Rataj V., Galambošová J., Vašek M., 2013. Determining the guidance system accuracy with the support of a large GNSS dataset. Res. Agr. Eng., 59 (Special Issue): S65-S70.Several methods are used presently to assess the accuracy of machinery guidance systems. However, these off er a limited number of records and are time and cost consuming. As the machinery is often equipped with a monitoring system for the management purposes, these data can be used. Th e aim of this work was to develop and verify a method to determine the accuracy of the machinery guidance systems based on a large dataset obtained from the machinery monitoring system. Th e proposed method uses the transformation of global navigation satellite systems (GNSS) data into a rectangular coordinate system SJTSK (National projection system -Krovak projection). Based on the geometry principle, the ideal line can be determined, and afterwards, the off -track error of each actual position can be calculated. After the verifi cation of this method, it can be concluded that it brings benefi ts in terms of further use of the data from the monitoring systems, the estimation of the error based on a robust dataset, elimination of subjective and measurement method errors, as well as spatial localisation of the off -track errors at the fi eld.Keywords: off -track error; geometry; rectangular coordinate system SJTSK Th e machinery movement in the fi eld is an important parameter of the machinery use effi ciency. Th e research into this topic has been conducted for many years by many authors from many aspects. Among the fi rst published results on the fi eld traffi c are those by Pavlov et al. (1973) and Ondej et al. (1989). Th e problem of the machinery movement is connected with the technical level of tractors and implements. Because of the kinematics of modern tractors, it is necessary nowadays to study and model the machinery movement. Several authors deal with the aspects of machinery movement itself including the kinematics (Backman et al. 2012;Rovira-Más 2012). Th e issue of the machinery movement within a defi ned space, described by the shape and the area of a fi eld, has been solved by Hameed et al. (2010). Besides the movement of machinery for the fi eld operations, those of machinery for service operations such as e.g. fi eld transport have been analysed (Bochtis et al. 2010a).Th e analyses of the machinery movement, optimal direction of movement, algorithms and mathematical models of machinery fi eld traffi c were conducted by Bochtis et al. (2007Bochtis et al. ( , 2010a, Oksanen and Visala (2007), Bochtis and Vougioukas (2008), Yakusshev and Yakusshev (2008), Spekken and Bruin (2011) and others.