Position determining unit for a land or sea based object, said position determining unit comprising or having access to a three dimensional map comprising three dimensional geo-referenced position data, said position determining unit further comprising map part selector means for selecting a part of the three dimensional map so as to obtain a geo-referenced position associated to the selected part, wherein the map part selector means comprises a user interface for manually selecting the part of the three dimensional map, at least one measurement instrument arranged to obtain bearing and/or distance information comprising at least two measurement values or at least three measurement values related to the land or sea based object, and a computing and control unit arranged to relate each obtained bearing and/or distance information to a corresponding obtained geo-referenced position and to determine a geographical position of the land or sea based object - in two dimensions based on the bearing and/or distance information, comprising at least two measurement values, and the corresponding obtained geo-referenced position(s), or - in three dimensions based on the bearing and/or distance information, comprising at least three measurement values, and the corresponding obtained geo-referenced position(s), wherein the computing and control unit is arranged to receive second position information related to the land or sea based object from a second source being a radio based global positioning system, e.g. GPS, to determine a difference between the second position information obtained from the second source and the determined geographical position of the land or sea based object, and - to detect an uncertainty in the obtained second position information when the difference exceeds a predetermined value. map part selector means for selecting a part of the three dimensional map so as to obtain a geo-referenced position associated to the selected part, wherein the map part selector means comprises a user interface for manually selecting the part of the three dimensional map, at least one measurement instrument arranged to obtain bearing and/or distance information comprising at least two measurement values or at least three measurement values related to the land or sea based object, and a computing and control unit arranged to relate each obtained bearing and/or distance information to a corresponding obtained geo-referenced position and to determine a geographical position of the land or sea based object - in two dimensions based on the bearing and/or distance information, comprising at least two measurement values, and the corresponding obtained geo-referenced position(s), or - in three dimensions based on the bearing and/or distance information, comprising at least three measurement values, and the corresponding obtained geo-referenced position(s), wherein the computing and control unit is arranged to receive second position information related to the land or sea based object from a second source being a radio based global positioning system, e.g. GPS, to determine a difference between the second position information obtained from the second source and the determined geographical position of the land or sea based object, and - to detect an uncertainty in the obtained second position information when the difference exceeds a predetermined value. - in two dimensions based on the bearing and/or distance information, comprising at least two measurement values, and the corresponding obtained geo-referenced position(s), or - in three dimensions based on the bearing and/or distance information, comprising at least three measurement values, and the corresponding obtained geo-referenced position(s), wherein the computing and control unit is arranged to receive second position information related to the land or sea based object from a second source being a radio based global positioning system, e.g. GPS, to determine a difference between the second position information obtained from the second source and the determined geographical position of the land or sea based object, and - to detect an uncertainty in the obtained second position information when the difference exceeds a predetermined value.