Farzad Moradi; Ali Reza Azmoudeh Ardalan; Parham Pahlavani
Abstract
Introduction Recently, National Cartographic Center, the Organizationfor Registrationof Deeds and Properties, and alsoon a limited scale some municipalities have developed systems to provide real-time differential positioning services. Although these systems have proved to be efficient for quick mapping ...
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Introduction Recently, National Cartographic Center, the Organizationfor Registrationof Deeds and Properties, and alsoon a limited scale some municipalities have developed systems to provide real-time differential positioning services. Although these systems have proved to be efficient for quick mapping purposes in this country, they do not provide accurate differential positioning in coastal and offshore areas and thus cannot meet the needs of navigation and exploration and extraction of marine resources in oil fields. However, Iran has long maritime boundary in its south and north, and maritime economy is considered to be a priorityin its development planning. Since site selection for permanent positioning stationsis considered to be the main step of creating a real-time differential positioning system, finding the most suitable location for permanent positioning stations in the south of the country was selected as the purpose of the present study. To reach this aim, pairwise comparison matrix of the required information layers was first constructed using Delphi methodbased on the opinion of 5 experts, and in the next step, computer coding was performedin MATLAB using Fuzzy Analytic Hierarchy Process to compute the weight of each layer and sublayer.Then, layers were classified in GIS environment based on the weights obtained from the analysis of pairwise comparison matrices for each sublayer. Finally, layers were integrated usingweighted index overlay analysis methodto select optimal sites for permanent stations based on the weights obtained for each layer. Details of the calculations and the results are presented in the article. Materials and Methods High efficiency of analytichierarchyprocess and spatial information systems in management and analysis of spatial data have led to the creation of a highly efficient environment in which various stages of different analysis such as site selection for permanent GNSS stations can be performed. One of the advantages of this procedure is that the analysis can beupdated in the shortest possible time and the result can be depicted visuallyat any stage of decision makingwith a simple changing of the values (weights) of each input data based on the expert opinion. Thisgreatly impacts experts' understanding of changes in the studyarea. Accordingly,fuzzy analytichierarchyprocess method is used within the GIS environment in the present study. Results and Discussion The present study addresses the issue of site selection for permanent GNSS stations. In the first step,pairwise comparison matrix was created for the criteria and sub-criteria and filled in by 5 experts. Then, layers were classified in GIS environment based on the weights obtained for each sub-layers of pairwise comparison matricesand the codes written in MATLAB. Finally, suitable locations for permanent GNSS stations were obtainedby integrating the layers usingweighted index overlay. Conclusion The present study has provided the results of optimal site selection for GNSS permanent stations. These selected sites meet the needsofprecise positioning in the coastal areas of the country and can be used in navigation and exploration and extraction of marine resources and oil fields. Afterthe selection of southern coasts as the study area, 7 criteria (proximity to urban areas and facilities, slope, distance from faults, distance from access roads, soil type, distance from rivers and distance from railways) were selected based on the expert opinion. A pairwise comparison matrix was createdfor these criteria and sub-criteria and 5 expert experts were consulted in this regard. Expert opinions were analyzed using codes written in MATLAB software andFuzzy Analytic Hierarchy Process method and thus, the weight of each criterion and sub-criterion was obtained. These weights were then integrated using the geometric mean method and the final weight of each layer and sublayer was determined. Using Arc map software, these weights were applied to different layers and sublayers, and finally, optimal locations for permanent GNSS stations were divided into 5 classesof very good, good, medium, bad, and very bad stations. Good and very good classes can be considered as optimal places forcontinuously operating reference stations.
Roohollah Karimi; Ali Reza Azmoude Ardalan; Siavash Yousefi
Abstract
Introduction
Components of verticaldeflection, i.e., North-South component and East-West component ,are used for accurate determination of geoid or quasigeoid. Moreover, vertical deflection components area useful source for determination of variations in subsurface density and geophysical interpretations. ...
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Introduction
Components of verticaldeflection, i.e., North-South component and East-West component ,are used for accurate determination of geoid or quasigeoid. Moreover, vertical deflection components area useful source for determination of variations in subsurface density and geophysical interpretations. Generally, there are two definitions for verticaldeflection. According to Helmert definition, vertical deflection at any given pointis the angle between the actualgravity vector (actual plumb line) and a line that is normal to the reference ellipsoid(a straight line perpendicular to the surface of reference ellipsoid). Another definition of vertical deflection is proposed by Molodensky. According this definition, vertical deflection at any given point is the angle between actualgravity vector and normal gravity vector (normal plumb line). Some relations have been introduced to convert Molodensky vertical deflection to Helmert vertical deflection. Helmert vertical deflection is estimated using astrogeodetic observations (combination of astronomical and geodetic observations).
Presently, global geopotential models (GGMs) have been expanded to the degree of2190, which is equivalenttoabout 5-min spatial resolution. Vertical deflectionat any point on the Earth can be calculated using the GGM. The resulting vertical deflection is consistent with Molodensky definition.Unfortunately, accuracy of GGMs is not sufficient for estimation of verticaldeflection.In other words, since GGMs are expanded up to a limited degree due to their resolution, omission error(or truncation error) occurs in computation of the earth’s various gravity field functionals, such as the geoidal height and verticaldeflection. Combining GGM with a digital terrain model (DTM) is a method used to reduce omission error.It should be noted that DTM has a higher spatial resolution as compared to GGM.In this method, the omitted signals of GGM can be modeled using residual terrain model (RTM) derived from subtracting high resolution DTM from a reference smooth surface. The reference smooth surface is obtained from eitherapplying average operator to DTM or expanding global topography into spherical harmonics. Fortunately, DTMs with spatial resolution of 3seconds or more,and reference smooth surface based on 2190 degree spherical harmonics are publicly available.
The present study seeks to assess vertical deflectionderived from a combination of GGM and DTM in Iran. Previously, Jekeli(1999) has studied EGM96 geopotential model with the aim of computingvertical deflection in the USA. Hirt(2010) and Hirt et al. (2010a) have assessed vertical deflection in Europe and the Alps using a combination of EGM2008 and RTM models.In Iran, GO_CONS_GCF_2_TIM_R4, a GOCE-only model, and EGM2008 geopotential model have been used toobtain vertical deflection and the results have been evaluated byKiamehr and Chavoshi-Nezhad(2014).
Materials & Methods
To implement the present study,a EGM2008 model with a spatial resolution of about 5-min is selected asGGM and a SRTM model with 3-sec spatial resolution is considered as DTM. To obtain RTM, DTM2006 model based on2190 degree spherical harmonicsis selected as the reference smooth surface.To compute the residual topography effect, prism method was used in an ellipsoidalmulti-cylindrical equal-area map projection system. First, we compute vertical deflectionusing EGM2008 model. It is also calculated using a combination of EGM2008 model and RTM(EGM2008/RTM method). In the next step, vertical deflection derived from the first method (EGM2008 model) and the second one (combination of EGM2008 model and RTM) are compared with vertical deflectionderived from astrogeodetic observations in 10 available Laplace stations in Iran.
Results & Discussion
Results indicate that there is a 1.2sec difference between North-South component of vertical deflection (i.e.) obtained from EGM2008 model and astrogeodetic observations.With RTM, this will reach 1 sec, which shows a 15% improvement. Moreover, there is a5.7secdifference between East-West component of vertical deflection () obtained from EGM2008 model and astrogeodetic observations, while this value will reach 5.6sec using RTM. Improvement in East-West component () is1.4%, which is smaller than the improvement of North-South component (). Based on the computations, we found that values of and in the Laplace stations canreach 17sec (RMS=7sec) and 15sec (RMS=8sec), respectively. Therefore, it is concluded that the relative error ofNorth-South component ()computation using EGM2008/RTM method is about 6% and the relative error ofEast-West component ()computation is about 37%.
Conclusion
The present research has studied the RTM effect on the improvement of GGM used for the determination of vertical deflectionin Iran. To performthe study, EGM2008 model with around 5-min spatial resolution was selected as GGM. RTM is also derived from subtracting the DTM2006 model (based on2190 degree spherical harmonics)from the 3-sec spatial resolutionSRTM model. Numerical findings indicate that a combination of RTM and GGM can improve the results of vertical deflectioncomputation, as compared to the results obtained from GGM-only approach. The improvement in North-South component of vertical deflection () is about15%and East-West component of the vertical deflection () undergoes about 1.4% improvement. In general, EGM2008 model and its combination with RTM have been more successful in the computation of component as compared to computationin the geographical region of Iran. There is no clear explanation for this difference, but it can be due to errors in theastronomical or geodetic observations oflongitude in Laplace stations.
Naser Abdi; Ali Reza Azmoude Ardalan; Roohollah Karimi
Abstract
Extended Abstract
Precise Point Positioning (PPP) is a technique to determine the position of a single receiver using un-differenced dual-frequency code and carrier phase observations. In this technique, the precise satellite orbit and clock products obtained from the GPS reference station network ...
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Extended Abstract
Precise Point Positioning (PPP) is a technique to determine the position of a single receiver using un-differenced dual-frequency code and carrier phase observations. In this technique, the precise satellite orbit and clock products obtained from the GPS reference station network are also required. Unlike the relative positioning techniques, the network needed for PPP is not necessary to be dense, and even a sparse network with long baselines like the International GNSS Service (IGS) network can be used. The IGS collects, archives, and distributes GPS observation data sets of sufficient accuracy to satisfy the objectives of a wide range of applications and experimentation. These data sets are used by the IGS to generate the data products which are made available to interested users through the IGS website. Moreover, in contrast to the relative positioning techniques, PPP can provide a uniform accuracy throughout the world without having the reference station observations. In the last decade, PPP has been widely used for the static and kinematic applications. The use of this technique in various applications requires to know its accuracy, processing software requirements, and performing methods. The aim of this paper is to study the performance of PPP by using the static and kinematic observations in comparison with the double difference relative solutions. For this purpose, the static observations of four dual-frequency receivers within Iranian Permanent GNSS Network (IPGN), namely AHVA, SFHN, SNDJ and TORQ, and the kinematic observations of GPS receiver installed on airplane were processed in the PPP and double difference relative solutions by the Bernese GNSS software version 5.0. The Bernese GNSS Software is a scientific, high-precision, GNSS data processing software developed at the Astronomical Institute of the University of Bern (AIUB). It is, e.g., used by Center for Orbit Determination in Europe (CODE) for its international (IGS) and European activities. In the double difference relative solution, the coordinates of 10 IGS stations in ITRF2008, which have been located around Iran, have been chosen as the weighted constraints, where the accuracy of constraints for horizontal and vertical components has been taken equal to 1 mm and 2 mm, respectively. The double difference relative results are assumed as reference values for comparisons. To find the optimum time interval of PPP for obtaining the accuracy better than 10 cm in the horizontal and vertical components, the various sessions have been taken in to account. The GPS station observations of each session are separately processed by the Bernese software in the PPP mode regarding the required parameters such as solid earth tide, ocean tidal loading, windup, antenna phase center offsets and variations for satellites and receivers, and satellite Differential Code Biases (DCBs). Then, the double difference relative results as reference values are subtracted from the obtained PPP results in X, Y and Z coordinates. To show the performance of PPP in both of horizontal and vertical components, the coordinate differences from Earth Centered Earth Fixed (ECEF) reference frame are transferred to the Local Geodetic (LG) reference frame in order to provide Northing (N), Easting (E) and Up (U) coordinates. From the PPP static results, we find that the minimum required time interval of the GPS observations is one hourin order to obtain the accuracy better than 10 cm. For assessment of the PPP performance in kinematic mode, the GPS observations collected by mounted GPS receiver on airplane are processed in relative and PPP modes. The duration of these observations is about 6 hours. In the relative kinematic processing by the Bernese software, the observations of 4 GPS reference stations within IPGN and IGS precise satellite orbit and clock products are used. The outputs of this step are three coordinates of GPS antenna mounted on airplane in 30-second epochs, which are considered as reference values. Like the static mode, the reference values are subtracted from the PPP kinematic results in X, Y and Z coordinates and transferred to the LG frame. The results show that the accuracy better than 10 cm and 20 cm can be obtained using the PPP kinematic technique in the horizontal and vertical components, respectively. These accuracies are enough for many applications such as hydrography, aerial photogrammetry and navigation. As a result, this study shows that the PPP technique can be an adequate alternative for the relative techniques.
Hamid Reza Ranjbar; Ali Reza Azmoude Ardalan; Hamid Dehghani; Mohamad Reza Serajeyan; Ali Alidousti
Abstract
Earthquake is one of the most catastrophic natural disasters to affect mankind. One of the critical problems after an earthquake is building damage assessment. The area, amount, rate, and type of the damage are essential information for rescue, humanitarian and reconstruction operations in the disaster ...
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Earthquake is one of the most catastrophic natural disasters to affect mankind. One of the critical problems after an earthquake is building damage assessment. The area, amount, rate, and type of the damage are essential information for rescue, humanitarian and reconstruction operations in the disaster area. On the other hand, to deal with the situation requires well organized and effective emergency planning. How quickly the event is responded and how efficiently response activities are managed are the main determinants of the overall costs of a disaster, both in terms of economic damages and fatalities. Remote sensing techniques play an important role in obtaining building damage information because of their non-contact, low cost, wide field of view, and fast response capacities. Now that more and diverse types of remote sensing data become available, various methods are designed and reported for building damage assessment. This paper provides a comprehensive review of these methods based on using optical images in three categories: mono, multi temporal and combination of images and vector map approach and also implements an automatic damage assessment method of buildings using high resolution satellite images and GIS layers. In this method, after extracting texture features of candidate buildings from both pre- and post-event images and defining optimized features, a neurofuzzy inference system was designed that determines buildings to four damage levels: Undamaged, Moderate damaged, Heavy damaged and Destroyed levels. Evaluation results show that the designed system has the overall accuracy of 89% in classifying buildings to the four damage levels.
