Harli Jürgenson

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.

Modelling the Influence of Terraced Landforms to the Earth’s Gravity Field

Medium resolution (1–3 arc-min) gravity anomaly grids do not reflect reality very accurately over terraced landforms, which in turn may affect the uncertainty of subsequent geoid modelling. This inaccuracy is due to many factors. The gravimetric datasets used in the gridding of gravity field models have a varying accuracy and coverage, especially in terraced and coastal areas. Further, the resolution of the terrain model used in the modelling of anomaly grids is usually too low to capture the complete gravimetric attraction of terraced landforms.

Determination and evaluation of the Estonian fitted geoid model EST-GEOID 2003

Abstract This paper focuses on issues related to the calculation of a high-precision fitted geoid model on Estonian territory. Model Est-Geoid2003 have been used in Estonia several years in geodesy and other applications. New data from precise levelling, new global models and terrestrial gravity data give plenty of possibilities for updates and accuracy evaluation. The model is based on a gravimetric geoid. From the gravimetric data gathered, a gravimetric geoid for Estonia was calculated as an approximately 3-km net using the FFT method. After including the new gravimetric data gathered, the gravimetric geoid no longer had any significant tilt relative to the height anomalies derived from GPS-levelling points. The standard deviation between the points was 2.7 cm. The surface of the calculated gravimetric geoid was fitted by high-precision GPS-levelling points. As a result, a height transformation model was determined to reflect the differences between the normal heights of BK77 and the ellipsoidal height...

Detecting the Baltic Sea Level Surface with GPS-Measurements and Comparing it with the Local Geoid Model

Gravimetric geoid NKG04 (Forsberg et al., NKG Geoid Meeting, Copenhagen) is derived by KMS using all available gravimetric data from the region. Some areas, also close to the Estonia are not completely covered by gravity measurements, example the eastern part of the Gulf of Finland. Baltic Sea is measured by airborn gravimetry with accuracy probably 2 mGal. Our idea was to compare the geoid on the sea areas against the independent method like GPS-levelling on the mainland. Main problem have been of course how to remove water tilt during the campaign