An ınvestigation on accuracy analysis of global and regional (high resolution) digital elevation models

Topografik yükseklikler birçok mühendislik uygulamasında ve yerbilimlerine ilişkin araştırmalarda kullanılmaktadır. Yüksek çözünürlüklü Sayısal Yükseklik Modelleri (SYM), günümüzde yükseklik verilerini elde etmenin en pratik ve ekonomik yoludur. SYM’lerinin üretiminde farklı yöntemler uygulanır. Bu modeller çeşitli hata kaynaklarından etkilenirler. Bu nedenle, SYM verilerini kullanmadan önce çalışma alanlarındaki performanslarını test etmek çalışmada gereksinim duyulan yükseklik doğruluğunun sağlanması için önemlidir. Genel bir yaklaşım olarak, Sayısal Yükseklik Modellerinin doğruluk analizinde topografyaya uygun dağılmış kontrol noktalarında Global Navigation Satellite System (GNSS) ve/veya nivelman yükseklikleri ile modelden elde edilen yükseklik farklarının karesel ortalama hata (k.o.h.) değerleri dikkate alınır. Bu çalışmada, yüksek çözünürlüklü global SYM’leri: ASTER GDEM (Advanced Spaceborne Thermal Emission and Reflection Radiometer-Gelişmiş Uzay Kaynaklı Termal Emisyon ve Yansıma Radyometresi), SRTM (Shuttle Radar Topography Mission-Mekik Radar Topografya Misyonu) ile bölgesel HGM DTED2 modellerinin doğrulukları GNSS/nivelman verileri kullanılarak analiz edilmiştir. Bu amaçla farklı topografik özellikteki alanları temsil eden üç ayrı GNSS/nivelman veri seti kullanılarak kontrol noktalarının dağılımının yanı sıra test alanı topografyasının SYM doğruluğuna etkisi incelenmiştir. Sonuçlar Türkiye'nin kuzeybatısındaki test edilen global ve bölgesel SYM’lerinin doğruluğunu topografyanın değişen özelliklerine bağlı olarak karşılaştırmakta ve değerlendirmektedir. Testler sonucu global SYM’lerinin doğruluğu 8.0 m iken bölgesel DTED2 SYM doğruluğu 6.0 m olarak bulunmuştur.The topographical heights are required in practice for a number of engineering applications as well as their specific use in many Earth science disciplines. Using a high-resolution Digital Elevation Model (DEM) is the most practical and economical way for obtaining the height data nowadays. These models include errors. So, it is important to clarify the quality as well as the accuracy of the DEM in the study areas before using its data. In general, validating DEMs using independent point-wise data such as GNSS and leveling heights provide an overall accuracy measure in terms of root means square error (r.m.s.e.) of the DEM derived heights. In this study three high-resolution digital elevation models ASTER, SRTM and Turkey Digital Topographic Data (DTED2) in 1 and 3 resolutions are assessed using GNSS/leveling data. Using three different sets of GNSS/leveling data invalidations it is aimed to clarify the role of the distribution of the ground-control points as well as the region’s characteristics, such as roughness of topography, land-cover, etc., in the validation results. The conclusions report the accuracy of the validated DEMs in northwest Turkey and hence include a categorization of DEM performances, generated from remotely sensed data and terrestrial techniques, depending on the topographical characteristics. In the test results the accuracies for global DTMs is 8.0 m, and for regional DTED2 DEM is 6.0 m

Assessment of recent global potential models with terrestrial data

Bu çalışmada gravite alanı belirleme amaçlı GRACE ve CHAMP uydularının verileri ile hesaplanan güncel global potansiyel modellerin (GGM02S, EIGEN-CHAMP03S, EIGEN-CG03C ve EIGEN-GL04C) Türkiye’deki performansları test edilmekte ve Türkiye bölgesel geoidi TG03’ün hesaplanmasında referans olarak kullanılmış EGM96 global potansiyel modeli ile karşılaştırılmaktadır. TG03 geoit modeli gravimetrik yöntemle belirlenmiştir ve Türkiye’deki jeodezik çalışmalarda kullanılmaktadır. Modelin mutlak doğruluğu desimetre mertebesindedir. Bu çalışmada amaç, farklı global potansiyel modeller ile Türkiye’de gravite alanının uzun dalga boylu bileşenindeki iyileşmenin araştırılmasıdır. Bu amaçla, farklı maksimum derecelerden küresel harmonik eşitlikler ile ifade edilen potansiyel modeller kullanılarak hesaplanan gravite anomalileri ve geoit yükseklikleri sırasıyla yersel gravite verileri ile ve GPS/nivelmandan elde edilen geoit yükseklikleri ile karşılaştırılarak test edilmiştir. Bunun yanı sıra yersel gravite anomalileri ve global potansiyel modeller kullanılarak bölgesel geoit modelleri hesaplanmış, global potansiyel modellerin gravimetrik bölgesel geoit modellerinin doğruluğuna katkısı böylelikle de test edilmiştir. Bunun için hesaplanan bölgesel gravimetrik geoit modellerinden türetilen geoit yükseklikleri bağımsız GPS/nivelman verileri ile karşılaştırılarak Türkiye geoidi için en uygun global potansiyel model belirlenmeye çalışılmıştır. Çalışmanın sonuçlarının presizyonlu Türkiye bölgesel geoidinin gelecek versiyonunun hesaplanmasında referans model olarak kullanılmak üzere en uygun global potansiyel modelin seçilmesinde faydalı olması beklenmektedir. Anahtar Kelimeler: Global potansiyel model, CHAMP, GRACE, bölgesel gravimetrik geoit modeli, yersel gravite anomalileri, GPS/nivelman.CHAMP and GRACE satellites are low Earth orbiters which are used for the determination of the Earth’s gravity field. The mission of the satellites is to provide the knowledge for the gravity field that leads to observe the Earth system for geodetic and geodynamic purposes with a sufficient accuracy. The characteristics specific to the satellites such as design, orbit, measurement and processing techniques have provided new approaches for the global gravity field determination. Earth potential models are representation of the global gravity field and they are divided into three classes, namely satellite-only Global Potential Models (GPMs) (derived from the tracking of artificial satellites), combined GPMs (derived from a combination of a satellite-only model, terrestrial gravimetry, satellite altimetry and/or airborne gravimetry) and tailored GPMs (derived by refining existing satellite-only or combined GPMs using regional gravity data). Satellite-only GPMs are known to be weak at the coefficients of degrees higher than 60 or 70 due to several factors such as the power-decay of the gravitational field with altitude, modeling of atmospheric drag, non-gravitational and third-body perturbations and incomplete tracking of satellite orbits from ground stations. Although the effects of some of these limitations on the GPMs decreased after the dedicated satellite gravity missions CHAMP and GRACE, the new satellite-only GPMs still have not got full power until a certain degree, and rapidly increasing errors make their coefficients unreliable at high-degrees. In this study, the most recent satellite-only and combined global potential models from the CHAMP and GRACE satellite missions released by GFZ (GeoForschungsZentrum) and the Center for Space Research of Texas University were tested. The older combined-GPM EGM96 was also included in the tests. Although EGM96 model was not calculated using data from CHAMP and GRACE, the study includes this model since it serves as the reference model for the official regional geoid model of Turkey. The Earth potential models from CHAMP, GRACE, and other data, assessed in the tests here, are GGM02S, EIGEN-CHAMP03S, EIGEN-CG03C and EIGEN-GL04C and they were compared with the older EGM96. The aim of this study is to investigate the improvements in the modelling of the long wavelength gravity field components for the area of Turkey. With this aim gravity anomaly and geoid height grids were generated for varying maximum degrees of the spherical harmonic expansions and the global potential models were compared with terrestrial gravity data as well as GPS/levelling data in Turkey. Furthermore, regional geoid models were computed by Fast Fourier Transform techniques using terrestrial gravity data and various geopotential models, and the results were again evaluated against GPS/levelling data. The intention of the study is to provide a valuable input for the selection of “the best reference geopotential model” for a high resolution hybrid geoid model for Turkey. In the investigation steps, firstly, the GPMs derived gravity anomalies were compared with the free-air gravity anomalies from the terrestrial data over Turkey. Than the GPMs derived geoid heights were interpolated and compared with the low-pass filtered GPS/levelling derived geoid heights at the co-located benchmarks in two test areas (İzmir and İstanbul) in the West part of Turkey. The statistics from these comparisons provided preliminary results in the assessment of the GPMs. According to this, EGM96, EIGEN-CG03C and EIGEN-GL04C combined geopotential models with the maximum degree and order of 360 fit slightly better in Turkey. In the second evaluation stage, the regional geoid models were computed in Turkey using Remove-Restore technique. The computed regional models refer to each of the GPMs (with their maximum expansion) as reference models. The differences between the geoid heights derived from the gravimetric geoid models and from the GPS/levelling data were investigated in Istanbul and Izmir test networks. According to statistical results, the EIGEN-CG03C is optimal GPM for pure gravimetric geoid model in West of Turkey. Finally, the regional geoid models were fitted to the GPS/levelling with a 2nd order polynomial using the residual geoid heights at the benchmarks. The tests of the regional gravimetric geoid models after corrector surface fitting against the GPS/levelling control data shown that the fitted geoid models has the similar performance in a test area. Keywords: Global potential model, CHAMP, GRACE, regional gravimetric geoid model, terrestrial gravity anomalies, GPS/levelling

Performance analysis of precise point positioning (ppp) technique in antarctica continent

Küresel ısınmanın etkisi ile kutup bölgelerinde meydana gelen iklim değişikliklerinin takibi, atmosfer fiziğinin daha iyi ortaya konulabilmesi, buzullardaki deformasyonların ve tektonik hareketlerin sürekli olarak izlenmesi büyük bir öneme sahiptir. Bu kapsamda GNSS (Global Navigation Satellite SystemKüresel Navigasyon Uydu Sistemi) ve uzaktan algılama uyduları küresel çaptaki afetlerin izlenmesinde yaygın olarak kullanılmaktadır. GNSS’i oluşturan uydu sistemlerinin en temel parçası olan GPS uydularının yörüngelerinin ekvatora göre 55 eğime sahip olmasından dolayı kutup bölgelerinde başucu doğrultusu çevresinde uydu gözlemleri kısıtlı olmaktadır. Ayrıca, kutup bölgelerinde yaz ve kış aylarında gün ışığı süresi ve hava koşulları büyük farklılıklar göstermektedir. Bu nedenlerden dolayı kutup bölgelerinde yapılacak GPS ölçmelerine etki eden atmosfer kaynaklı hataların giderilmesi daha fazla önem arz etmektedir. Bu çalışma kapsamında PPP tekniğinin Antarktika Kıtasındaki performansı araştırılmıştır. Bu amaçla, Antarktika Kıtasında UNAVCO’ya ait sürekli gözlem yapan HUGO ve HOWE sabit GPS istasyonlarına ait farklı mevsimlerdeki 30 saniye aralıklı günlük gözlem verileri statik ve kinematik PPP yöntemiyle değerlendirilmiştir. Çalışmada, günümüzün en yaygın kullanılan online GNSS değerlendirme servislerinden birisi olan Canadian Spatial Reference System-Precise Point Positioning (CSRS-PPP) servisi ve açık kaynak kodlu akademik bir yazılım olan RTKLIB kullanılmıştır. Yapılan çalışma sonuçlarına göre PPP tekniği ile elde edilen koordinatların, AUSPOS online rölatif konumlandırma servisi ile elde edilen ve bilinen olarak kabul edilen koordinatlarla statik çözümlerde milimetre, kinematik çözümlerde ise santimetre mertebesinde farklara sahip olduğu görülmüştür.It is of great importance to observe the climate changes that occur in the polar regions with the effect of global warming, to better reveal the atmospheric physics, and to continuously monitor the deformations and tectonic movements in the glaciers. In this context, GNSS (Global Navigation Satellite System) and remote sensing satellites are widely used in the monitoring of global disasters. As one of the most fundamental components of GNSS, GPS satellites have inclined orbits 55 with respect to the equator, therefore around the zenith direction the satellite observations are limited in the polar regions. Also, the daylight duration and weather conditions vary greatly in the polar regions in summer and winter times. For these reasons, eliminating the effects of atmospheric errors on GPS measurements in polar regions is much more crucial. In this study, the performance of PPP technique in Antarctica Continent was investigated. For this purpose, the daily observations with 30-second sampling rates from different seasons belong to HUGO and HOWE continuous GPS stations of UNAVCO in the Continent, were processed with static and kinematic PPP methods. In the study, Canadian Spatial Reference System-Precise Point Positioning (CSRS-PPP) online service, and RTKLIB, an open source academic software, were used. In results of the study, it is seen that the coordinates obtained with PPP technique have differences with the known coordinates of the points obtained from the AUSPOS online relative positioning service are in millimeter for static solutions and in centimeter for kinematic solutions, respectively

Evaluation of High-Precision Sensors in Structural Monitoring

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

An Experimental Study of a New Keypoint Matching Algorithm for Automatic Point Cloud Registration

Light detection and ranging (LiDAR) data systems mounted on a moving or stationary platform provide 3D point cloud data for various purposes. In applications where the interested area or object needs to be measured twice or more with a shift, precise registration of the obtained point clouds is crucial for generating a healthy model with the combination of the overlapped point clouds. Automatic registration of the point clouds in the common coordinate system using the iterative closest point (ICP) algorithm or its variants is one of the frequently applied methods in the literature, and a number of studies focus on improving the registration process algorithms for achieving better results. This study proposed and tested a different approach for automatic keypoint detecting and matching in coarse registration of the point clouds before fine registration using the ICP algorithm. In the suggested algorithm, the keypoints were matched considering their geometrical relations expressed by means of the angles and distances among them. Hence, contributing the quality improvement of the 3D model obtained through the fine registration process, which is carried out using the ICP method, was our aim. The performance of the new algorithm was assessed using the root mean square error (RMSE) of the 3D transformation in the rough alignment stage as well as a-prior and a-posterior RMSE values of the ICP algorithm. The new algorithm was also compared with the point feature histogram (PFH) descriptor and matching algorithm, accompanying two commonly used detectors. In result of the comparisons, the superiorities and disadvantages of the suggested algorithm were discussed. The measurements for the datasets employed in the experiments were carried out using scanned data of a 6 cm × 6 cm × 10 cm Aristotle sculpture in the laboratory environment, and a building facade in the outdoor as well as using the publically available Stanford bunny sculpture data. In each case study, the proposed algorithm provided satisfying performance with superior accuracy and less iteration number in the ICP process compared to the other coarse registration methods. From the point clouds where coarse registration has been made with the proposed method, the fine registration accuracies in terms of RMSE values with ICP iterations are calculated as ~0.29 cm for Aristotle and Stanford bunny sculptures, ~2.0 cm for the building facade, respectively

An Assessment of the GOCE High-Level Processing Facility (HPF) Released Global Geopotential Models with Regional Test Results in Turkey

The launch of dedicated satellite missions at the beginning of the 2000s led to significant improvement in the determination of Earth gravity field models. As a consequence of this progress, both the accuracies and the spatial resolutions of the global geopotential models increased. However, the spectral behaviors and the accuracies of the released models vary mainly depending on their computation strategies. These strategies are briefly explained in this article. Comprehensive quality assessment of the gravity field models by means of spectral and statistical analyses provides a comparison of the gravity field mapping accuracies of these models, as well as providing an understanding of their progress. The practical benefit of these assessments by means of choosing an optimal model with the highest accuracy and best resolution for a specific application is obvious for a broad range of geoscience applications, including geodesy and geophysics, that employ Earth gravity field parameters in their studies. From this perspective, this study aims to evaluate the GOCE High-Level Processing Facility geopotential models including recently published sixth releases using different validation methods recommended in the literature, and investigate their performances comparatively and in addition to some other models, such as GOCO05S, GOGRA04S and EGM2008. In addition to the validation statistics from various countries, the study specifically emphasizes the numerical test results in Turkey. It is concluded that the performance improves from the first generation RL01 models toward the final RL05 models, which were based on the entire mission data. This outcome was confirmed when the releases of different computation approaches were considered. The accuracies of the RL05 models were found to be similar to GOCO05S, GOGRA04S and even to RL06 versions but better than EGM2008, in their maximum expansion degrees. Regarding the results obtained from these tests using the GPS/leveling observations in Turkey, the contribution of the GOCE data to the models was significant, especially between the expansion degrees of 100 and 250. In the study, the tested geopotential models were also considered for detailed geoid modeling using the remove-compute-restore method. It was found that the best-fitting geopotential model with its optimal expansion degree (please see the definition of optimal degree in the article) improved the high-frequency regional geoid model accuracy by almost 15%

Improvement of GOCE-Based Global Geopotential Models for Gravimetric Geoid Modeling in Turkey

This study investigates the contribution of global geopotential models which are calculated with GOCE satellite mission data to the improvement of gravimetric geoid models in Turkey. In this context, direct (DIR), time-wise (TIM), space-wise (SPW), and GOCO satellite-only model series were considered. The research was carried out in two parts. The first part includes the validation of models in each series at 100 homogeneously distributed GNSS/leveling stations over the country utilizing spectrally enhanced geoid heights to determine the best performing model and its optimal expansion degree. According to obtained statistics, the TIM-R6 model was selected as the best model with an optimal expansion degree of 204. In the second part, the TIM-R6 model up to 204 degree/order was linearly blended with EGM2008 to obtain an improved version up to 360 degree/order of expansion. To clarify the contribution of the linearly blended model to the improvement of the regional geoid model, the gravimetric geoid models were computed adopting TIM-R6 up to 204 degree/order and its improved version up to 360 degree/order as reference models. To further emphasize the contribution of the GOCE mission’s data, the gravimetric geoid computations were repeated relying on EGM2008 up to 204 and 360 degrees of expansions, since EGM2008 does not contain GOCE data. In addition, we computed gravimetric geoids based on another combined model that includes GOCE mission data, the EIGEN-6C4 model. The calculated regional geoids were compared to each other and validated using GNSS/leveling data set. The obtained results revealed a ∼23% improvement in regional geoid model accuracy when the blended GOCE-based geopotential model was used as a reference. In addition, the results of this study presented the significance of GOCE contribution to mapping the gravity field in Turkey. The best accuracy obtained from this study was 7.7 cm for the Turkey geoid