).
The main reason for this change is to achieve uniformity in the designation of the reference plane with the other North Sea countries. It is emphasized that only the name is changed and that the reference plane itself remains unchanged.
The changes will be gradually implemented in new editions of Hydrographic Service products.
Terms of use
This product is free to download. Use of the product for any purpose is subject to the following conditions:
The data provided may not be used for navigation, because the data sets are not linked and are not updated according to the Notices to Mariners.
The Hydrographic Service reserves the right to exercise its moral rights (Article 6bis, Berne Convention) in respect of the reproduced materials that contain the data provided.