With an increasing demand for accurate and reliable estimates of sea surface heights (SSH) from c... more With an increasing demand for accurate and reliable estimates of sea surface heights (SSH) from coastal and marine applications, approaches based on GNSS positioning have become favored, to bridge the gap between tide gauge (TG) and altimetry measurements in the coastal zone, and to complement offshore altimetry data. This study developed a complete methodology for jointly deriving and validating shipborne GNSS-determined SSH, using a geoid model and realistic dynamic topography estimates. An approach that combines the properties of hydrodynamic models and TG data was developed to obtain the latter. Tide gauge data allow estimating the spatiotemporal bias of a hydrodynamic model and, thus, linking it to the used vertical datums (e.g., a novel geoid-based Baltic Sea Chart Datum 2000). However, TG data may be erroneous and represent different conditions than offshore locations. The qualities of spatiotemporal bias are, hence, used to constrain TG data errors. Furthermore, a rigid syst...
Tide gauge (TG) time series and GNSS measurements have become standard datasets for various scien... more Tide gauge (TG) time series and GNSS measurements have become standard datasets for various scientific and practical applications. However, the TG and geodetic networks in the Baltic Sea region are deforming due to vertical land motion (VLM), the primary cause of which is the glacial isostatic adjustment. Consequently, a correction for VLM, either obtained from a suitable VLM model or by utilizing space-geodetic techniques, must be applied to ensure compatibility of various data sources. It is common to consider the VLM rate relative to an arbitrary reference epoch, but this also yields that the resulting datasets may not be directly comparable. The common height reference, Baltic Sea Chart Datum 2000 (BSCD2000), has been initiated to facilitate the effective use of GNSS methods for accurate navigation and offshore surveying. The BSCD2000 agrees with the current national height realizations of the Baltic Sea countries. As TGs managed by national authorities are rigorously connected ...
Animal physiology and development both rely on biological clocks, but the extent to which feedbac... more Animal physiology and development both rely on biological clocks, but the extent to which feedback loops among core components of the circadian clock and conserved microRNAs operate within developmental timers is not well understood. Here, we show that a negative feedback loop between NHR-23/RORα and let-7 modulates the PER-dependent rhythm of the C. elegans molting cycle. Related quiescent intervals are delayed and protracted in nhr-23 knockdowns, advanced and abbreviated in particular let-7 mutants, and yet scheduled more regularly in double mutants. NHR-23 binds upstream ROR Response Elements (REs) and activates transcription of primary let-7 when larvae are active, whereas let-7 targets an LCS in the 3′UTR and represses expression of nhr-23 transcripts when larvae are quiescent. Moreover, NHR-23 and let-7 have scores of shared targets that are cyclically expressed and mediate related transitions in cell and animal behavior. ROREs are also found upstream of vertebrate let-7 homologs, while LCSs are found in 3′UTRs of ROR transcripts. Conservation of this feedback loop has implications for human clocks and related malignancies and disorders of sleep and metabolism.
ABSTRACT Varbla, S.; Ellmann, A., and Delpeche-Ellmann, N., 2020. Utilizing airborne laser scanni... more ABSTRACT Varbla, S.; Ellmann, A., and Delpeche-Ellmann, N., 2020. Utilizing airborne laser scanning and geoid model for near-coast improvements in sea surface height and marine dynamics. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1339-1343. Coconut Creek (Florida), ISSN 0749-0208. For most coastal engineering applications sea surface height (SSH) is commonly obtained from land bounded tide gauges (TG). Instead, this study now explores a methodology that utilizes airborne laser scanning (ALS) in conjunction with a hydrodynamic model (HDM), TGs and a high-resolution marine geoid model to obtain (i) accurate SSH at coasts; (ii) deeper insight into the marine dynamics that may be present and (iii) identification of deficiencies in the HDM and/or geoid model. The methodology employed a robust processing of ALS data, followed by referencing all sources of data to the geoid and subsequent statistical analysis of the discrepancies. The comparison between HDM and ALS shows maximum discrepancies of 15 cm, which reveals deficiencies in the used HDM. In addition, the ALS data shows promising results for identifying surface waves properties that otherwise would not be captured in such detail.
The Baltic Journal of Road and Bridge Engineering, 2019
Unmanned aerial vehicle photogrammetry is a surveying technique that enables generating point clo... more Unmanned aerial vehicle photogrammetry is a surveying technique that enables generating point clouds, 3D surface models and orthophoto mosaics. These are based on photos captured with a camera placed on an unmanned aerial vehicle. Within the framework of this research, unmanned aerial vehicle photogrammetry surveys were carried out over a sand and gravel embankment with the aim of assessing the vertical accuracy of the derived surface models. Flight altitudes, ground control points and cameras were varied, and the impact of various factors on the results was monitored. In addition, the traditional real-time-kinematic Global Navigation Satellite System surveys were conducted for verifications. Surface models acquired by different methods were used to calculate volumes and compare the results with requirements set by Estonian Road Administration. It was found that with proper measuring techniques an accuracy of 5.7 cm for the heights were achieved.
The first GNSS (Global Navigation Satellite System) CORS (Continuously Operating Reference Statio... more The first GNSS (Global Navigation Satellite System) CORS (Continuously Operating Reference Station) in Estonia became operational in 1996. Starting from 2008 four Estonian CORS are incorporated into the EPN (EUREF Permanent GNSS Network). During the years 2014-2015 modernization of the GNSS CORS in Estonia was carried out. Currently the total number of the resulting ESTPOS reference stations is 28. These ESTPOS stations were interconnected to the I order national geodetic network by a special GNSS campaign in 2017. The maintenance and compilation of the ESTPOS based research products is explained. Aspects of the ESTPOS usage, including also the real time surveys, are discussed. Comparisons with SWEPOS (Swedish national GNSS CORS) and FinnRef (Finnish national GNSS CORS) networks indicate possibilities for further ESTPOS developments.
Deduction of a regularly spaced gravity anomaly grid from scattered survey data is studied, addre... more Deduction of a regularly spaced gravity anomaly grid from scattered survey data is studied, addressing mainly two aspects: reduction of gravity to anomalies and subsequent interpolation by various methods. The problem is illustrated in a heterogeneous study area and contrasting test areas including mountains, low terrain and a marine area.
ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2016
Mobile laser scanning (MLS) is a faster and cost-effective alternative to static laser scanning, ... more Mobile laser scanning (MLS) is a faster and cost-effective alternative to static laser scanning, even though there is a slight trade-off in accuracy. This contribution describes a compact mobile laser scanning system mounted on a vehicle. The technical parameters of the used system components, i.e. a small LIDAR sensor Velodyne VLP-16 and a dual antenna GNSS/INS system Advanced Navigation Spatial Dual, are reviewed, along with the integration of these components for spatial data acquisition. Calculation principles of 3D coordinates from the real-time data of all the involved sensors are discussed. The field tests were carried out in a controlled environment of a parking lot and at different velocities. Experiments were carried out to test the ability of the GNSS/INS system to cope with difficult conditions, e.g. sudden movements due to cornering or swerving. The accuracy of the resulting MLS point cloud is evaluated with respect to high-accuracy static terrestrial laser scanning dat...
International Association of Geodesy Symposia, 2015
For accurate regional gravity field modelling it is vital to have dense and high quality data cov... more 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.
Yeryuvarının gerçek şekli olan jeoidin 1 cm doğrulukla belirlenmesi, günümüz jeodezi topluluğunun... more Yeryuvarının gerçek şekli olan jeoidin 1 cm doğrulukla belirlenmesi, günümüz jeodezi topluluğunun en önemli hedeflerinden biridir. Gravimetrik jeoit belirleme belirtilen hedefe ulaşmak için tercih edilen yöntemlerin başında gelir. Ne var ki; fiziksel yeryüzünde ölçülen gravite değerleri jeoit belirleme sürecine doğrudan dahil edil(e) mez. Yüzey gravite değerlerinin öncelikle gravite anomalilerine indirgenmesi, bu işlem sırasında verilerin toplandığı yeryüzü parçasının topoğrafik niteliklerini yitirmemesi istenir. Öte yandan, düzenli bir grid verisi oluşturmak için enterpolasyonda dayanak verisi olarak kullanılacak gravite anomalilerinin topoğrafyaya bağımlılığı en alt düzeyde olmalıdır. Boşlukta gravite anomalileri jeoit modellemenin temel veri grubunu oluştururken, daha yumuşak bir yeryuvarı şeklini betimleyen Bouguer anomalileri ise gravite verilerinin enterpolasyonuna elverişlidir. Uygulamada basit Bouguer anomalileri yaygın kullanıma sahip olsa da, düzensiz topoğrafik alanların bozucu etkilerini az ya da çok içerdikleri bilinmektedir. Bu çalışmada basit ve tamamlanmış Bouguer anomalileri arasındaki farklılıklar, bir test alanındaki gravite verileri yardımıyla ortaya konulmuştur. Boşlukta gravite anomalileri üzerinden her iki yaklaşım için elde edilen farklar, topoğrafya ile korelasyonlu olarak 16 mGal'e kadar çıkmaktadır. Bu sonuç, Türkiye gibi düzensiz topoğrafyaya sahip bölgelerde, enterpolasyon işlemlerinde tamamlanmış Bouguer anomalilerinin kullanılması gerektiğini işaret etmektedir.
We report on testing the UNB (University of New Brunswick) software suite for accurate regional g... more 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.
The application of topographic corrections to gravity anomalies and disturbances, and their use i... more The application of topographic corrections to gravity anomalies and disturbances, and their use in formulating and solving the gravimetric inverse problem are reinvestigated. The gravity anomaly, whose definition is based on the disturbing potential by means of the fundamental gravimetric equation, rather than by the vertical derivative of the disturbing potential, differs from the gravity disturbance, which also has implications to the application of the topographic correction. We demonstrate that the application of the topographic ...
Abstract. Recently the Estonian gravity datasets have been thoroughly revised by the joint resear... more Abstract. Recently the Estonian gravity datasets have been thoroughly revised by the joint research group. The purpose of this study was on assessing the quality of historic gravity data observed on the bottom of the Gulf of Riga from 1966 to 1967 and on the frozen Lakes Võrtsjärv and Peipsi from 1986 to 1987. For the evaluation of historic gravity datasets, new gravity campaigns were carried out on large ice-covered Lakes Peipsi and Võrtsjärv in winter 2009 and on the ice of sea along the Gulf of Riga coast in winter 2010. Scintrex ...
With an increasing demand for accurate and reliable estimates of sea surface heights (SSH) from c... more With an increasing demand for accurate and reliable estimates of sea surface heights (SSH) from coastal and marine applications, approaches based on GNSS positioning have become favored, to bridge the gap between tide gauge (TG) and altimetry measurements in the coastal zone, and to complement offshore altimetry data. This study developed a complete methodology for jointly deriving and validating shipborne GNSS-determined SSH, using a geoid model and realistic dynamic topography estimates. An approach that combines the properties of hydrodynamic models and TG data was developed to obtain the latter. Tide gauge data allow estimating the spatiotemporal bias of a hydrodynamic model and, thus, linking it to the used vertical datums (e.g., a novel geoid-based Baltic Sea Chart Datum 2000). However, TG data may be erroneous and represent different conditions than offshore locations. The qualities of spatiotemporal bias are, hence, used to constrain TG data errors. Furthermore, a rigid syst...
Tide gauge (TG) time series and GNSS measurements have become standard datasets for various scien... more Tide gauge (TG) time series and GNSS measurements have become standard datasets for various scientific and practical applications. However, the TG and geodetic networks in the Baltic Sea region are deforming due to vertical land motion (VLM), the primary cause of which is the glacial isostatic adjustment. Consequently, a correction for VLM, either obtained from a suitable VLM model or by utilizing space-geodetic techniques, must be applied to ensure compatibility of various data sources. It is common to consider the VLM rate relative to an arbitrary reference epoch, but this also yields that the resulting datasets may not be directly comparable. The common height reference, Baltic Sea Chart Datum 2000 (BSCD2000), has been initiated to facilitate the effective use of GNSS methods for accurate navigation and offshore surveying. The BSCD2000 agrees with the current national height realizations of the Baltic Sea countries. As TGs managed by national authorities are rigorously connected ...
Animal physiology and development both rely on biological clocks, but the extent to which feedbac... more Animal physiology and development both rely on biological clocks, but the extent to which feedback loops among core components of the circadian clock and conserved microRNAs operate within developmental timers is not well understood. Here, we show that a negative feedback loop between NHR-23/RORα and let-7 modulates the PER-dependent rhythm of the C. elegans molting cycle. Related quiescent intervals are delayed and protracted in nhr-23 knockdowns, advanced and abbreviated in particular let-7 mutants, and yet scheduled more regularly in double mutants. NHR-23 binds upstream ROR Response Elements (REs) and activates transcription of primary let-7 when larvae are active, whereas let-7 targets an LCS in the 3′UTR and represses expression of nhr-23 transcripts when larvae are quiescent. Moreover, NHR-23 and let-7 have scores of shared targets that are cyclically expressed and mediate related transitions in cell and animal behavior. ROREs are also found upstream of vertebrate let-7 homologs, while LCSs are found in 3′UTRs of ROR transcripts. Conservation of this feedback loop has implications for human clocks and related malignancies and disorders of sleep and metabolism.
ABSTRACT Varbla, S.; Ellmann, A., and Delpeche-Ellmann, N., 2020. Utilizing airborne laser scanni... more ABSTRACT Varbla, S.; Ellmann, A., and Delpeche-Ellmann, N., 2020. Utilizing airborne laser scanning and geoid model for near-coast improvements in sea surface height and marine dynamics. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1339-1343. Coconut Creek (Florida), ISSN 0749-0208. For most coastal engineering applications sea surface height (SSH) is commonly obtained from land bounded tide gauges (TG). Instead, this study now explores a methodology that utilizes airborne laser scanning (ALS) in conjunction with a hydrodynamic model (HDM), TGs and a high-resolution marine geoid model to obtain (i) accurate SSH at coasts; (ii) deeper insight into the marine dynamics that may be present and (iii) identification of deficiencies in the HDM and/or geoid model. The methodology employed a robust processing of ALS data, followed by referencing all sources of data to the geoid and subsequent statistical analysis of the discrepancies. The comparison between HDM and ALS shows maximum discrepancies of 15 cm, which reveals deficiencies in the used HDM. In addition, the ALS data shows promising results for identifying surface waves properties that otherwise would not be captured in such detail.
The Baltic Journal of Road and Bridge Engineering, 2019
Unmanned aerial vehicle photogrammetry is a surveying technique that enables generating point clo... more Unmanned aerial vehicle photogrammetry is a surveying technique that enables generating point clouds, 3D surface models and orthophoto mosaics. These are based on photos captured with a camera placed on an unmanned aerial vehicle. Within the framework of this research, unmanned aerial vehicle photogrammetry surveys were carried out over a sand and gravel embankment with the aim of assessing the vertical accuracy of the derived surface models. Flight altitudes, ground control points and cameras were varied, and the impact of various factors on the results was monitored. In addition, the traditional real-time-kinematic Global Navigation Satellite System surveys were conducted for verifications. Surface models acquired by different methods were used to calculate volumes and compare the results with requirements set by Estonian Road Administration. It was found that with proper measuring techniques an accuracy of 5.7 cm for the heights were achieved.
The first GNSS (Global Navigation Satellite System) CORS (Continuously Operating Reference Statio... more The first GNSS (Global Navigation Satellite System) CORS (Continuously Operating Reference Station) in Estonia became operational in 1996. Starting from 2008 four Estonian CORS are incorporated into the EPN (EUREF Permanent GNSS Network). During the years 2014-2015 modernization of the GNSS CORS in Estonia was carried out. Currently the total number of the resulting ESTPOS reference stations is 28. These ESTPOS stations were interconnected to the I order national geodetic network by a special GNSS campaign in 2017. The maintenance and compilation of the ESTPOS based research products is explained. Aspects of the ESTPOS usage, including also the real time surveys, are discussed. Comparisons with SWEPOS (Swedish national GNSS CORS) and FinnRef (Finnish national GNSS CORS) networks indicate possibilities for further ESTPOS developments.
Deduction of a regularly spaced gravity anomaly grid from scattered survey data is studied, addre... more Deduction of a regularly spaced gravity anomaly grid from scattered survey data is studied, addressing mainly two aspects: reduction of gravity to anomalies and subsequent interpolation by various methods. The problem is illustrated in a heterogeneous study area and contrasting test areas including mountains, low terrain and a marine area.
ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2016
Mobile laser scanning (MLS) is a faster and cost-effective alternative to static laser scanning, ... more Mobile laser scanning (MLS) is a faster and cost-effective alternative to static laser scanning, even though there is a slight trade-off in accuracy. This contribution describes a compact mobile laser scanning system mounted on a vehicle. The technical parameters of the used system components, i.e. a small LIDAR sensor Velodyne VLP-16 and a dual antenna GNSS/INS system Advanced Navigation Spatial Dual, are reviewed, along with the integration of these components for spatial data acquisition. Calculation principles of 3D coordinates from the real-time data of all the involved sensors are discussed. The field tests were carried out in a controlled environment of a parking lot and at different velocities. Experiments were carried out to test the ability of the GNSS/INS system to cope with difficult conditions, e.g. sudden movements due to cornering or swerving. The accuracy of the resulting MLS point cloud is evaluated with respect to high-accuracy static terrestrial laser scanning dat...
International Association of Geodesy Symposia, 2015
For accurate regional gravity field modelling it is vital to have dense and high quality data cov... more 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.
Yeryuvarının gerçek şekli olan jeoidin 1 cm doğrulukla belirlenmesi, günümüz jeodezi topluluğunun... more Yeryuvarının gerçek şekli olan jeoidin 1 cm doğrulukla belirlenmesi, günümüz jeodezi topluluğunun en önemli hedeflerinden biridir. Gravimetrik jeoit belirleme belirtilen hedefe ulaşmak için tercih edilen yöntemlerin başında gelir. Ne var ki; fiziksel yeryüzünde ölçülen gravite değerleri jeoit belirleme sürecine doğrudan dahil edil(e) mez. Yüzey gravite değerlerinin öncelikle gravite anomalilerine indirgenmesi, bu işlem sırasında verilerin toplandığı yeryüzü parçasının topoğrafik niteliklerini yitirmemesi istenir. Öte yandan, düzenli bir grid verisi oluşturmak için enterpolasyonda dayanak verisi olarak kullanılacak gravite anomalilerinin topoğrafyaya bağımlılığı en alt düzeyde olmalıdır. Boşlukta gravite anomalileri jeoit modellemenin temel veri grubunu oluştururken, daha yumuşak bir yeryuvarı şeklini betimleyen Bouguer anomalileri ise gravite verilerinin enterpolasyonuna elverişlidir. Uygulamada basit Bouguer anomalileri yaygın kullanıma sahip olsa da, düzensiz topoğrafik alanların bozucu etkilerini az ya da çok içerdikleri bilinmektedir. Bu çalışmada basit ve tamamlanmış Bouguer anomalileri arasındaki farklılıklar, bir test alanındaki gravite verileri yardımıyla ortaya konulmuştur. Boşlukta gravite anomalileri üzerinden her iki yaklaşım için elde edilen farklar, topoğrafya ile korelasyonlu olarak 16 mGal'e kadar çıkmaktadır. Bu sonuç, Türkiye gibi düzensiz topoğrafyaya sahip bölgelerde, enterpolasyon işlemlerinde tamamlanmış Bouguer anomalilerinin kullanılması gerektiğini işaret etmektedir.
We report on testing the UNB (University of New Brunswick) software suite for accurate regional g... more 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.
The application of topographic corrections to gravity anomalies and disturbances, and their use i... more The application of topographic corrections to gravity anomalies and disturbances, and their use in formulating and solving the gravimetric inverse problem are reinvestigated. The gravity anomaly, whose definition is based on the disturbing potential by means of the fundamental gravimetric equation, rather than by the vertical derivative of the disturbing potential, differs from the gravity disturbance, which also has implications to the application of the topographic correction. We demonstrate that the application of the topographic ...
Abstract. Recently the Estonian gravity datasets have been thoroughly revised by the joint resear... more Abstract. Recently the Estonian gravity datasets have been thoroughly revised by the joint research group. The purpose of this study was on assessing the quality of historic gravity data observed on the bottom of the Gulf of Riga from 1966 to 1967 and on the frozen Lakes Võrtsjärv and Peipsi from 1986 to 1987. For the evaluation of historic gravity datasets, new gravity campaigns were carried out on large ice-covered Lakes Peipsi and Võrtsjärv in winter 2009 and on the ice of sea along the Gulf of Riga coast in winter 2010. Scintrex ...
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Papers by Artu Ellmann