Dimitrios Rossikopoulos

Local Geoid Computation and Evaluation

Abstract A local geoid solution for the northern part of Greece is presented based on a recent processing of newly available gravity data in the area 40.25 ≤ /o ≤ 41.00, 22.5 ≤λ ≤ 24.25. The derived gravimetric geoid heights are compared with geoid heights computed at recently measured GPS/ leveling benchmarks. A 4-parameter transformation model is applied to the differences between the two aforementioned geoid height sets, and a discussion is given on the current state of the leveling datum in the test area and the Greek territory. Regional and local transformation parameters are computed and some numerical tests are performed. A common adjustment of gravimetric geoid heights and corresponding GPS/leveling heights will be carried out in another study following an integrated procedure in order to study problems arising from the combination of different height data sets for geoid determination. Finally, some conclusions are drawn on the problems related to the optimization of a local geoid solution.

The Realization of a Semi-Kinematic Datum in Greece Including a New Velocity Model

The current geodetic network in Greece, which is realised by both the official Hellenic Geodetic Reference System of 1987 (HGRS87) and the GPS permanent network of the contemporary static Hellenic Terrestrial Reference System of 2007 (HTRS07), experience significant inhomogeneous ground displacements. As time passes, the distortion of both networks results in increasing degradation of positioning accuracy and datum stability. For these reasons the velocity field of the Earth’s crust in Greece has to be rigorously estimated and taken into account.

Combination Schemes for Local Orthometric Height Determination from GPS Measurements and Gravity Data

One of the most interesting and challenging tasks in the field of geodetic surveying is the accurate determination of orthometric heights from GPS measurements taking into account leveling data and additional gravity field information. This paper focuses on the presentation of the currently available various solution strategies which are then properly applied. The first method is based on the integrated geodetic model, where gravity field parameters are treated as signals. A second solution is based on a combination scheme employing least squares collocation as the optimal heterogeneous combination method for gravity and height data. Another method is the spectral domain equivalent of least squares collocation, namely the Multiple Input Multiple Output System Theory, where gravity and height data are treated as stochastic signals with full variance covariance information. The last method consists in a polynomial interpolation model of various orders expressing different geoid representations.

Local Vertical Crustal Movements in the Mygdonian Basin - North Greece, Resulting from Gravity and GPS Measurements

Abstract Repeated gravity measurements in the tectonically active Mygdonian basin, close to Thessaloniki - North Greece, showed gravity changes ranging from 420 to 810 nm s−2, within a period of 20 years. These gravity changes, which were interpreted as local vertical crustal movements, using the free-air relation, correspond to changes from -26 to 13 cm. The rate of the gravity (and correspondingly height) change is higher in the rebound period, after the 1978 seismic sequence near Thessaloniki. The rate of the vertical changes estimated from the gravimetric observations was compared with corresponding rate estimated from GPS measurements. Conclusions were drawn, concerning the fault system of the region.

A methodology for the performance evaluation of low-cost accelerometer and magnetometer sensors in geomatics applications

Abstract This paper presents a methodology and its software implementation for the performance evaluation of low-cost accelerometer and magnetometer sensors for use in geomatics applications. A known mathematical calibration model has been adopted. The method was completed with statistical methodologies for adjusting observations and has been extended to calculate accuracies for the attitude, heading, and tilt angles estimation that are of interest to geomatics applications. The evaluation method consists of two stages. First, the evaluation method reviews the total magnitude of acceleration or the strength of the magnetic field. Second, the evaluation is more detailed and concerns the determination of mathematical parameters that describe both accelerometer and magnetometer working model. A software tool that implements the evaluation model has been developed and is applied both in accelerometer and magnetometer measurement data-sets acquired from a low-cost sensor system.

A New Velocity Field of Greece Based on Seven Years (2008–2014) Continuously Operating GPS Station Data

Greece is characterized by complex and intense geodynamics, because it is located between the collision boundaries of two tectonic plates (Africa-Nubia and Eurasia), with major active tectonic features such as the Hellenic Arc, the Anatolian fault in North Aegean Trough and the Kefalonia fault in the Ionian Sea. GPS is a well-established tool for geophysical research purposes, because it is able to provide continuous measurements for monitoring displacements of the Earth’s crust. The aim of the present study is to create a modern and improved geodetic velocity field for Greece using GPS observations from continuously operating reference stations. The new set of geodetic velocities is derived from the processing of 7 years (2008–2014) of daily GPS data, using 155 stations distributed in the broader Greek territory and 30 IGS-EPN GPS stations. The GAMIT/GLOBK software package was used to process the GPS measurements. The results are expressed in the ITRF2008 reference frame. The analysis showed that the northern region of Greece is the most stable and has identical movement with the Eurasian plate in contrast with the region of the southern part and the Aegean Sea. According to the results, the estimated horizontal geodetic velocities show completely different pattern between northern and southern Greece with significant differences both in magnitude and direction. The derived site values were used for a velocity grid creation in order to predict velocities within the Greek area and to enforce proper realization of GNSS reference systems in Greece.

Assessing a new velocity field in Greece towards a new semi-kinematic datum

Greece has always been an interesting study area for the geodesists due to its intense geodynamic behaviour. Especially, the exploitation of a modern geodetic velocity field using GNSS becomes a crucial requirement for geodetic purposes. The main aim of the present study is the assessment of a modern geodetic velocity model. This procedure is based on the unification of various reference frames, implemented from individual extended GNSS campaigns in different epochs with the effective involvement of the Hellenic Military Geographic Service. The implementation of geodetic velocities in the inhomogeneous Greek velocity field improves significantly the reliability of transformations, more than 60% with respect to the horizontal component, applying a vice versa time shift to a common epoch. We also emphasise the necessity to adopt a velocity model in order to process regional GNSS networks and therefore to establish a modern semi-kinematic reference frame in Greece.

A Fusion Method for Combining Low-Cost IMU/Magnetometer Outputs for Use in Applications on Mobile Devices

This paper presents a fusion method for combining outputs acquired by low-cost inertial measurement units and electronic magnetic compasses. Specifically, measurements of inertial accelerometer and gyroscope sensors are combined with no-inertial magnetometer sensor measurements to provide the optimal three-dimensional (3D) orientation of the sensors’ axis systems in real time. The method combines Euler–Cardan angles and rotation matrix for attitude and heading representation estimation and deals with the “gimbal lock” problem. The mathematical formulation of the method is based on Kalman filter and takes into account the computational cost required for operation on mobile devices as well as the characteristics of the low-cost microelectromechanical sensors. The method was implemented, debugged, and evaluated in a desktop software utility by using a low-cost sensor system, and it was tested in an augmented reality application on an Android mobile device, while its efficiency was evaluated experimentally.

The Role of GNSS Vertical Velocities to Correct Estimates of Sea Level Rise from Tide Gauge Measurements in Greece

ABSTRACT In this study, we show how the Global Navigation Satellite System (GNSS)-derived vertical velocities contribute to the correction of tide gauge (TG) measurements used for the sea level rise estimation in Greece. Twelve sites with records of local sea level heights are processed in order to estimate their trend. Certain error sources related to TGs, e.g. equipment changes, data noise, may lead to biased or erroneous estimations of the sea level height. Therefore, it would be preferred to follow a robust estimation technique in order to detect and reduce outlier effects. The geocentric sea level rise is estimated by taking into account the land vertical motion of co-located GNSS permanent stations at the Hellenic area. TGs measure the height of the water relative to a monitored geodetic benchmark on land. On the other hand, using GNSS-based methods the vertical land motion can be derived. By means of extended models fitted to the GNSS time-series position, obtained from seven years of continuous data analysis, periodic signals are well described. The synergy of the two co-located techniques results in the correction of TG relative sea level heights taking into account the GNSS vertical velocities and consequently obtaining the conversion to absolute (geocentric) sea level trend.