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Öğe Accuracy assessment of aster and SRTM digital elevation models: A case study in Turkey(International Multidisciplinary Scientific Geoconference, 2017) İl, Hatice Tuba Arlı; Abbak, Ramazan AlpayDigital Elevation Model (DEM) describes the physical and topographic information of earth’s surface. Today, DEM produced by different methods is successfully utilized in many geospatial applications such as gravimetric geoid determination in geodesy, morphology analysis in geology, topographic map production in cartography etc. As known, every year, the new satellites are launched to space and provide the most current information about the earth’s surface with different working principles and different image taking configurations and offer this information to end users from large variety disciplines. SRTM (Shuttle Radar Topography Mission) and ASTER (Advanced Space-based Thermal Emission and Reflection Radiometer) are among these methods. ASTER has high resolution and superimposed image with Terra platform. SRTM aims to create a high resolution DEM of 80% of the Earth surface from radar images taken by Space Shuttle Endeavour. In this study, SRTM and ASTER DEM at one-arc second resolution are compared with local heights obtained from GPS-levelling data and accuracies of both DEMs are investigated in the territory of Turkey. The numerical results show that SRTM DEM gives better statics than ASTER DEM with respect to the local height data. © SGEM 2017. All Rights Reserved.Öğe Assessment of Shuttle Radar Topography Mission Elevation Data Based on Topographic Maps in Turkey(CARTOGRAPHY & GEOGRAPHIC INFOR SOC, 2009) Bildirici, İbrahim Öztuğ; Üstün, Aydın; Selvi, Hüseyin Zahit; Abbak, Ramazan Alpay; Buğdaycı, İlkayDepending oil scale, topographic maps depicting the shape of the land surfaces Of the Earth are produced from different data sources. National topographic maps at a scale of 1:25 000 (25K maps) produced by General Command of Mapping are used as the base map set in Turkey. This map set, which consists of approximately 5500 sheets, covers the Whole country and is produced using photogrammetric methods. Digital Elevation Models (DEMs) created From these maps are also available. Recently, another data source, Synthetic Aperture Radar (SAR) interferometric data, has become more important than those produced by conventional methods. The Shuttle Radar Topography Mission (SRTM) contains elevation data with 3 arc-second resolution and 16 in absolute height error (90 percent confidence level). These data are freely available via the Internet for approximately 80 percent of the Earth's land mass. In this study, SRTM DEM was compared with DEM derived From 25K topographic maps for different parts of Turkey. The study areas, each covering four neighboring 25K maps, and having all area of approximately 600 km(2), were chosen to represent various terrain characteristics. For the comparison, DEMs created from the 25K maps were obtained and organized as files for each map sheet in vector format, containing the digitized contour lines. From these data, DEMs in the resolution of 3 arc-second were created (25K-DEM), in the same structure as the SRTM DEM, allowing the 25K-DEMs and the SRTM DEM to be compared directly. The results show that the agreement of SRTM IN M to the 25K-DEM is within about 13 in, which is less than the SRTM's targeted error of 16 in. The spatial distribution of the height differences between SRTM-DEM and the 25K-DEM and correlation analysis show that the differences were mainly related to the topography of the test. areas. In some areas, local height shifts were determined.Öğe Assessment of shuttle radar topography mission elevation data based on torographic maps in Turkey(2009) Bildirici, İbrahim Öztuğ; Üstün, Aydın; Selvi, Hüseyin Zahit; Abbak, Ramazan Alpay; Buğdaycı, İlkayDepending on scale, topographic maps depicting the shape of the land surfaces of the Earth are produced from different data sources. National topographic maps at a scale of 1:25 000 (25K maps) produced by General Command of Mapping are used as the base map set in Turkey. This map set, which consists of approximately 5500 sheets, covers the whole country and is produced using photogrammetric methods. Digital Elevation Models (DEMs) created from these maps are also available. Recently, another data source, Synthetic Aperture Radar (SAR) interferometric data, has become more important than those produced by conventional methods. The Shuttle Radar Topography Mission (SRTM) contains elevation data with 3 arc-second resolution and 16 m absolute height error (90 percent confidence level). These data are freely available via the Internet for approximately 80 percent of the Earth's land mass. In this study, SRTM DEM was compared with DEM derived from 25K topographic maps for different parts of Turkey. The study areas, each covering four neighboring 25K maps, and having an area of approximately 600 km2, were chosen to represent various terrain characteristics. For the comparison, DEMs created from the 25K maps were obtained and organized as files for each map sheet in vector format, containing the digitized contour lines. From these data, DEMs in the resolution of 3 arc-second were created (25K-DEM), in the same structure as the SRTM DEM, allowing the 25K-DEMs and the SRTM DEM to be compared directly. The results show that the agreement of SRTM DEM to the 25K-DEM is within about 13 m, which is less than the SRTM's targeted error of 16 m. The spatial distribution of the height differences between SRTM-DEM and the 25K-DEM and correlation analysis show that the differences were mainly related to the topography of the test areas. In some areas, local height shifts were determined.Öğe ASTER GDEM AND SRTM DEM IN TURKISH TERRITORY: AN EVALUATION IN TERMS OF HEIGHT ACCURACY AND 3D VISUALIZATION(BULGARIAN CARTOGRAPHIC ASSOC, 2016) Bildirici, Ibrahim Oztug; Abbak, Ramazan Alpay; Ulugtekin, Nesibe NeclaASTER is a Japanese sensor which is one of the five equipments that are on board of the Terra satellite launched by NASA in 1999. The sensor has been collecting satellite imagery since 2000. The ASTER GDEM was released to the public after a joint operation between NASA and METI (Japan's Ministry of Economy, Trade and Industry). It is the most complete DEM of the earth ever made, covering 99% of its surface in 1 arc second resolution. The previous most comprehensive DEM, Shuttle Radar Topography Mission (SRTM) DEM, covered approximately 80% of the Earth's surface, with a resolution of 3 arc seconds, and 1 arc seconds. The GDEM covers the earth from 83 degrees North to 83 degrees South (SRTM's coverage is from 56 degrees S to 60 degrees N), becoming the first DEM that covers the Polar Regions. Nowadays the second version is in use, which is corrected and enhanced in terms of several artifacts. In spite of these corrections it is reported that there are still some artifacts such as wells and spikes in the data. In this study ASTER DEM and SRTM DEM are analyzed against local height data. The ground truth data is local DEMs created by using 25K national topographic maps. We will do an area based comparison between ASTER DEM, SRTM DEM and local DEMs. For this purpose we selected 37 25K map sheets randomly distributed over the country. Furthermore we create 3D visualizations and compare them in terms of detail richness of the topography. Finally it is concluded that SRTM DEM seems to be superior to ASTER DEM over the Turkish territory.Öğe Comparison of ASTER and SRTM digital elevation models at one-arc-second resolution over Turkey(Selçuk Üniversitesi Mühendislik Fakültesi, 2017) Bildirici, İbrahim Öztuğ; Abbak, Ramazan AlpayIn February 2000, the “Shuttle Radar Topography Mission (SRTM)” satellite captured elevation data by scanning the Earth landmasses between the 60o North and South latitudes. After the mission of 11 days, the collected data were processed, and a Digital Elevation Model (DEM) within one arc-second resolution for United States and three arc-second resolutions for the other parts of the globe was created and published on the NASA servers. Recently, a global SRTM DEM with one-arc-second resolution has been released. Additionally, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is a sensor boarded on the Terra satellite in 1999. The sensor has been collecting satellite imagery since 2000. The ASTER GDEM at one-second resolution was released to the public, which is the most complete DEM of the earth ever made. In this study, SRTM and ASTER DEMs with one arc-second resolution over Turkish territory was evaluated by means of a local DEM produced from 1:25K national topographic maps. Results show that the accuracy of the SRTM DEM is better than the ASTER GDEM with respect to the local DEM.Öğe Comparison of the KTH and remove-compute-restore techniques to geoid modelling in a mountainous area(PERGAMON-ELSEVIER SCIENCE LTD, 2012) Abbak, Ramazan Alpay; Erol, Bihter; Ustun, AydinIn mountainous regions, a gravimetric geoid determination is a difficult task which needs special attention to obtain reliable results satisfying the demands, e.g. of the engineering infrastructure. The present study compares the KTH (Royal Institute of Technology) method with the remove-compute-restore (RCR) technique to attain a precise geoid model for the Konya Closed Basin (KCB) and thus aims to clarify the theoretical and numerical differences between these techniques. The KCB located in the central part of Turkey is a mountainous area with a very limited amount of the ground gravity data. Various data sources, such as the Turkish Digital Elevation Model with 3 '' x 3 '' resolution (TRDEM3), a recently published satellite-only global geopotential model and terrestrial gravity observations are combined in the gravimetric geoid models of the KCB by applying the KTH and RCR techniques, separately. The both gravimetric geoid models (KG-KTH11 and KG-RCR11) are compared with GNSS/levelling data at the control points in the target area, resulting the RMS (Root Mean Square Error) differences of +/- 6.7 cm and +/- 9.8 cm in the absolute sense, respectively. These results show that the KTH method gives more reasonable accuracy than the RCR technique in a mountainous area with scarce terrestrial gravity data. (C) 2012 Elsevier Ltd. All rights reserved.Öğe Compilation of digital elevation model for Turkey in 3-arc-second resolution by using SRTM data supported with local elevation data(Springer Berlin Heidelberg, 2010) Bildirici, İbrahim Öztuğ; Üstün, Aydın; Uluğtekin, Nesibe Necla; Selvi, Hüseyin Zahit; Abbak, Ramazan Alpay; Buğdaycı, İlkay; Doğru, Ahmet ÖzgürIn February 2000, the Space Shuttle Endeavour of the “Shuttle Radar Topography Mission” (SRTM) launched by NASA collected elevation data by scanning the earth landmasses between ~60º north and south parallels. After this 11 days mission, the collected data were processed, and a Digital Elevation Model (DEM) at 1-arc-second – and also 3-arc-second – resolution was created. SRTM DEM contains data voids because of several disturbing effects. The proportion of data voids in Turkey (0.17% for second corrected version) is approximately same as the global proportion (0.15%). The data voids are the most important obstacles for efficient use of the data. DEMs with high resolution are important data sources for all disciplines in geosciences. Since collecting such data and creating DEM are both time consuming and expensive processes, many scientists have been studied on filling these voids in a reliable way. The aim of this paper is to present the results of a project, which aims to validate the SRTM within Turkish territory, and to create a DEM based on the SRTM data covering the same area without data voids. The validation of SRTM was performed using following ground truth data sets: topographic maps (25K maps) and DGPS tracks. The executed statistical analysis for accuracy assessment of SRTM within Turkish territory gives the results consistent with those of global analyses performed by several researchers. The voids are visually classified as the clustering and the scattered voids. The clustering voids were filled with a local data source. For this purpose the 1:25000 scaled national topographic map set (25K maps) was used. The scattered voids were filled with interpolation. As a result of the project, which was completed in September 2008, a complete DEM of Turkey at 3-arc-second resolution has been created and published via Internet for the use of researchers and professionals in geosciences. © Springer-Verlag Berlin Heidelberg 2010.Öğe Deniz düzeyi gözlemlerinin en küçük kareler yöntemiyle spektral analizi(Selçuk Üniversitesi Fen Bilimleri Enstitüsü, 2005-07-28) Abbak, Ramazan Alpay; Yerci, MehmetBu çalışmada En Küçük Karelerle Spektral Analiz (EKKSA) tekniği kullanılarak saatlik deniz düzeyi gözlemlerinin analizi yapılmıştır. Deniz düzeyi gözlemleri jeodezide zaman dizilerine güzel bir örnektir. Bu gözlemler, diğer deneysel zaman dizilerinde olduğu gibi içerisinde kısa boşluklar, doğrusal trend, datum kayıklıkları bulundurur. Bu etmenler çoğu zaman analiz için zorlaştırıcı etki yapar. Geleneksel yöntemlerde (Hızlı Fourier Dönüşümü gibi) doğrusal trend ve datum kayıklıkları gibi bazı etmenler yok sayılırken, kısa boşluklar ve eşit olmayan veri aralıkları gibi etmenler de enterpolasyon teknikleri (harmonik veya doğrusal) kullanılarak boşluklar doldurulur. Bu durum, hem dizilerin içindeki gerekli bilgilerin yok sayılmasına hem de yapay sinyallerin elde edilmesine neden olabilir. Bu da analiz işlemi için istenmeyen sonuçlar doğması anlamına gelir. İşte sözü edilen türdeki zorlukların üstesinden gelebilecek başka bir yöntem vardır ki; o da EKKSA dır. EKKSA yöntemi sayısal analize uygun olduğundan bu tür problemlerin çözümünde ve farklı ağırlıktaki verilerin analizinde kolayca uygulanabilir. Saatlik deniz düzeyi gözlemlerindeki gizli peryodikliklerin belirlenmesi ve bu peryodikliğe neden olan fiziksel olayların araştırılması çalışmanın temel konusunu oluşturmaktadır. Bu amaçla Antalya ve Menteş mareograf istasyonuna ait saatlik gözlemler bu yöntemle analiz edilmiştir. Verileri ön-işlemeye (pre-process) gerek duymadan, dizilerin analizi sonucunda 3 değişik peryodiklik belirlenmiştir.Öğe Effect of ASTER DEM on the prediction of mean gravity anomalies: a case study over the Auvergne test region(SPRINGER, 2014) Abbak, Ramazan AlpayA precise gravimetric geoid determination requires height information and terrestrial gravity data with high accuracy and resolution. The height data is utilized for predicting mean free-air gravity anomalies as well as computing the topographic, atmospheric and downward continuation effects which are fundamental components of any geoid model. Nowadays the Digital Elevation Model (DEM) obtained from the Shuttle Radar Topography Mission (SRTM) has been widely used when an accurate regional DEM does not exist. In addition the DEM generated from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) was newly released by researchers from Japan and United States. In this study the effect of ASTER DEM on the estimating mean free-air gravity anomalies in geoid determination were investigated in the Auvergne test area where one of its regions exhibits one of the most rugged topography over the world. The numerical results show that ASTER DEM gives worse statistics than SRTM DEM with respect to the accuracy of the height. Using ASTER DEM introduces discrepancies (compared to SRTM DEM) in the range of -4 to 10 mGal in the interpolation of free-air gravity anomalies. It is also proven that the geoid differences due to the use of ASTER DEM are a few centimeters, which remain below the accuracy level of the external GPS-levelling data.Öğe EFFECT OF DIFFERENT NETWORK GEOMETRY ON GNSS RESULTS(Selçuk Üniversitesi, 2018) Ahmed, Shamal Fatah Ahmed; Abbak, Ramazan AlpayGlobal Navigation Satellite System (GNSS) is mostly used to establish geodetic networks in surveying engineering. To establish a geodetic network, one should have an understanding of the various types of geodetic networks, their design, accuracy requirements, and essence. The main area where GNSS networks are needed include mapping, tracking crustal movements, planning large engineering projects, implementing cadastral works, designing urbanization activities, GIS, etc. In GNSS network, stations are generally located where they are needed, but the observation schema between stations are important. The main goal of this research was to select the best observation schema of GNSS networks according to the number of receivers and the redundancy of the observation. In this research, six points were established after reconnaissance the field of the project. After preparation the sessions according to the number of receivers, time, and distance between points observations were made by using static method. Data collections were made by using two and three receivers. From data collected in three days four types of geometric design of GNSS network were selected. The first was Hub method that is from one fixed point the new points (i.e. six points in this study) were observed. The second one was Star method that is one fixed point in the center of the new network and observed unknown points. The third was Loop method 1 (using two receivers) where all baselines (i.e. 21 baselines in this study) were observed from one known point and the last was Loop method 2 (using three receivers) from one fixed point. These methods had some advantage and disadvantage according to the type of the project that are selected. Due to no redundancy, no close loop, and no nontrivial line between adjacent points the first and second methods are not recommended for the establishment of the precise GNSS network in our study.Öğe Effect of different network geometry on GNSS results(Selçuk Üniversitesi Teknik Bilimler Meslek Yüksekokulu, 2018) Ahmed, Shamal Fatah Ahmed; Abbak, Ramazan AlpayGlobal Navigation Satellite System (GNSS) is mostly used to establish geodetic networks in surveying engineering. To establish a geodetic network, one should have an understanding of the various types of geodetic networks, their design, accuracy requirements, and essence. The main area where GNSS networks are needed include mapping, tracking crustal movements, planning large engineering projects, implementing cadastral works, designing urbanization activities, GIS, etc. In GNSS network, stations are generally located where they are needed, but the observation schema between stations are important. The main goal of this research was to select the best observation schema of GNSS networks according to the number of receivers and the redundancy of the observation. In this research, six points were established after reconnaissance the field of the project. After preparation the sessions according to the number of receivers, time, and distance between points observations were made by using static method. Data collections were made by using two and three receivers. From data collected in three days four types of geometric design of GNSS network were selected. The first was Hub method that is from one fixed point the new points (i.e. six points in this study) were observed. The second one was Star method that is one fixed point in the center of the new network and observed unknown points. The third was Loop method 1 (using two receivers) where all baselines (i.e. 21 baselines in this study) were observed from one known point and the last was Loop method 2 (using three receivers) from one fixed point. These methods had some advantage and disadvantage according to the type of the project that are selected. Due to no redundancy, no close loop, and no nontrivial line between adjacent points the first and second methods are not recommended for the establishment of the precise GNSS network in our study.Öğe En küçük karelerle spektral analiz ve Fourier tekniğinin karşılaştırılması(Selçuk Üniversitesi Teknik Bilimler Meslek Yüksekokulu, 2012) Abbak, Ramazan Alpay; Yerci, Mehmetİstatistik biliminin en önemli amaçlarından birisi, deneysel zaman dizilerinin analizi yapılarak fiziksel süreçler hakkında bilgiler çıkarılmasıdır. Bu amaca uygun olarak, literatürde birçok matematiksel araç geliştirilmiştir. Fourier tekniği ve En Küçük Karelerle Spektral Analiz (EKKSA) bunlardan sadece iki tanesidir. Deneysel zaman dizilerinin büyük bir kısmı içerisinde trend, kısa boşluklar, datum kayıklıkları ve eşit olmayan veri aralıkları ile ağırlıklarını bulundurur. Çoğu kez bunlar analiz aşamasında zorlaştırıcı etki yapar. Fourier tekniğinde trend ön analizle kaldırılırken, kısa boşluklar ve eşit olmayan veri aralıkları enterpolasyon teknikleri kullanılarak doldurulur. Söz konusu prosedürler, hem dizilerin içindeki gerekli bilginin yok sayılmasına hem de yapay sinyallerin elde edilmesine neden olur. Bu durum analiz işlemi için istenmeyen sonuçlar doğması anlamına gelir. Bu tür zorlukların üstesinden gelebilecek alternatif yöntemlerden birisi de EKKSA’dır. Bu çalışmada sözü edilen zorlukları içeren deniz düzeyi gözlemleri hem EKKSA hem de Fourier tekniği kullanılarak analiz edilmiş, sayısal sonuçlar karşılaştırılarak EKKSA’nın üstün yönlerine dikkat çekilmiştir. Sonuçlar, deneysel zaman dizilerinin spektral analizinde EKKSA’nın Fourier tekniğinden daha güçlü bir matematiksel araç olduğunu göstermektedir.Öğe Global yerpotansiyel modellerin gravimetrik jeoit belirlemeye katkısı(2018) Demir, Selda; Abbak, Ramazan Alpay; İl, Hatice Tuğba ArlıGravimetrik jeoit belirlemede global yerpotansiyel modellerinden, sayısal yükseklik modellerinden ve yersel gravite gözlemlerinden yararlanılır. Yeryuvarının tümüne homojen olarak yayılmış gravite gözlemlerinin değerlendirilmesiyle global yerpotansiyel modeller üretilir. Global yerpotansiyel modellerin doğruluğu, bu modeller kullanılarak oluşturulacak çalışmaların başarısını doğrudan etkileyecektir. Bu uygulamada yakın tarihlerde CHAMP, GRACE ve GOCE uydu verileri yardımıyla oluşturulan yeni nesil global yerpotansiyel modellerinden türetilen jeoit ondülasyonları, GPS/Nivelman verilerinden elde edilen eşlenikleri ile karşılaştırılıp seçilen güncel global yerpotansiyel modellerinin doğrulukları bölgesel olarak değerlendirilmiştir. Ele alınan yeni nesil global yerpotansiyel modeller kullanılarak KTH tekniği ile Konya Kapalı Havzasında 12 adet gravimetrik jeoit modeli oluşturulmuş ve oluşturulan her bir model 7 parametreli benzerlik dönüşümü yardımıyla GPS/Nivelman verileri kullanılarak karşılaştırılmıştır. Sonuç olarak yeni nesil global yerpotansiyel modelinin doğruluğunun hesaplanan gravimetrik jeoit modelinin doğruluğuna yaptığı katkılar sayısal anlamda ortaya konmuştur.Öğe Global yerpotansiyel modellerinin spektral yöntemlerle değerlendirilmesi ve jeoit belirleme için yerel olarak iyileştirilmesi(Selçuk Üniversitesi Fen Bilimleri Enstitüsü, 2011-06-24) Abbak, Ramazan Alpay; Üstün, AydınBilim ve tekniğin gelişmesiyle birlikte, farklı kurumlar tarafından her yıl onlarca global yerpotansiyel modeli oluşturulmaktadır. Yeryuvarının dış çekim alanını temsil eden bu modeller global anlamda homojen olarak dağılmış uydu ve yersel gravite ölçmelerinin sentezi olarak karşımıza çıkar. Bu çalışmada söz konusu global yerpotansiyel modellerinin değerlendirilmesi ve iyileştirilmesi için gerekli işlem adımları açıklanmış ve bu bilgiler ışığında bölgesel ölçekte bir jeoit belirleme çalışması gerçekleştirilmiştir. Global yerpotansiyel modellerinin değerlendirilmesinde dış ve iç doğruluk ölçütleri kullanılmış, özellikle model çözünürlüğüne bağlı doğruluk araştırması yapılmıştır. İç doğruluk analizinde son zamanlarda gündeme gelen spektral yöntemlere (araçlara) yer verilmiştir. Karşılaştırmalar sonucunda GRACE uydu misyonu verilerine dayanan modeller daha iyi sonuçlar verdiği ve karşılaştırma için seçilen modeller arasında yerel jeoit belirleme için en iyi referans modelin ITG-GRACE2010S olduğu değerlendirilmiştir. Bunun devamında sınırlı sayıda yersel gravite gözlemleri (22 km^2 ye bir nokta) çözüme katılarak bu model KTH (İsveç Kraliyet Teknoloji Enstitüsü) yaklaşımı yardımıyla iyileştirilmiştir. İyileştirme sonucunda Konya Kapalı Havzası'nı kaplayan 68 000 km^2'lik alanda 3'\times 3' çözünürlüklü gravimetrik bölgesel jeoit modeli KJ2011 elde edilmiştir. Yeni model (KJ2011) GNSS/nivelman verileriyle hem mutlak hem de bağıl olarak değerlendirilmiştir. KJ2011 modelinin mutlak ve bağıl doğruluğu sırasıyla 5.6 cm ve 1.7 ppm olarak hesaplanmıştır. Buna karşın Türkiye sınırlarında en iyi sonuçları veren EGM2008 global kombine modeli ile yapılan analizler sonucunda EGM2008 modelinin proje sahasında mutlak ve bağıl doğruluğu sırasıyla 7.8 cm ve 1.8 ppm olarak belirlenmiştir. Bu sonuçlar global modellerin, yerel veriler yardımıyla iyileştirileceği ve bu iyileşmenin her ne kadar yerel verilerin yeterli sıklık ve kapsamda olmamasına karşın anlamlı değerlere ulaşabileceğini göstermiştir.Öğe Height biases of SRTM DEM related to EGM96: from a global perspective to regional practice(TAYLOR & FRANCIS LTD, 2018) Üstün, Aydın; Abbak, Ramazan Alpay; Öztürk, Emel ZerayIt is investigated to what extent EGM96 affects the accuracy of digital elevation model (DEM) produced from the shuttle radar topography mission (SRTM). Global and regional analysis of EGM96 compared with EGM2008 indicate that locally there are large differences distorting to the accuracy level of SRTM DEM. In the absolute sense, the overall geoid differences throughout 1 x 1 arc-degree tiles reach -5m in the northeast and 2-3m in the southern parts of Turkey. A numerical investigation over the test profiles of 200-700 km length running at various directions proves that a possible vertical datum change from EGM96 to EGM2008 yields systematically more accurate height information with an improvement of up to 2.5 m. A GPS-levelling traverse of about 900 km length points out some key patterns of this recovery. Consequently, a correction for the present version of SRTM DEM should be considered in critical implementations of Earth sciences like geoid or water flow modelling, especially for areas where EGM96 shows weak performance.Öğe On Global and Regional Spectral Evaluation of Global Geopotential Models(IOP Publishing Ltd, 2010) Üstün, Aydın; Abbak, Ramazan AlpaySpectral evaluation of global geopotential models (GGMs) is necessary to recognize the behaviour of gravity signal and its error recorded in spherical harmonic coefficients and associated standard deviations. Results put forward in this wise explain the whole contribution of gravity data in different kinds that represent various sections of the gravity spectrum. This method is more informative than accuracy assessment methods, which use external data such as GPS-levelling. Comparative spectral evaluation for more than one model can be performed both in global and local sense using many spectral tools. The number of GGMs has grown with the increasing number of data collected by the dedicated satellite gravity missions, CHAMP, GRACE and GOCE. This fact makes it necessary to measure the differences between models and to monitor the improvements in the gravity field recovery. In this paper, some of the satellite-only and combined models are examined in different scales, globally and regionally, in order to observe the advances in the modelling of GGMs and their strengths at various expansion degrees for geodetic and geophysical applications. The validation of the published errors of model coefficients is a part of this evaluation. All spectral tools explicitly reveal the superiority of the GRACE-based models when compared against the models that comprise the conventional satellite tracking data. The disagreement between models is large in local/regional areas if data sets are different, as seen from the example of the Turkish territory.Öğe A precise gravimetric geoid model in a mountainous area with scarce gravity data: a case study in central Turkey(SPRINGER, 2012) Abbak, Ramazan Alpay; Sjoberg, Lars E.; Ellmann, Artu; Üstün, AydınIn mountainous regions with scarce gravity data, gravimetric geoid determination is a difficult task that needs special attention to obtain reliable results satisfying the demands, e.g., of engineering applications. The present study investigates a procedure for combining a suitable global geopotential model and available terrestrial data in order to obtain a precise regional geoid model for Konya Closed Basin (KCB). The KCB is located in the central part of Turkey, where a very limited amount of terrestrial gravity data is available. Various data sources, such as the Turkish digital elevation model with 3 '' x 3 '' resolution, a recently published satellite-only global geopotential model from the Gravity Recovery and Climate Experiment satellite (GRACE) and the ground gravity observations, are combined in the least-squares sense by the modified Stokes' formula. The new gravimetric geoid model is compared with Global Positioning System (GPS)/levelling at the control points, resulting in the Root Mean Square Error (RMS) differences of +/- 6.4 cm and 1.7 ppm in the absolute and relative senses, respectively. This regional geoid model appears to he more accurate than the Earth Gravitational Model 2008, which is the best global model over the target area, with the RMS differences of +/- 8.6 cm and 1.8 ppm in the absolute and relative senses, respectively. These results show that the accuracy of a regional gravimetric model can be augmented by the combination of a global geopotential model and local terrestrial data in mountainous areas even though the quality and resolution of the primary terrestrial data are not satisfactory to the geoid modelling procedure.Öğe Regional analysis of recent global geopotential models: A case study in Turkey(International Multidisciplinary Scientific Geoconference, 2017) Demir, Selda; Abbak, Ramazan AlpayGeoid is an equipotential surface of Earth’s gravity field, which partially coincides with the mean sea level and extended inside continents. In the gravimetric geoid determination; global geopotential models (GGMs), digital elevation models and terrestrial gravity observations are combined in a proposed technique (e.g. Remove-Compute-Restore). The external Earth's gravity field is represented by a GGM that consists of the globally and homogeneously distributed terrestrial and satellite gravity measurements. Currently, CHAMP, GRACE, and GOCE satellite missions have been used to determine the gravity field. Satellite-based GGMs that mainly represent the long wavelength components of the gravity field can be evaluated externally by using terrestrial data such as ground gravity, GPS-leveling, etc. Then, the standard deviation of the differences (e.g. NGGM minus NGPS-LEV) is considered as the accuracy of the model. In this study, geoid undulations derived from recently published GGMs are compared with equivalents obtained from GPS-leveling data, and the accuracies of selected GGMs are investigated in the territory of Turkey. Numerical results show that GGMs combining GRACE and GOCE data are more accurate than the others. © SGEM2017. All Rights Reserved.Öğe A software package for computing a regional gravimetric geoid model by the KTH method(SPRINGER HEIDELBERG, 2015) Abbak, Ramazan Alpay; Ustun, AydinNowadays, the geodetic community has aimed to determine 1-cm accuracy gravimetric geoid model, which satisfies the demands of most engineering applications. However, the gravimetric geoid determination is a difficult mission which needs an exclusive attention to obtain reliable results for this purpose. Today, Least-Squares Modification of Stokes (LSMS) formula which is so-called the KTH method (Swedish Royal Institute of Technology) has been performed in the regional geoid studies. Based upon the earlier investigations, the KTH method provides more reasonable results than the Remove Compute Restore technique, especially in roughly terrain with sparse terrestrial gravity data. Nevertheless, a compact and practical software package is now not available for users and researchers in geosciences. Thus, in this paper, a scientific software called "LSMSSOFT" is developed and presented by adding a new algorithm which speeds up the evaluation of Stokes' integral. Afterwards, the LSMSSOFT is applied to a case study for the construction of a geoid model over the Auvergne test area in France. Consequently, the algorithm treated in the software and its results imply that the LSMSSOFT is an alternative software package for modelling the gravimetric geoid by the KTH method.Öğe SRTM1 ve ASTER sayısal yükseklik modellerinin gravimetrik jeoit belirlemeye katkısı(2018) İl, Hatice Tuba Arlı; Abbak, Ramazan Alpay; Bildirici, İbrahim Öztuğ; Demir, SeldaGravimetrik jeoit modellerinin oluşturulma sürecinde gravite verilerinin yanı sıra topografik yüksekliklere de ihtiyaç vardır. Çünkü jeoit modelleme çalışmalarında topografya etkisi en önemli parametrelerden biridir ve dünyanın fiziksel yüzeyinden toplanan gravite ölçüleri jeoit modellemede doğrudan kullanılamaz. Serbest hava gravite anomalilerinin enterpolasyonu için herhangi bir Sayısal Yükseklik Modelinden (SYM) alınan grid merkezlerinin ortalama yüksekliklerine gereksinim duyulur. Bu çalışmada SRTM1 ve ASTER SYM’lerin bölgesel gravimetrik jeoit modellemedeki performansları değerlendirilmiştir. Öncelikle çalışma sahasına homojen olarak dağılmış nivelman noktalarıyla modellerin doğrulukları bölgesel anlamda araştırılmıştır. Daha sonra, test bölgesinde SYM’ler sırasıyla kullanılarak KTH (İsveç Kraliyet Enstitüsü) yöntemi ile iki farklı gravimetrik jeoit modeli hesaplanmıştır. Karşılaşılan çeşitli sistematik hataları ortadan kaldırmak ve en uygun kombinasyonu elde etmek için gravimetrik ve GPS-nivelman jeoit modellerini düzeltici yüzeye dayalı olarak optimum yüzeyde birleştirilmiş ve jeoit modellerinin doğrulukları analiz edilmiştir. Sonuç olarak her iki jeoit modeli arasında anlamlı bir fark olmadığı görülmüştür. Modellerin benzer sonuçları vermesi, her iki SYM’nin doğruluklarının yakın olmasından kaynaklandığı değerlendirilmektedir. Ancak SRTM1 Modelinin olmadığı yerlerde ASTER Modelinin alternatif olarak kullanılabileceği önerilmektedir.
