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    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 Necla
    ASTER 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.
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    An iterative approach for inverse transformation of map projections
    (TAYLOR & FRANCIS INC, 2017) Bildirici, Ibrahim Oztug
    Map projections are given by forward transformation equations. Inverse transformation is derived from forward transformation analytically or numerically. In this paper, a numerical approach for inverse transformation of map projections is proposed, which is based on numerical differentiation and Newton-Raphson root finding method. This approach can facilitate the program developments for map projections when inverse transformation is needed. Numerical differentiation is tested with three map projections. It is seen that seven-digit precision or more can be reached. Boundary conditions and initial guess problem in inverse transformation are discussed. In terms of initial guess, map projections are divided into three categories, and appropriate initial guess values for cylindrical, pseudocylindrical, azimuthal, and conical projections in normal aspect are suggested. Newton-Raphson method with numerical differentiation is tested with 20 different map projections by using test data sets. The results show that the proposed approach is applicable if appropriate initial guess is available.
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    Quasi indicatrix approach for distortion visualization and analysis for map projections
    (TAYLOR & FRANCIS LTD, 2015) Bildirici, Ibrahim Oztug
    Tissot's indicatrix or ellipse of distortion is a diagram that is the projection of an infinitesimal circle on the original surface. It is normally an ellipse of which elongation depends on the amount of distortion caused by map projection. It provides a medium for analyzing existing projections and developing new ones. The ellipse can be scaled and depicted on the map for visualization purposes. This paper presents an alternative approach, in which the projection of a finite small circle on the sphere is used. Its projection is normally an ellipse that can be very close to Tissot's indicatrix, and is called quasi indicatrix, here. Its parameters can be derived from the forward projection equations without using partial derivatives. Therefore, it is a useful and practical approach from a programmer's point of view. The quasi indicatrix approach is also numerically tested on Aitoff-Hammer projection with a set of points. The indicatrix parameters obtained by using this approach deviate 0.5% from the ground truths at most, being the average less than 0.2%.