INTERNATIONAL HYDROGRAPHIC REVIEW 
MAY 2020 
66 
The MSL-based chart datum is only well-known at the tide gauge location. With increasing 
distance it is hard to predict the actual height difference of the sea level with respect to the tide 
gauge location. The uncertainty of the sea level predictions may reach the order of several 
decimeters and limits the accuracy of the depth information in nautical charts. Updating nautical 
charts and references for different kind of water level information implied large work in the past, 
especially in Sweden and Finland (see Section 4). 
Meanwhile, digital maps, satellite positioning and wireless internet have fundamentally changed 
the way we navigate in daily life. Regarding marine traffic, electronic nautical charts (ENC) 
replace paper charts. GNSS is used in hydrographic surveying not only to define the position in 
the horizontal direction but also in the vertical direction when measuring depths. To get the 
benefits of GNSS, the chart datum has to be well defined and compatible with GNSS positioning. 
Modern hydrography is more than ship navigation and charted depths with respect to tide gauges. 
Spreading from environmental protection to economy, novel applications are typically multidisci 
plinary connecting the fields of hydrography and modern space-borne geodesy with for instance 
geophysics and telemetry. This is stimulated by increasing use of the sensitive coastal zones 
(e.g., offshore energy, safe vessel navigation, etc.). Another most relevant example is, of course, 
the monitoring of (global and regional) sea level changes by combining classical and modern 
geodetic space techniques (i.e., water level stations, geometric levelling, satellite altimetry and 
GNSS). Finally, GNSS based 3-D navigation becomes more common in commercial shipping and 
accuracy requirements of the depth information will increase. Future marine traffic will potentially 
see autonomous vessels which will require remote GNSS-based surveillance of the under keel 
clearance (UKC). 
All these applications rely on observations on or of the sea surface by modern space-geodetic 
techniques. Thanks to GNSS and satellite radar altimetry, the height and the changes of both sea 
surface and land surface can nowadays be observed in a global Earth-fixed three-dimensional 
coordinate system with high spatial resolution. Heights obtained from these 3-D coordinates are 
related to a mathematical defined global mean earth ellipsoid, which can differ as much as 100m 
from mean sea level. Therefore, the ellipsoidal GNSS heights are not directly suitable for the most 
practical applications and have to be transformed to a meaningful physical height reference 
surface (HRS). This enables to realize a datum for nautical charts based on a geodetic height 
reference surface (see Section 3). 
For practical applications on land, the use of GNSS-based height determination together with a 
compatible model of the HRS (called geoid in geodesy) have become standard and have 
replaced classic surveying techniques for height determination (mainly geometric levelling) in 
many geodetic applications. The European spatial and vertical reference systems 
=s> European Terrestrial Reference System 1989 (ETRS89) and 
=s> the European Vertical Reference System 2000 (EVRS2000) 
are well developed and widely distributed. They are part of the INSPIRE directive of the European 
Commission and are supported by the member states. The respective national geodetic infra 
structures have either adopted these reference frames or are in very close agreement to it (see 
Table 1 and Figures. 2a-e). The same reference systems and technologies are already in 
use out at sea to reference modernized hydrographic surveys.