V okviru projekta Vzpostavljanje evropskega prostorskega referenčnega sistema v Sloveniji je bila izračunana tudi ploskev testnega geoida. V prispevku bo predstavljena primerjava določitve geoidnih višin iz geoida 2000 in testnega geoida iz leta 2010. Analizo natančnosti ploskev geoida Slovenije smo opravili s primerjavo geoidnih višin, ki jih dobimo kot razlike merjenih eliposoidnih in nadmorskih višin (»merjene geoidne višine«) in tistih, ki jih interpoliramo iz modelov. Primerjali smo »merjene« in interpolirane geoidne višine na 352 GNSS/nivelman kontrolnih točkah. Z izračunom ploskve testnega geoida smo pridobili možnost predhodne analize kakovosti prihodnjega geoida, ki je zelo pomembna za izvajanje GNSS-višinomerstva v geodetski praksi ; This paper presents a quality analysis and comparison of two height reference surfaces. The first is the actual geoid model from the year 2000, and the second is the test geoid model determined in the frame of the project Establishment of the European Reference System in Slovenia. Quality analysis is based on the comparison of geoid heights determined from measured ellipsoidal and mean-sea-level heights and geoid heights interpolated from the model. A comparison was made on 352 GNSS/levelling points.

This paper presents a quality analysis and comparison of two height reference surfaces. The first is the actual geoid model from the year 2000, and the second is the test geoid model determined in the frame of the project Establishment of the European Reference System in Slovenia. Quality analysis is based on the comparison of geoid heights determined from measured ellipsoidal and mean-sea-level heights and geoid heights interpolated from the model. A comparison was made on 352 GNSS/levelling points.

A new gravimetric geoid (OCTAS07v2) is generated using Stokes’ formula with gravity data as input. As local gravity data, a combination of land gravity data, new and old airborne gravity data, and adjusted marine gravity data has been used. All marine gravity data has been error screened and quality assured by removing dubious data and adjusting the data when necessary. Voids in the gravity data distribution were patched with gravity data from satellite altimetry. The OCTAS07v2 geoid was estimated using the remove-compute-restore technique. The long-wavelength signal of the local gravity data was reduced using a Wong-Gore modified Stokes’ function. The long-wavelength part was represented by a global gravity field model based on GRACE data. The OCTAS07v2 geoid model was combined with the OCTAS07_MSS model to create a synthetic Mean Dynamic Topography (MDT) model. In comparison with the OCCAM MDT, our new synthetic MDT model gave a std. dev. of the residuals of 11 cm. A comparison to the main northern North Atlantic currents show many similar features

The OCTAS project, Ocean Circulation and Transport Between North Atlantic and the Arctic Sea, funded by the Norwegian Research Council, is a multidisci- plinary project combining geodesy, satellite altimetry and oceanography. The main objective is to enhance the Norwegian capacity in Earth observation technolo- gies through determining the ocean circulation and trans- port by using satellite techniques in combination with geodesy. The primary study area is the Fram Strait be- tween Svalbard and Greenland. A vital objective is the determination of a high precision gravimetric geoid for the OCTAS study area. This re- quires an error free high quality gravimetric dataset. The process of establishing such a data set by adjusting older marine data through comparison with modern airborne and marine gravity data sets is described. Combining this updated gravity data set with data from the CHAMP and GRACE satellites an OCTAS geoid has been computed. The updated gravity field and the derived geoid may be used in validating the GOCE products. The challenges and efforts undertaken in deriving a high precision mean sea surface in a region with an abun- dance of sea ice and limited number of altimetric satel- lites is described. The derived geoid and mean sea sur- face is combined to form the mean dynamic topography, MDT. These MDT's are assessed by intercomparing with oceanographically derived MDT models. The status and an overview of the project is given includ- ing identification of challenges that must be addressed in order to achieve the project objectives.

Initially, existing mean dynamic topography (MDT) models were collected and reviewed. The models were corrected for the differences in averaging period using the annual anomalies computed from satellite altimetry. Then a composite MDT was derived as the mean value in each grid node together with a standard deviation to represent its error. A new synthetic MDT was obtained from the new mean sea surface (MSS) KMS04 combined with a regional geoid updated using GRACE gravity and gravimetric data from a recent airborne survey. Compared with the composite MDT the synthetic MDT showed very similar results.Then combination methods were tested for the computation of MDT models from gravity data and MSS data. Both a rigorous and an iterative combination method have been tested in the GOCINA region. At this stage, the iterative combination method with its efficient handling of large data sets covering the whole region appears to give the best solution. Naturally, the errors associated with the MDT can be obtained using the rigorous method only.

GPS-levelling points are widely used to control gravimetric geoid or quasigeoid models. Direct comparison is often interpreted to reveal the accuracy of the gravimetric model, using GPS-levelling as a reference. However, both GPS and levelled heights contain errors, and in order to achieve a centimeter-accuracy geoid, these should be investigated. The Norwegian Height System NN1954 is known to contain large systematic errors due to postglacial land uplift in the area. In this study, the current height system and two revised versions, corrected for uplift, are applied to compute three sets of control quasigeoid heights in the southern part of Norway. These heights are then compared to various Nordic gravimetric quasigeoid models generated during the last two decades. In contradiction to some earlier studies, the accuracy of gravimetric quasigeoid models for this area are found to improve near-linearly with time. This is in accordance with expectations, since both data coverage and computation methods have progressed during this time. However, this study shows the importance of establishing accurate and error-free control data for geoid comparisons.

While variations in the sea surface height and thus in the ocean currents can be derived directly from satellite altimeter data, an assessment of the absolute value of the ocean dynamic topography (and hence the absolute surface circulation) requires that the elevation of a hypothetical ocean at rest, i.e., the geoid , be subtracted from the altimetric mean sea surface height. The typical elevation scale of the dynamic topography is of the order of 0.1 to 1 m. The dilemma is that the precision of present geoid models is of the same size on the scale of many ocean-circulation features. Hence, the calculation of the mean dynamic topography using mean sea surface height and geoid information can not be satisfactory performed for wavelengths less than about 1000 km. In that spatial domain, the geoid model error becomes equal to or larger than the dynamic topography signals. The application of imprecise geoid models to the determination of dynamic topography at shorter spatial-scales can consequently result in the computation of false topographic signals (< 1m) which, in turn, correspond to erroneous transport calculations of several Sv (where 1 Sv = 10 m/s). Transport uncertainties of this magnitude are of significance in climate studies. In this context the Gravity and Ocean Circulation Explorer (GOCE) mission planned for launch in early 2007 is therefore considered to be of significant importance. 1) Also at The University of Bergen, Geophysical Institute, Bergen, Norway

Vertical reference systems based on national precise levelling networks have been realised in Europe since the 1860’s. The heights are related to gravity, which by convention may be measured values or amodel. The Norwegian Height System was adjusted in 1956 when the southern part of the country had been levelled once. Because of the lack of measured gravity, Clairaut’s formula for gravity was used in the orthometric correction. The intention was to establish an orthometric height system, but this was never tested or confirmed. In the literature the system is often referred to as orthometric, but some times as normal orthometric. This study shows that the Norwegian Height System was realised by a strongly deformed net due to lack of land uplift corrections. The derived heights are neither normal nor orthometric. When the land uplift is handled in a proper way, the heights are close to normal heights. The remaining small differences are shown to depend on the free air anomaly.

Varicella is a common viral infection in childhood, and acute osteomyelitis is one of the rare but serious complications. We report two cases of osteomyelitis as a complication of varicella. The possibilities and limitations of the different imaging modalities are discussed, as well as imaging findings during the course of this condition.

In July 1998 an airborne gravity survey was carried out in the Svalbard region as a joint effort of Norwegian and Danish organisations. The primary purpose was to improve the gravity field description of the archipe lago. A secondary goal and the topic of this paper, was to test the usefulness of the airbome gravity system in exploration geophysics. This was achieved by flying along a seismic and marine gravity line in the Barents Sea.

An updated high-resolution geoid model for the Scandinavian and Baltic region has been done by spherical FFT approaches, taking topography into account by a hybrid RTM/Helmert condensation approach. Significant new data sources have entered the geoid solution: a new spherical harmonic reference model, new gravity data over many regions, including the Baltic republics and the Russian border regions, and new high-resolution topographic sources. ERS-1 geodetic mission satellite altimetry has been included in the Baltic, using Fourier-based draping techniques, and taking sea surface topography from tide gauge levellings into account. Compared to more than 300 GPS-levelling points across the region the geoid shows a fit at the 10 cm level, thus being a major improvement over earlier models. In local regions with good gravity coverage, such as Denmark, the geoid fitted to the national GPS net yields accuracies at the 1 cm-level, thus allowing accurate operational levelling by GPS.