A number of new satellite-only Global Gravity Models (GGMs) become progressively available based on the CHAMP and GRACE satellite mission data. These models promise higher (compared to older GGMs) accuracy in the determination of the low and medium harmonics of the Earth's gravity field. In the present study, the latest GGMs generated from CHAMP and GRACE data (namely EIGEN2, EIGEN3p, GGM0IC, GGM0IS and GRACED IS) have been studied with respect ro their accuracy and performance when used in gravity field approximation. A spectral analysis of the new models has been carried out, employing their degree and error-degree variances. In this way, their performance against each other and with respect to EGM96 was assessed, and the parts of the gravity field spectrum that each model describes more accurately have been identified. The results of the analysis led to the development of a combined geopotential model, complete to degree and order 360, whose coefficients were those of CHAMP until degree 5, then GRACE until degree 116, and EGM96 for the rest of the spectrum. Finally, a validation of all models (the combined included) has been performed by comparing their estimates against GPS/levelling data in land areas and TOPEX/Poseidon sea surface heights in marine regions. All rests have taken place over Greece and the eastern part of the Mediterranean Sea. From the results obtained it was concluded that the combined GGM developed provides more accurate results (compared to EGM96), in terms of the differences with the control datasets, at the level of 1-2 cm geoid and 1-2 mGal for gravity (ICT). Furthermore, the absolute geoid accuracy that the combined GGM offers is 12.9 cm (ICT) for 11 = 120, 25 cm for 11 = 200 and 33 cm for n = 360, compared to 29 cm, 36 cm and 42 cm for EGM96, respectively.

Natural disasters have an effect on many important economic and social parameters and are related to a wide spectrum of sciences. Their kind, form, scale, intensity and other characteristics vary in different areas on Earth. One of the most common fields where such disasters are of great importance (either as threats or as resulting damage) is the urban environment (either the buildings or the infrastructure). The complexity of the natural disasters concerning each of the five phases in their lifecycle (prevention, mitigation, preparedness, emergency management, recovery) leads to the selection of strong and most capable tools, in order to predict their results, i.e. the damage. Since nowadays the geospatial technologies have undergone an effective shift to become better suited to the internet, the most appropriate tools for this purpose are the Web-based GIS. In this review paper, a Web-based GIS, which is under development, named SyNaRMa (Information System for Natural Risk Management in the Mediterranean) is being presented. SyNaRMa features include collection and analysis of data related to earthquakes, landslides and forest fires, simulation of natural disaster effects resulting from realistic scenarios and prediction of their impact on the natural and anthropogenic environment of the wider implementation areas. It is to be noted that its open architecture offers many benefits and conveniences for future plans as for example the incorporation within the system of data related to other natural disasters (e.g. drought, desertification, tsunamis, volcanic eruptions etc.) and the potential to be used all over the world.