Keywords: ASTER, SRTM, Digital Elevation Model, GPS-Levelling data, Validation.

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1 ACCURACY ASSESSMENT OF ASTER AND SRTM DIGITAL ELEVATION MODELS: A CASE STUDY IN TURKEY H. Tugba Arli IL 1 Dr. R. Alpay ABBAK 2 1 Ministry of Health - Turkey 2 Selcuk University - Turkey ABSTRACT Digital 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 Spacebased 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. Keywords: ASTER, SRTM, Digital Elevation Model, GPS-Levelling data, Validation.

2 17 th International Multidisciplinary Scientific GeoConference SGEM 2017 INTRODUCTION Developments in remote sensing technologies provide new possibilities for digital height data. It has become a need in today's world to determine the Digital Elevation Models as quickly and reliably as. Apart from geodetic, photogrammetric and remote sensing methods, Digital Elevation Models can also be constructed from topographic maps providing high position accuracy. In this method, numerical data is obtained by digitizing the maps. Although the digitization of maps is automatic, it often requires manual intervention of problems. This disadvantage means a large amount of time, which requires intensive effort as well as high cost. However, the application areas of conventional geodetic surveying techniques are limited depending on the physical conditions. Different height measurement methods have been developed using a wide variety of measuring equipment in some techniques that require laborious and intensive work. Methods such as precise leveling, global positioning system, stereo SAR, and InSAR (Interferometric SAR) are used to determine the temporal variation and spatial distribution of elevation. The nature of the altitude data, the accuracy expectations, the economic and other factors that are influential affect the choice of geodetic technique to be applied. Another important advantage of these techniques is that data collection is not required to be in the region and work locally. Digital Elevation Model (DEM) is used in many applications such as earth sciences, natural resources management, engineering projects, military applications, threedimensional visualization. The accuracy of the produced digital elevation model is important for different applications. Therefore, the investigation of the accuracy of the Digital Elevation Models produced remains important as a current field of study. In this study, it is aimed to determine the accuracy of height values obtained from SRTM (Shuttle Radar Topography Mission) and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) Models and to find out which height data has better precision. GLOBAL DIGITAL ELEVATION MODEL The elevation data of many regions are in the form of contour a map on the earth. Height values which are located on the maps with contour lines and irregular points, are transferred to the computer in the digital format and these files are converted to regular grid points. Grid points consist of a square shape covering the entire map surface, containing the height information of the coordinates in which they are located. The map created by these points is called grid map. The result is a model called the digital elevation model, in which the heights can be obtained as matrix points equally spaced horizontally and vertically [7]. The Global Digital Elevation Model is a suitable structure for showing the surface of the earth continuously changing in topography. This model is a general data source for terrain analysis and other 3D applications. In largescale standard topographic maps, it is desirable to keep the point density below roughly 30 m in Turkey. Considering the above criteria, a DEM which is between parallel and meridians for Turkey with a resolution of 1" 1" ( 30 30) consists of about 1.4 billion grid points. The width and intensity data corresponding to a field in Turkey width can be gathered by satellite techniques only in today's conditions. In this context, SRTM and ASTER are among the examples that can be given to GDEM Global Digital Elevation Models.

American National Aeronautics and Space Administration (NASA).

3 SRTM Model Shuttle Radar Topography mission (SRTM) is a joint project between US National Geographic Intelligence Agency (NGIA), German and Italian Space agencies under the leadership of the American National Aeronautics and Space Administration (NASA). The aim of the project is to create a high-resolution Digital Elevation Model of 80% of the world's surface (area between 60 degrees north and 50 degrees south latitudes) from radar images taken with the aid of a space shuttle. With the project, a section almost all of the ground except for the regions close to the poles was selected as a target and aimed at establishing a DEM that has reached the highest level of integrity in this field by then. Figure 1. SRTM Coverage Map (URL-1) The SRTM project was carried out by the Space Shuttle Endeavor on February Continuously collect data by viewing 99.97% of the earth in the mission area at least once (94.59% at least twice, 50% at three or more) with a 10-day orbital flight. Figure 1 is scanned area on the world map and the number of times it is displayed depending on the colors. The red areas on the map represent the not displayed land area. The areas covered by ocean waters are not taken into account (JPL, 2008). Figure 2. Space Shuttle used for SRTM (URL-2)

4 17 th International Multidisciplinary Scientific GeoConference SGEM 2017 SRTM equipment to obtain two radar images taken from different locations consists of two radars attached to one under the shuttle and the other is attached to a shuttle and attached to a 60 m long bar extending out from the shuttle (Maathuis, 2004). SRTM heights are published on NASA servers in 3 different resolutions which are 1 grid interval (~30 meters resolution), 3 (~90 meters) and 30 (about 1 km). The SRTM1 is in hgt format with geographic lat/long coordinates and a 1 arc-second (30 m) grid of elevation postings. It is referenced to the WGS84/EGM96 geoid. Accuracies for this global product were ± 16 meters for vertical data. ASTER Model Terra satellite which is part of NASA's Earth Observing System (EOS), was launched from Van Der Berg Air Base in California in December 1999 and began operations in February There are five different modules on the satellite, ASTER, MODIS, CERES, MOPITT and MISR. It is generated from data collected from Advanced Space Based Thermal Emission and Reflection Radiometer (ASTER) module, a spaceborne earth observing optical instrument with single high resolution and superimposed image capture capability. ASTER is the only high-resolution device on the Terra platform. ASTER module is an important device in that it serves as a zoom lens for other Terra devices at change detection, calibration/validity. It was produced in partnership with American NASA, the Japanese Ministry of Trade and Industry and scientific and industrial organizations of both countries (Abrams vd., 2003). Today, it is still operated by NASA Earth Observing System (EOS), Ministry of Economy Trade and Industry of Japan (METI) and Japan Earth Remote Sensing Data Analysis Center (ERSDAC). Table 1. General information about Terra Repeat Time 16 Day Revolution in a Day 14 Elevation Orbital Period 705 km min. Inclination 98.3 o Image capture is done only by ASTER'S VNIR sensor from Terra satellite. ASTER has 14 bands in total. For the 14 bands, which serve different purposes, the resolution values change to 15, 30 and 90 m. The first 3 bands of the ASTER suit are referred to as VNIR (Visible Near Infrared) bands and include visible and near infra-red spectral range. The spatial resolution is 15 m in the horizontal plane. The ASTER GDEM covers land surfaces between 83 N and 83 S and is composed of 22,600 1 Ï1 tiles. The ASTER GDEM is in GeoTIFF format with geographic lat/long coordinates and a 1 arc-second (30 m) grid of elevation postings. It is referenced to the WGS84/EGM96 geoid. Pre-production estimated accuracies for this global product were 20 meters at 95 % confidence for vertical data and 30 meters at 95 % confidence for horizontal data (URL-3).

5 NUMERICAL ANALYSIS Study Area The study area covers approximately 70,000 km² which is between Northern latitudes and Eastern longitudes (the provinces of Konya, Niğde, Aksaray and Karaman). Heights in the area are ranging from 50 m in the flat area to 3000 m in the mountainous areas (Fig. 3). This study area is an ideal study area in terms of containing different types of land cover and usage including water surface, urban, mountainous and agricultural areas. Figure 3. Study area Data Used As it is well known, the users should be aware of the accuracy of any model before use it in the project area. Accuracy is analyzed by reference data (ground truth). The reference data to be used in this study are 3074 GPS-Levelling heights with known positions provided by the Map General Command. Vertical datum is determined

6 17 th International Multidisciplinary Scientific GeoConference SGEM 2017 according to Antalya Tide gauge station. ASTER and SRTM1 data are available free of charge to users worldwide via electronic download. ASTER Model which global vertical accuracy is in the range of 7-14 meters and SRTM1 Model which Global vertical accuracy is 16 meters. Data can be downloaded from national agency web addresses. Table 2. General characteristics of SRTM1 And ASTER Data General Characteristics of Data Source SRTM1 ASTER Generation and distribution METI/NASA NASA/USGS Release year 2003 ~ 2009 ~ Temporal Extend 11 days (in 2000) 2000 on going Coordinate System Geographical latitude and longitude Horizontal Datum WGS84 WGS84 Vertical Datum EGM96 EGM96 Geographic Dimension 1 x 1 1 x 1 File Formats hgt Geotiff Geographical latitude and longitude Posting interval 1 arc-second (30 m) 1 arc-second (30 m) DEM accuracy ± 16 m 7 14 m Coverage Area of missing data 60 degrees north ~ 56 degrees south Topographically steep area (due to radar characteristics) 83 degress north 83 degrees south Areas with no ASTER data due to constant cloud cover (supplied by other DEM) Comparison In this study, after the ASTER and SRTM DEMs of 3074 geographical coordinates are transferred to the Global Mapper program separately with the required interpolation calculations, the heights obtained from the ASTER and SRTM models were compared with the GPS-Nivelman heights and the accuracy analyzes were made with respect to each other. The software generates difference maps by calculating the differences between height values with the same x, y coordinates for two height data and provides statistical information. The most important feature of this program is the use of many height data and vector data without the need to merge them together. The statistical data calculated according to the heights obtained from the ASTER and SRTM1 Models with reference to the control data are shown in Table 3.

7 Table 3. Statistical summary [m] Elevation Data Number of Points Min. Max. Median Mean Δh /n RMSE SRTM ASTER The outliers in the observation were determined by the 3-sigma rule. Outliers were eliminated then accuracy assessments were performed with 3019 points for SRTM1 and 2992 points for ASTER. The accuracy of SRTM1 is 1.06 times more accurate than that of ASTER in Turkey. CONCLUSION This study aimed to determine the accuracy of ASTER and SRTM1 against to a ground truth data in Turkey. In this context the heights of 3074 data, of which geographical coordinates are known, are interpolated from both DEMs and compared with GPSleveling height. According to statistical tests, the root mean square error of differences obtained from ASTER is ± m, and the root mean square error of differences obtained from SRTM1 is ± m. According to these results, the accuracy of the SRTM1 is times more accurate than that of the ASTER. If we assume that the leveling data in our hand is a few meters defective, the accuracy achieved for SRTM1 and ASTER remains within global accuracy limits. From the results of this study, it can be seen that the height data obtained from both Digital Elevation Models can be preferred in studies using scale maps of 1/ and above. REFERENCES [1] Abrams, M., Hook, S., Ramachandran, B., 2003, ASTER User Handbook Version 2. [2] Maathuis B., 2004, DEM from Active Sensor SRTM, WRS [3] JPL, The Shuttle Radar Topography Mission. Jet Propulsion Labratory, California Institute of Technology, , [4] URL [5]URL-2 [6] URL-3 [7] Venkatachalam, P,. Mohan, B.K., Kotwal, A., Mıshra, V., Muthuramakrıshnan, V., Pandya M., Automatic Delineation of Watersheds for Hydrological Applications Proc. ACRS nd Asian Conference on Remote Sensing, 5-9 November 2001, Singapore. Vol. 2, pp

ACCURACY ASSESSMENT OF ASTER GLOBAL DEM OVER TURKEY

ACCURACY ASSESSMENT OF ASTER GLOBAL DEM OVER TURKEY E. Sertel a a ITU, Civil Engineering Faculty, Geomatic Engineering Department, 34469 Maslak Istanbul, Turkey sertele@itu.edu.tr Commission IV, WG IV/6