Global geopotential models of spherical harmonic coefficients are used to determine the external gravitational field of the Earth. These coefficients are derived from satellite orbit perturbations, terrestrial gravity anomalies and altimeter data. Hundreds of thousands of coefficients and standard deviation values for these coefficients are estimated from millions of observation. Measurement amount, homogenous distribution of the measurements of global scale, different measurement types reflecting the different frequencies of the gravity signal and measuringassessment techniques affect the model accuracy directly. Starting from 1960's and lasts to the present day and also gaining new acceleration with the satellite gravity field missions, every outcome of the studies related to the determination of the global Geopotential model is experienced by a series of validation tests. Accuracy of the model can either be determined from the estimated error degree variances concerning the coefficients (interior validation) or comparison of geoid heights, gravity anomalies, gravity disturbances and components of vertical deflection calculated from the model with terrestrial measurements directly (outer validation). In this paper, recent global geopotential models are primarily explained. Global geopotential models are compared with GNSS/levelling data of the study area. The objective of this comparison is to determine the best fit global geopotential model which will contribute to the study of Turkish geoid determination.

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High-degree geopotential models of spherical harmonic coefficients are used for modelling the exterior gravity field of the Earth. These coefficients are derived from satellite tracking data, altimeter data, and terrestrial and airborne gravity data. Hundreds of thousands of coefficients and standard deviation values for these coefficients are estimated from millions of measurements. The geopotential model accuracy is affected by the amount, the distribution and the type of measurements. The satellite gravity field missions have provided accurate data forming geopotential models since 1960's. The geopotential models related to the satellite gravity field missions are experienced by interior validation (estimated error degree variances of fully-normalized coefficients) or outer validation (comparison of model based gravity anomalies and geoid heights with terrestrial measurements). In this paper, recent high-degree geopotential models are primarily explained and evaluated by GNSS/levelling data of a selected study area. The objective of this evaluation is to determine the high-degree geopotential model giving a better fit to the GNSS/levelling data over the study area for the contribution to the regional geoid determination studies in Turkey. Türkiye'de Bölgesel Jeoid Tespiti için Yüksek Dereceli Jeopotansiyel Modellerin Değerlendirilmesi Anahtar kelimelerJeoid; Jeopotansiyel model; GNSS/nivelman. ÖzetKüresel harmonik katsayılardan oluşan yüksek dereceli jeopotansiyel modeller Dünya'nın dış gravite alanının modellenmesi için kullanılır. Bu katsayılar uydu takip verisinden, altimetre verisinden ve yersel graviteden türetilir. Yüzbinlerce katsayı ve bu katsayıların standart sapma değerleri milyonlarca ölçümden hesaplanır. Jeopotansiyel modelin doğruluğu ölçümlerin miktarından, dağılımından ve türünden etkilenir. Uydu gravite görevleri, 1960 lardan beri jeopotansiyel modelleri oluşturan doğru veri sağlamaktadır. Uydu gravite görevleri ile bağlantılı jeopotansiyel modeller iç geçerlilik (tam normalleştirilmiş katsayıların hesaplanan hata derece varyansları) veya dış geçerlilik (model bazlı gravite anomalilerinin ve jeoid yüksekliklerinin yersel ölçüler ile karşılaştırılması) ile değerlendirilir. Bu çalışmada, güncel yüksek dereceli jeopotansiyel modeller öncelikli olarak açıklanmış ve seçilmiş bir çalışma alanında GNSS/nivelman verisi kullanılarak değelendirilmiştir. Bu değerlendirmenin amacı, Türkiye'deki bölgesel jeoid tespiti çalışmalarına katkıda bulunmak için, çalışma alanındaki GNSS/nivelman verisine daha iyi uyan yüksek dereceli jeopotansiyel modelin belirlenmesidir.

The eastern Anatolia provides one of the best examples of an area of rapid deformation and intense contraction that is the consequence of an active continental collision between the Arabian and Eurasian plates leading to large and devastating earthquakes. The latest evidence of the active tectonism in the region is revealed by two remarkable seismic events; Van-Tabanli (Mw 7.2, October 23, 2011) and Van-Edremit (Mw 5.6, November 9, 2011) earthquakes. The study of the earthquake cycle and observation of geodetic and seismic deformation in this region is very important to hazard assessments. In this study, the inter-seismic, co-seismic, and post-seismic movements caused by the above-mentioned earthquakes were investigated using the time series of 2300 days of Global Navigation Satellite Systems (GNSS) observations of the local stations selected from the network of the Continuously Operating Reference Stations, Turkey (CORS-TR). For the inter-seismic period, approximately 1100 daily data were obtained from 21 CORS-TR stations (prior to the earthquakes between October 1, 2008 and October 23, 2011) and evaluated using the GAMIT/GLOBK software. The behaviour of these stations was investigated by processing 1 Hz data from the GNSS stations during the earthquakes on the GAMIT/TRACK software. In addition to October 23 and November 9, the GNSS data on one day before and after the earthquakes was assessed to determine co-seismic deformations. During the October 23 earthquake, hanging-wall deformation of about 60 mm was detected in the SW direction at the MURA station. However, at the VAAN station, deformation of 200 mm (value predicted by time series) was observed in the footwall block in the NW direction. There were not any significant changes at the stations during the November 9 earthquake. For the post-seismic period, the GNSS data from 2012 to 2015 was evaluated. According to the observations, post-seismic deformation continued at the stations close to the epicenter of the earthquake.

The establishment of Turkish National Fundamental GPS Network (TNFGN) was completed in 2001 and Large Scale Map and Map Information Production Regulation (LSMMIPR) came into force in 2005 in parallel with the establishment of TNFGN and the increase in the use of GPS applications. TNFGN has been designed as first order GPS network and it comprises second-, third-and fourth-order GPS densification networks.LSMMIPR has required determining the positions of first-, second-and third-order GPS densification (C1, C2 and C3) points with the reference epoch besides the measurement epoch. Therefore, it is necessary to estimate the velocity vectors of the densification points. In practise, the velocity vectors of C1, C2 and C3 points are estimated from TNFGN points or higher-order densification points velocity vectors by interpolation methods but LSMMIPR did not specify the interpolation method for this procedure. The objective of this study is to use a back propagation artificial neural network (BPANN) that has been more widely applied in engineering among all other neural network models for estimating the velocity of the densification point as an alternative to the interpolation methods. BPANN and selected ten interpolation methods are evaluated over a test area, in terms of root mean square error (RMSE). The results showed that the employment of BPANN estimated the densification point velocity (V X,Y,Z ) with a better accuracy (±5.0 mm, ±4.0 mm, ±3.9 mm, respectively) than the interpolation methods in the test area and indicated that BPANN can be a useful tool for estimating point velocity in the densification networks as a real alternative to the interpolation methods.