Geoportal.lt is a Lithuanian state information system whose scope is to allow all data users to freely access geographic data, maps and e-services. The portal also allows drawing relief altitude profile and set the altitude of point on location. It is performed by applying the tool “Set altitude of point on location”. It is one of the newest tools of geoportal.lt website, whose accuracy has not been analysed, and which, according to the information provided by GIS Centre, complies with the requirements set for 1:50000 scale maps. This tool is selected as an object of research, while the aim of the research is to analyse the accuracy of data provided by Lithuanian spatial information portal tool “Set altitude of point on location” in case of different land covers. The following methods of investigation have been employed: literary analysis; the analysis of cartographic material; field measurements and data processing; comparative analysis of data. The data is processed using Geomap and Microsoft Excel programmes. After the analysis has been carried out, it was established that the most common errors in all types of land covers are from 0.5 m to 1.5 m. Such errors comprise 70 per cent in forest areas, 35 per cent in built-up territory, and 53 per cent in thin land cover. Taking into consideration that the website geoportal.lt operates on the basis of orthographic map whose accuracy is 1 meter and the discussed tool shows the altitude of the nearest known point, it can be stated that the obtained presumptions are permissible. To summarise the obtained data, the tool is reliable. The reliability of the data is 91 per cent in thin land cover, 86 per cent in forest area, 75 per cent in built-up area. To compare it with topographic maps of analogous format, where the errors of altitudes may reach up to 10 meters, the tool is reliable even in case of major errors.

Land cover objects are reflected in a set of georeferential data and are constantly changing. These changes can be accurately examined by computer and interactive information systems. One of the main advantages of computer information systems is the fact that their maps can be constantly improved and updated. The update of georeferential data was conducted in a selected area using the ArcGIS software. After the analysis of the Lithuanian land cover data, the area meeting the following criteria was selected: a diverse landscape, the abundance of different objects (built-up areas, forests, bodies of water), an adjacent city and good access to major metropolitan areas. The article presents the updated georeferential data and tracks changes in the updated data of built-up areas, areas overgrown with trees and shrubs, dams, swimming pools, lakes, ponds and roads in the selected area within the period from 2008 to 2015. The results revealed that changes occurred in all analysed layers. It proves that land cover objects are constantly changing. The greatest change was observed in the data of built-up areas. In comparison with 2008, in 2015 even 41% of built-up areas was changed (i.e. the old boundaries were revised, new and defunct built-up areas were discovered), 125 new areas have overgrown with trees and bushes and 46 changes were observed in ponds and pools.