Artu Ellmann

A precise gravimetric geoid model in a mountainous area with scarce gravity data : a case study in central Turkey

In mountainous regions with scarce gravity data, gravimetric geoid determination is a difficult task that needs special attention to obtain reliable results satisfying the demands, e.g., of engineering applications. The present study investigates a procedure for combining a suitable global geopotential model and available terrestrial data in order to obtain a precise regional geoid model for Konya Closed Basin (KCB). The KCB is located in the central part of Turkey, where a very limited amount of terrestrial gravity data is available. Various data sources, such as the Turkish digital elevation model with 3 ″ × 3″ resolution, a recently published satellite-only global geopotential model from the Gravity Recovery and Climate Experiment satellite (GRACE) and the ground gravity observations, are combined in the least-squares sense by the modified Stokes’ formula. The new gravimetric geoid model is compared with Global Positioning System (GPS)/levelling at the control points, resulting in the Root Mean Square Error (RMS) differences of ±6.4 cm and 1.7 ppm in the absolute and relative senses, respectively. This regional geoid model appears to be more accurate than the Earth Gravitational Model 2008, which is the best global model over the target area, with the RMS differences of ±8.6 cm and 1.8 ppm in the absolute and relative senses, respectively. These results show that the accuracy of a regional gravimetric model can be augmented by the combination of a global geopotential model and local terrestrial data in mountainous areas even though the quality and resolution of the primary terrestrial data are not satisfactory to the geoid modelling procedure.

Determining sea surface heights using small footprint airborne laser scanning

Small footprint airborne laser scanning (ALS) is widely used to collect topographic data over large areas. ALS point clouds provide high resolution datasets for variety of scientific and engineering applications, e.g. geomorphology, geodynamics and forestry. ALS can also be used for monitoring coastal processes. For many marine applications, however, the sea surface heights (SSH) are often requested. Satellite altimetry (SA) has been used to monitor SSH globally. But in regional scale, especially in the coastal areas and enclosed water bodies, the usability of SA is limited due to poor accuracy. Alternatively, our experiments have demonstrated that the water surface in the nadir range can be registered using small footprint ALS. Therefore, a special case study was carried out to analyze SSH determination from ALS measurements. Three profile-wise ALS measurements were carried out in the eastern shores of the Baltic Sea. Along flight trajectories 100 m wide corridors of ALS points were formed. Shorter wavelength signals, like sea wave oscillation, were removed by a low-pass averaging filter. The achieved SSH were verified against a high resolution regional geoid model and also with high-frequency tide gauge observations. Comparisons revealed that the ALS-based sea level-corrected SSH agree with the regional geoid model with standard deviation as of ±1…±2 cm. Thus, small footprint ALS measurements could be applied to determine SSH in regions where SA has limited quality, e.g. in coastal areas and enclosed water bodies.

Precise Hydrodynamic Leveling by Using Pressure Gauges

This study investigates the applicability of hydrodynamic leveling by means of contemporary pressure gauges for achieving geodetic accuracy in height determination. The main problems associated with pressure gauges are rigorous connection to a national leveling network and data processing, for example, determination of time-dependent drift and data filtering principles. The equipment and methodology were tested in a test area in the Baltic Sea. It can be concluded that the year-long sea level series may provide ±2.0 cm accuracy for hydrodynamic leveling within the water stretches up to 65 km. This is confirmed by alternative height determination methods and additional field experiments.

Evaluation of a GRACE‐based combined geopotential model over the Baltic countries

Abstract Nowadays, many geodetic and engineering applications require that the two essential components of the vertical positioning – the height and the corresponding reference surface (geoid), are determined precisely. The recent advancements of satellite technology have resolved the long‐wavelength component of the global geoid with an accuracy of a few cm. The tracking data of the GRACE twin‐satellites are the basis for the new combined geopotential model EIGEN‐GL04c. This contribution assesses the quality of this model through comparisons with an earlier geopotential model (EGM96) of the Baltic countries. The method of spherical harmonic expansions is used in numerical investigations. The results of evaluation revealed significant discrepancies between the long wavelength contributions of the models, which may reach several decimetres in terms of the geoidal heights. There are also some notable improvements of numerical statistics (assessed by the GPS‐levelling data) in the target area when utilizing ...

Geoid profiles in the Baltic Sea determined using GPS and sea level surface

Abstract The idea was to compare the geoid of sea areas by an independent method, like GPS levelling, on the mainland. On the earth surface we can compare the gravimetric geoid with GPS levelling to get an accuracy estimation and tilt information. On the sea we can do it by the GPS methodology and eliminating the current water tilt corrections and the sea surface topography effect. A modern GPS device on board a ferry can store data every second and determine heights with an accuracy of a few centimetres (using the kinematic method with the postprocessing of data obtained from several base stations close to the ferry line). As a result, it is possible to observe the current water levels relative profile in reference to the ellipsoid. Some areas close to Estonia, such as the eastern part of the Gulf of Finland, are not completely covered by gravity measurements. The Baltic Sea has been measured using airborne gravimetry with the accuracy of about 2 mGal. Therefore, the gravimetric geoid is not fully relia...

Improving and Validating Gravity Data Over Ice-Covered Marine Areas

For accurate regional gravity field modelling it is vital to have dense and high quality data coverage. Ice gravimetry is a viable alternative to ship- and airborne gravimetry to help fill gaps over marine areas. A number of factors affect the accuracy of gravimetry on ice, thus special survey and data processing methods are needed. Nevertheless with appropriate methods an accuracy of ±0.16 mGal was achieved on coastal ice. An efficient method for positioning of survey points is RTK GNSS which takes no more than a few minutes on each point and the accuracy achieved is at least ±0.15 cm, while 10 min static surveys also yield acceptable results.This study reports ice gravity surveys proceeded on shore-fast ice in the Vainameri Basin, Estonia. Acquired gravity data agree with existing airborne data while covering a larger area. As a result of the survey it was possible to confirm and specify the extents of an area of positive anomalies. An effort to determine the geoid heights over Vainameri Basin directly via using the GNSS data gathered during gravity surveys on ice was made. For now it proved to be less reliable than classical geoid determination from gravity data.

Performance analysis of freeware filtering algorithms for determining ground surface from airborne laser scanning data

Abstract Numerous filtering algorithms have been developed in order to distinguish the ground surface from nonground points acquired by airborne laser scanning. These algorithms automatically attempt to determine the ground points using various features such as predefined parameters and statistical analysis. Their efficiency also depends on landscape characteristics. The aim of this contribution is to test the performance of six common filtering algorithms embedded in three freeware programs. The algorithms’ adaptive TIN, elevation threshold with expand window, maximum local slope, progressive morphology, multiscale curvature, and linear prediction were tested on four relatively large (4 to 8     km 2 ) and diverse landscape areas, which included steep sloped hills, urban areas, ridge-like eskers, and a river valley. The results show that in diverse test areas each algorithm yields various commission and omission errors. It appears that adaptive TIN is suitable in urban areas while the multiscale curvature algorithm is best suited in wooded areas. The multiscale curvature algorithm yielded the overall best results with average root-mean-square error values of 0.35 m.

Testing Stokes-Helmert geoid model computation on a synthetic gravity field: experiences and shortcomings

We report on testing the UNB (University of New Brunswick) software suite for accurate regional geoid model determination by use of Stokes-Helmert’s method against an Australian Synthetic Field (ASF) as “ground truth”. This testing has taken several years and has led to discoveries of several significant errors (larger than 5mm in the resulting geoid models) both in the UNB software as well as the ASF. It was our hope that, after correcting the errors in UNB software, we would be able to come up with some definite numbers as far as the achievable accuracy for a geoid model computed by the UNB software. Unfortunately, it turned out that the ASF contained errors, some of as yet unknown origin, that will have to be removed before that ultimate goal can be reached. Regardless, the testing has taught us some valuable lessons, which we describe in this paper. As matters stand now, it seems that given errorless gravity data on 1′ by 1′ grid, a digital elevation model of a reasonable accuracy and no topographical density variations, the Stokes-Helmert approach as realised in the UNB software suite is capable of delivering an accuracy of the geoid model of no constant bias, standard deviation of about 25 mm and a maximum range of about 200 mm. We note that the UNB software suite does not use any corrective measures, such as biases and tilts or surface fitting, so the resulting errors reflect only the errors in modelling the geoid.

Terrestrial laser scanning for the monitoring of bridge load tests – two case studies

Terrestrial laser scanning (TLS) technology has various applications due to its capability of acquiring detailed 3D information about objects within a limited time-period. Yet, it is not widely used for the deformation monitoring of structures. To investigate the suitability of TLS for such tasks, a time-of-flight type Leica ScanStation C10 was used to determine the vertical deformations for two bridge load tests in Estonia, namely for the Loobu highway bridge and the Tartu railway overpass. The TLS results were verified with precise levelling, reflectorless tacheometry and dial gauges. Generally, deformation estimates obtained from TLS and other measurement techniques show sub-millimetre agreements (in terms of standard deviations). The maximum differences between the TLS and precise levelling results were 3.4 and 0.8 mm for the Loobu and the Tartu study, respectively. Since TLS has not yet reached the same accuracy as conventional geodetic high-precision techniques, it cannot fully replace them in high accuracy applications. However, TLS can be considered a complementary survey method for load tests as it provides valuable entire surface covering 3D information.

Combining airborne and terrestrial laser scanning to monitor coastal processes

This study explores the potential of joint use of terrestrial (TLS) and airborne laser scanning (ALS) to quantify rapid and spatially inhomogeneous changes to the subaerial beach and to characterize the intensity of coastal processes. This remote sensing technology that uses scanning laser pulses for acquiring high-resolution three-dimensional surface of the measured object is applied to beach segment of the Pirita Beach (Tallinn Bay, the Baltic Sea). The extent and distribution of erosion and accumulation spots are analyzed by means of creating and comparing two digital terrain models of these areas from scanning point clouds obtained in different seasons. After elimination of systematic errors the ALS/TLS combination yields sub-decimeter accuracy for height determination of the beach. The analysis reveals not only the corresponding volume changes in the study area but also several features of internal dynamics of the beach across and along the waterline that are overlooked by classical monitoring methods. The benefits and shortcomings of combining the two laser scanning methods for monitoring coastal processes and the accuracy of the results are also discussed.