The paper presents the application of a high precision positioning system ASG-EUPOS and its service NAWGEO for agricultural machines positioning. A measurement set was mounted on a cereal combine harvester and consisted of a GNSS antenna and receiver with a GSM modem for RTK corrections transfer. The positioning system was validated in a field during the 2011 harvest period in selected farms in southern and western Wielkopolska region in Poland. The total area of the field under study was 75 hectares. The quality of determining the machine’s position was monitored. It was understood as standard deviation values for longitude, latitude and altitude above the mean sea level. The hypothesis about the importance of impact of the adopted criteria on the level of changes in the recorded deviation errors was tested. Field tests show usefulness of the ASG-EUPOS network and its VRS NAWGEO service for precise positioning of agricultural machinery in dynamic conditions. The obtained data can be used to create numerical models of fields on-line, for example, in selective cereals harvesting technology, but they require filtration to remove the points affected by positioning error exceeding the acceptable value.