Aydin Ustun

A precise gravimetric geoid model in a mountainous area with scarce gravity data : a case study in central Turkey

In 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 ″ × 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 be 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.

On global and regional spectral evaluation of global geopotential models

Spectral 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.

Land subsidence in Konya Closed Basin and its spatio-temporal detection by GPS and DInSAR

Konya Closed Basin that includes two depression blocks, Konya and Tuz Gölü, is the biggest endorheic basin in Turkey. A lowering in the ground levels of the depositional areas has been shown with respect to the surrounding mountains. In the past, this process was controlled by geological and hydrogeological phenomena; nowadays, it has gained a new dimension due to the excessive pumping of groundwater to the Earth’s surface. The geodetic techniques such as Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) reveal a considerable land subsidence resulting in the rate of 1–4 cm/year in Konya sub-basin. It seems that the calculated amounts are responsible for changing environmental conditions like seasonality. Both GPS and differential InSAR techniques verify these relatively small fluctuations in the subsidence development. Target districts that should be observed primarily are agricultural zones where groundwater is overused for irrigation and regions where human population density is high. The geocoded interferogram and vegetation index map verify the spatial relation between the subsidence and green areas. In this study, a project dealing with the investigation of the land subsidence occurrences and of their possible causes and consequences within a specific part of Konya Closed Basin is introduced.

Assessment of Shuttle Radar Topography Mission Elevation Data Based on Topographic Maps in Turkey

Depending 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 Earths 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 SRTMs 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.

Compilation of Digital Elevation Model for Turkey in 3-Arc-Second Resolution by Using SRTM Data Supported with Local Elevation Data

In 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 (25 K 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 (25 K 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.

Initial Results of a Surface Deformation by Using InSAR Techniques: Case Study of Babadağ (Denizli), Turkey

InSAR (Interferometric synthetic aperture radar) techniques and applications are very useful for monitoring and mapping different types of deformations such as landslides, subsidence and volcanic activities on the Earth surface. In this research, we investigated Babadag landslide (Denizli in western Turkey) by using the InSAR techniques and compare our previous research from Karaman city, central Turkey. An active landslide in Babadag county has been issued in the study by using DInSAR to monitor the vertical deformation amount. According to initial results, the application of DInSAR has not have sensitive like PSInSAR in Karaman for the recognition of subsidence. In the further step, PSInSAR technique will be applied for the Babadag landslide.

A software package for computing a regional gravimetric geoid model by the KTH method

Nowadays, 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.

Static and kinematic PPP using online services: a case study in Florida

Abstract In the not very distant past, many organisations began providing online GNSS data-processing services. Some of these services work on the differential positioning principle and some use precise point positioning. Moreover, these services provide the results for both static and kinematic modes – users only need to specify the mode of processing. In this study, both long-period static and short-period kinematic GNSS data are collected at seven NGS sites and processed using GAMIT, Track and online services. Time series from online processing services show a performance of a few decimetres for static data and sub-metre accuracy for 2 min of kinematic data.

Height biases of SRTM DEM related to EGM96: from a global perspective to regional practice

It 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 arc-degree tiles reach −5 m in the northeast and 2–3 m 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.

Multipath interference cancelation in GPS time series under changing physical conditions by means of adaptive filtering

Multipath, a highly autocorrelated signal is observable phenomena during time periods longer than the sidereal period of Global Positioning System (GPS) satellites in their constellations. Multipath pattern and sidereal repeat time interaction are examined in terms of time series correlations of topocentric coordinates using GPS code and phase observations collected at high and low frequencies. The horizontal and vertical components at 5 and 30 s sampling rates are analyzed to detect and remove multipath classified by their MP1 levels and baseline lengths. For the selected 11 stations that have 24-hour data from CORS stations in the USA and Turkey, the repeat times of the GPS constellation were identified as a function of maximum cross-correlation of adjacent time series. The multipath interference signal from two-day time series data is removed by an adaptive filter to improve time series of coordinate estimates. After the filtering process, an effective epoch based position tracking has been accomplished, especially in the height component. The position errors disappeared to maximum possible extent in the pseudo-range measurements. Up to 70% improvement for the phase derived coordinates in reflective environment has been achieved over the baselines ranging from 1 km to 175 km. For the phase observables, the filter success directly related to the multipath level which is specific to the station.