Knowledge about the surface of the Earth, about vertical movements of the Earth’s crust, and about long-term variations of the sea level is essential for many scientific purposes as well as administrative and engineering applications. The variation of the mean sea level is considered by many scientific institutions to be an indication of global climate change. However, reliable values of long-term sea-level variations can be derived only if high-quality observation data are available. Moreover, besides observations and measurements by sensors, the application of geodetic scientific principles and systems is of crucial importance. The use of geodetic reference frames allows to present the observations of different sensors in uniform regional or even global reference systems and thus combines the different types of information gained by a variety of sensors. In addition, time series of observation data are required that are not affected by systematic effects, so that systematic influences of sensors and systematic geodetic deviations have to be analyzed and compensated. If such systematic impacts remain unnoticed or are ignored, they may be misinterpreted as long-term sea-level variations, while real changes due to hydrological or climate-related causes remain unidentified. This thesis highlights several aspects of sea-surface measurements and the derivation of long-term sea-level variations. These include the underlying geodetic fundamentals and the scientific principles that govern sea level variations. Many studies and publications about long-term sea-level variations rely on observations of sea-level gauges in tidal waters. However, because the main purpose of these tidal gauges is the safety and ease of navigation, their observations do not have the degree of homogeneity that is needed to derive long-term sea-level variations. To achieve such homogeneity of data, it is essential to analyse the geometric relations between the level of the gauge fixture on the ground, the gauge zero point, and the actual vertical land movements or apparent changes in height that are due to modifications in official height reference systems. In order to distinguish vertical land movements from long-term variations of the mean sea-level, selected tidal gauging stations have been equipped with permanently working GNSS systems that make continuous height monitoring possible. This action was combined with the referencing of the absolute gauge-zero points, both to the height-reference system of the national land-survey network and to a global height-reference system. The latter is crucial for transboundary analyses, for the integration of other satellite-based sea-level observations, and the monitoring of the sea-level rise. Another focus of this thesis is on comparisons between tidal-gauge readings and satellite altimetry observations regarding the ITRF2005 data in the German Bight. In the vicinity of the gauges, the results of satellite altimetry are in good agreement with the local gauge readings, provided only instantaneous observations are used in the data processing. It was found that due to certain specific features of the German Bight, the conventional models used to correct meteorological and oceanic tidal effects are neither able to model the actual high-frequency sea-level variations in sufficient accuracy nor to identify and eliminate unsuitable data from the altimetric observations, especially in areas near the coastline. The combination of point-wise but continuous records of tidal gauges with line-by-line satellitealtimetry observations provides the option of a geometric description of the sea surface and its long-term variations. In particular, the use of improved retracking algorithms and high-frequency observations allows to use satellite altimetry also in coastal areas. Evaluations in this study show a very good agreement between tidal-gauge readings and satellite-altimetry observations in coastal areas. Spatial gaps between the satellite observations over the open sea and the coastal gauge data can be reduced, resulting in a better description of the marine system of the North Sea.