Transformation of ellipsoidal heights derived from the Global Positioning System (GPS) to orthometric heights using geoid models is investigated in the north and west parts of Turkey. Although the transformation depends on a simple relation between ellipsoidal h, orthometric H and geoid N heights, the accuracy of the resulting orthometric heights after transformation is crucial in geodetic and surveying applications. Various factors which affect this accuracy, such as measurement errors, datum inconsistencies and theoretical assumptions, are investigated in this study, while testing different methods in three test networks (Sakarya in the Northwest, Çankırı in the North and Izmir in the West). The study consists of three steps. In the first step the regional Turkey geoids TG99A, TG03 and the European gravimetric geoid EGG97 are tested comparing geoid heights derived from models and GPS/levelling at co−located benchmarks. In the second step, regional geoid models are combined with GPS/levelling using Least Squares Adjustment of height differences and corrector surface models. In this step, additionally, Variance Component Estimation (VCE) using Minimum Norm Quadratic Unbiased Estimation (MINQUE) approach is performed, in order to combine the height sets. In the last step, GPS/levelling surface type local geoids are determined and their performances are tested in transformation of GPS−heights. Finally, the resulting accuracies are compared and practical aspects of those approaches in deriving orthometric heights from GPS measurements in geodetic and surveying applications are discussed.

The continuous efforts on establishment and modernization of the geodetic control inTurkey include a number of regional geoid models that have been determined since 1976. The recently released gravimetric Geoid of Turkey, TG03, is used in geodetic applications where GPS-heights need to be converted to the local vertical datum. To reach a regional geoid model with improved accuracy, the selection of the appropriate global geopotential model is of primary importance. This study assesses the performance of a number of recent satellite-only and combined global geopotential models (GGMs) derived from CHAMP and GRACE missions' data in comparison to the older EGM96 model, which is the underlying reference model for TG03. In this respect, gravity anomalies and geoid heights from the global geopotential models were compared with terrestrial gravity data and low-pass filtered GPS/levelling data, respectively. Also, five new gravimetric geoid models, computed by the Fast Fourier Transform technique using terrestrial gravity data and the geopotential models, were validated at the GPS/levelling benchmarks. The findings were also compared with the validation results of the TG03 model.The tests showed that as it was expected any of the high-degree combined models (EIGEN-CG03C, EIGEN-GL04C, EGM96) can be employed for determining the gravity anomalies over Turkey. In the west of Turkey, EGM96 and EIGEN-CHAMP03S fit the GPS/levelling surface better. However, all the tested GGMs revealed equal performance when they were employed in gravimetric geoid modelling after de-trending the gravimetric geoid model with corrector surface fitting. The new geoid models have improved accuracy (after fit) compared to TG03.

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One of the most intricate branches of metrology involves the monitoring of displacements and deformations of natural and anthropogenic structures under environmental forces, such as tidal or tectonic phenomena, or ground water level changes. Technological progress has changed the measurement process, and steadily increasing accuracy requirements have led to the continued development of new measuring instruments. The adoption of an appropriate measurement strategy, with proper instruments suited for the characteristics of the observed structure and its environmental conditions, is of high priority in the planning of deformation monitoring processes. This paper describes the use of precise digital inclination sensors in continuous monitoring of structural deformations. The topic is treated from two viewpoints: (i) evaluation of the performance of inclination sensors by comparing them to static and continuous GPS observations in deformation monitoring and (ii) providing a strategy for analyzing the structural deformations. The movements of two case study objects, a tall building and a geodetic monument in Istanbul, were separately monitored using dual-axes micro-radian precision inclination sensors (inclinometers) and GPS. The time series of continuous deformation observations were analyzed using the Least Squares Spectral Analysis Technique (LSSA). Overall, the inclinometers showed good performance for continuous monitoring of structural displacements, even at the sub-millimeter level. Static GPS observations remained insufficient for resolving the deformations to the sub-centimeter level due to the errors that affect GPS signals. With the accuracy advantage of inclination sensors, their use with GPS provides more detailed investigation of deformation phenomena. Using inclinometers and GPS is helpful to be able to identify the components of structural responses to the natural forces as static, quasi-static, or resonant.

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