Working with Geographic Information Systems (GIS), geo-reference is a methodology to
give the coordinates of all objects of the system and
define the coordinate system of these coordinates.
Naturally, as the coordinate systems can be of several kinds, the transformation methods between these coordinates are also a part of this field. The objects can be of vector or raster types; in the first case, the coordinates of the vertices should be given. Working with raster datasets, the coordinates of every pixel should be defined.
The first sentence of the above paragraph is very similar to the basic exercise of the surveying. However, the GIS application supposes that the field survey has been completed, so the geo-reference is – with a very few exceptions – mostly office, computer-aided work. Besides, as it will be detailed later, the accuracy claims are often different – less – than the needs in the classical geodesy. Perhaps this is the reason, why the developing of these methods handled less important by the geodesists, albeit the methods are well known for them. However, in the GIS, the coordinate handling and conversion methods are highly needed, even if their accuracy is around one meter or even a few meters. Therefore these methods are less introduced in the literature.
The geo-reference is a crucial part of the GIS: it is the key of the uniform handling of many different input data; the key of the spatial data integration (Fig. 1). Every GIS user has already faced this problem, if his data was not in just one spatial coordinate system. I hope this book can be helpful in solving these problems correctly and exercises with the desired accuracy.
It is necessary to give here, in the introduction, the definition of the accuracy in the geo-information. It is a relative subject; in the everyday GPS practice it is mostly the one meter-few meters error, that is an acceptable level. While we work with scanned maps, it should be known that during the map making and printing process, the post-printing drying and the final scanning, the best accuracy could be around half a millimeter in the map. That’s why, in this case, the aimed accuracy of the applied methods is a function of the scale of the scanned map: at 1:10000 scale, it is 5 meters while if the map has a scale of 1:50000, it is enough to apply methods with an accuracy limit of 25 meters. In most cases, it is not only unnecessary to apply better methods as they are less cost-effective: the input data are burdened by higher errors than our precious method is optimized for.