Aive Liibusk

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.

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...

Sea-Level Change and Flood Risks at Estonian Coastal Zone

This paper reviews Estonian relative sea level, land uplift and coastal floods data and provides sea-level scenarios and risk assessment of coastal flooding in urban areas for the twenty-first century. Considering the present post-glacial land uplift rates of Estonian coastal areas and the global ocean level rise projections, the long-existing trend of relative sea-level lowering may very probably be replaced by a relative sea-level rising trend during the twenty-first century. By the end of the twenty-first century we project the relative sea level to be c. 20 to 40 cm or c. 40 to 60 cm higher in the case of the International Panel for Climate Change Representative Concentration Pathways (RCP) 4.5 or RCP 8.5 scenario, respectively. The sea-level rise together with the increased storm frequency and decreased winter ice cover period will very probably increase the extent of floods during the twenty-first century. A significant coastal flooding risk affects four cities, Parnu, Kuressaare, Haapsalu and Tallinn and eight smaller towns. The largest coastal flooding in Estonia is recorded in Parnu, with the highest sea level 275 cm in 2005. Calculations show that due to the impact of predicted climate change and in the case of certain weather conditions, coastal floods in Parnu may affect areas up to 400 cm above the present sea level by the end of the twenty-first century. The scenarious of future flood limits are needed for sustainable planning of the coastal zone and for development of rescue strategies.There are already several land use and urban planning instruments and laws for climate adaptation, such as environmental impact assessment, risk assessment and restriction zones for construction in certain buffer and flood areas. Flooding risk measures consist of risk mapping and a national emergency plan. However, further integration of climate issues into existing laws, strategies and land use plans is essential to have a targeted approach in reducing the vulnerability of populated areas and strengthening the adaptive capacity of the urban system against climate change.

Validation of Marine Geoid Models by Profile-wise GNSS Measurements on Ice Surface

Ship-board global navigation satellite system (GNSS) measurements are widely used to determine sea surface heights, marine geoid validation, and/or satellite altimetry calibration. However, the use of a vessel could be complicated near coastal areas due to shallow water. Therefore, in the area of sea ice formation, GNSS measurements on the ice surface could be a viable alternative to vessel-borne surveys. Importantly, the ice-covered water is not affected by short-term winds, which otherwise could have systematic influence on the instantaneous sea surface topography. This study tackles methodology and validation of marine geoid models by profile-wise GNSS measurements on ice in an archipelago of the Baltic Sea. The GNSS measurements were carried out on the three ice roads with total length 48 kilometers. The along-route standard deviation between the gravimetric geoid model and profile-wise GNSS heights remained within ±2.1 centimeters.

Transfer of heights to islands in West-Estonian Archipelago using hydrodynamic levelling

During the last decade sea level measurements by means of pressure sensors have become wide-spread, since they are compact, can withstand icy conditions, are easy to integrate with data loggers and communication systems. This study uses two years (2010-2011) long sea level series collected by pressure gauges for hydrodynamic levelling. As a result, the accuracy of hydrodynamic levelling was estimated to be ±1.5 cm in West-Estonian Archipelago. Therefore the height differences calculated from sea level series of contemporary pressure gauges were used as reference for historic staff gauge readings with data sampling 12h and for some days sea level observations in calm weather and ice covered sea conditions. The comparisons indicated that the height transfer accuracy ±1.5 cm with different observation periods and data samplings is guaranteed only up to 20 km distances. Over longer distances the frequent data sampling and at least one year sea level observations must be carried out for geodetic applications.

Exploring sea surface heights by using Airborne Laser Scanning

Resolution of satellite altimetry derived sea surface heights (SSH) is relatively low, whereas near coastal areas the data have poor accuracy. For monitoring SSH regionally, Airborne Laser Scanning (ALS) in conjunction with kinematic GPS-positioning can be a more accurate and high-resolution alternative. A case study was carried out at the southern shores of Gulf of Finland, the Baltic Sea. An ALS profile flown at an altitude of ~400 m was used for SSH determination. Two different algorithms for ALS trajectory calculations were compared and clear systematic discrepancies between the two were determined. Also, significant variations in backscattering of ALS pulses yield large data gaps even with near ideal flight conditions. The causes for this phenomena were analyzed and a possible explanation given. The accuracy of the sea level corrected SSH results was validated with a regional geoid and sea surface models. The ALS derived SSH values agree within 2 cm (in terms of standard deviation) with the geoid model.

Correcting tide gauge series due to land uplift and differences between national height systems of the baltic sea countries

Different types of tide gauges (TG) are used to monitor sea level dynamics around the Baltic Sea. They are usually connected to national levelling network and several of them are linked into regional networks (e.g. Baltic Operational Oceanographic System). The sea level readings are used for nautical navigation, modelling and forecasting of sea level changes. Long-term and historical sea level series are also useful in studying regional land uplift/subsidence or calibrating satellite altimetry data. Both tasks require precise knowledge of interconnections between height systems of countries surrounding the same sea. Presently, however, six different height reference systems are in official use in the Baltic Sea countries. Even though all these systems are based on mean sea level (MSL) observations averaged over different time-periods, but different reference TG and tidal systems have been adopted for national height systems. The differences of national height systems between the countries around the Baltic Sea can reach up to 20 cm. Overlooking this yields undesirable systematic biases between regional TG data. Additionally, the entire Fennoscandia is affected by apparent land uplift at the velocity rate up to +9 mm/year, primarily due to the viscoelastic response of the solid Earth resulting from the de-glaciation of the Pleistocene ice-sheets. Over a time span this causes notable distortions of height system realisations even within a country. Therefore, the land uplift corrections should be also taken into account in sea level series, which are used for modelling and forecasting of sea level changes. A case study in West-Estonian Archipelago involves a recently developed land uplift model EST2013LU, which is based on four repeated high-precision levelling data from 1933 to 2010. Also connections between levelling network and TG series are analysed. The results reveal that discrepancies due to obsoleteness of the heights in the national height system may cause discrepancies in series of nearby located tide gauges up to 7 cm.

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