Influence of Replacement Used Reference Coordinate System for Georeferencing of the Old Map of Europe

The article describes the effect of the replacement of the used reference coordinate system in the georeferencing of an old map of Europe. The map was georeferenced into three types of projection – the equal-area conic (original cartographic projection), cylindrical Plate Carrée and cylindrical Mercator map projection. The map was georeferenced by means of the affine and the second-order polynomial transformation. The resulting georeferenced raster datasets from the Plate Carrée and Mercator projection were projected into the equal-area conic projection by means of projection equations. The output is the comparison of drawn graphics, the magnitude of standard deviations for individual projections and types of transformation. Keywords—Georeferencing, reference coordinate system, transformation, standard deviation. I. PROBLEMATIC ASPECTS IN GEOREFERENCING HE georeferencing of old maps is a process consisting of a series of individual acts. The process of digitizing map data usually does not put much emphasis on georeferencing, and georeferencing itself is subject to numerous errors. Frequent errors occurring during georeferencing are pointed out by [3]. The process of georeferencing old maps is described by [11], [10], and [12]. Provided we want to georeference an old paper map, we must first scan it. Scanning is done on large scale calibrated scanners. A map is usually scanned with a resolution of 400 DPI. Higher resolutions mostly only bring an increase in the volumes of data. The risk associated with the choice of a lower resolution, on the contrary, is losing map details. In the case of a resolution of 400 DPI, the size of one pixel is approximately 0.06 mm and with regard to the minimum width of a line on the map of 0.1 mm this resolution is sufficient. The next step is to locate the scanned map into a reference coordinate system. Ground control points are identified on the scanned map for which coordinates in the respective reference coordinate system are available. This study addresses the issues of the influence of replacement used reference coordinate system for georeferencing of the old map of Europe [8]. In practice, this means that we have scanned an original old map in a specific reference coordinate system and are trying to transform it into a map located in another reference coordinate system by means of ground control points. In the case that the coordinate systems used on both maps are Ing. Jakub Havlicek and Doc. Ing. Jiri Cajthaml, Ph.D. are with the Czech Technical University in Prague, Faculty of Civil Engineering, Department of Geomatics, Thakurova 7, 166 29, Prague (e-mail: [email protected], [email protected]). different, the results of the transformation are affected without accuracy of ground control points. It is important to obtain as much information about the scanned map as possible, especially regarding the used reference coordinate system, including all parameters of the cartographic projection. The possibilities of determining the used reference coordinate system were studied in more detail by [1], [2], and [4]. This study develops the influence of a cartographic projection on the results of georeferencing for conic projection and cylindrical projections for area of Europe. After obtaining information about a map and determining ground control points, the next important step is to choose the type of transformation method. There are two types of transformation methods used in practice. The first of them are global transformation methods by means of which one transformation key is calculated from all ground control points using the Least Squares Method. This key is applied to the whole area of the map, and therefore, ground control points do not have their position in the final result as some residuals arise at these points in the case of redundant ground control points. The second group of used transformation methods is local methods. A unique transformation key is calculated for each location on the map. Ground control points have their position in the final result in this method. A considerable disadvantage is that in the case of a wrongly determined ground control point, this point cannot be identified and a relatively large distortion of the map arises in its vicinity. The following step usually involves saving information about georeferencing. Currently, there are three saving options used. The first of them is a world file where information is stored in an auxiliary file to a raster file by using six digits – the pixel size in the x and y direction, rotation about the x and y axes, and the coordinates of the upper left corner of the raster. The disadvantage of this type of storage is that only global transformation methods, namely identity, similarity and affine transformation, can be saved in these six values. While using higher order polynomial transformations or local transformation methods, the results can no longer be stored using six unknown parameters. Another option is to store information in an XML file where information about the reference coordinate system, ground control points, transformation type, etc. is also stored. This type of storage is the best considering the usage. Unfortunately, there is currently no single XML for different geographical information systems, and for example, the ESRI Company uses its own AUX.XML file. The third option is resampling the georeferenced raster. In this case, a new raster arises which Influence of Replacement Used Reference Coordinate System for Georeferencing of the Old Map of Europe Jakub Havlicek, Jiri Cajthaml T World Academy of Science, Engineering and Technology International Journal of Environmental, Chemical, Ecological, Geological and Geophysical Engineering Vol:9, No:6, 2015 759 International Scholarly and Scientific Research & Innovation 9(6) 2015 scholar.waset.org/1999.6/10002015 In te rn at io na l S ci en ce I nd ex , G eo m at ic s an d C iv il E ng in ee ri ng V ol :9 , N o: 6, 2 01 5 w as et .o rg /P ub lic at io n/ 10 00 20 15 has always a worse resolution than the original as the pixel values are derived from the surrounding pixels using a variety of methods. The disadvantage of this option is the loss of information on ground control points. For more details on the storage possibilities of the results of georeferencing see publications by [6], [7]. II. INFLUENCE OF REPLACEMENT USED REFERENCE COORDINATE SYSTEM USED FOR GEOREFERENCING OF AN OLD MAP OF EUROPE As mentioned above, the principal focus of the article is the effect of the replacement of the used reference coordinate system in the georeferencing of an old map of Europe. Based on cartographic experience and measurements on the map frame, it was identified that the old map of Europe had been produced in the equal-area conic projection (Albers equal-area projection) with one standard parallel and the prime meridian passing through the map centre. This map was georeferenced using three types of reference coordinate systems. The map was displayed in the equal-area conic projection (Albers), the cylindrical Plate Carrée projection and the cylindrical Mercator projection. The georeferencing applied the affine and the second order polynomial transformation. The information on the reference coordinate system used was assigned to the raster dataset. The georeferenced raster dataset was projected into the correct reference coordinate system – the equal-area conic map projection (Albers). All transformations were supplied with transformation protocols with standard deviations, computed in the external Alltran programme, developed by the Department of Special Geodesy of the Faculty of Civil Engineering, Czech technical University in Prague [9]. In addition, standard deviations for cylindrical projections were identified in case the map did not contain any positional deviations for the equal-area conic projection. A. Conic Cartographic Projection Albers Projection (Conic Equivalent Projection) Albers projection is the most used for maps lying between the Equator and the Pole. Conic equivalent projection or conic equal-area projection for whole world is displayed in Fig. 1. In mathematical terms, this projection may be written as (1): )). ( cos(sin sinU 2sinU 2 + cot R cotU R Y )), ( sin(sin sinU 2sinU 2 + cot R = X