Driven by technological developments, miniaturisation and the cost-efficient construction of sensors such as GNSS-receivers, inertial measurement units, magnetometers and barometers, so called unmanned aerial vehicles (UAVs) are increasingly utilised in the field of close range airborne photogrammetry. Those UAVs might have the potential to close the gap between conventional terrestrial measurement methods and complex manned missions in aerial photogrammetry. In the context of this thesis, extensive investigations on the direct geo-referencing of aerial images, i.e. the determination of the current position and orientation of images in 3D-space, were carried out using a particular UAV manufactured by the company twins.nrn. The main goals of this thesis were the continuation of previous work carried out at the research group photogrammetry of the Department of Geodesy and Geoinformation at the Vienna university of Technology on the one hand, and to improve the accuracy of direct geo-referencing on the other hand. By implementing a dual-frequency GNSS receiver on the UAV, the current position at the exposure time of the images should be determined at an accuracy level of several centimetres. The main advantage of direct geo-referencing resides in the possibility to reduce the number of required control points on the ground, which are difficult to measure especially in impassable or forested areas, or even to omit them completely. The elements of the exterior orientation can serve as additional observation within the bundle block adjustment in the case of insufficient numbers of control points and to avoid the deformation of image bundles. The application for UAVs in the field of direct geo-referencing for the mapping of linear infrastructure facilities (such as rails) describes a second goal of the thesis. Therefore, the possibility to survey a railroad line in Herzogenburg (Lower Austria) was tested at flight altitudes of 40 and 70 metres in cooperation with the ÖBB-Infrastruktur AG, Department of Research and Development and Department of Surveying and Data Management. For the integration of the synchronized measurement data an extended Kalman filter was applied. Hereby, not only the parameters of the direct geo-referencing, but also the biases of the inertial measurement unit were estimated. The quality of the method could be verified by analyzing the differences between the positions and orientations that were calculated directly on the one hand, and indirectly derived with the useage control-points on the other hand. The resulting standard deviations for both flights are in the range of <1 for the roll and pitch angles, <2 for the yaw angle and <3cm for the 3D position. In a further step, the impact of the accuracy for direct geo-referencing on ground-based object points was evaluated and discussed. It could be shown, that uncertainties within the interior camera orientation strongly affect the coordinates of the object points in the range of up to 12 centimetres for the position components, and up to 45 centimetres for the height component. The consideration of some control-points within the bundle-block adjustment cause an obvious reduction of the differences, and standard deviations in position and height of <2cm could be achieved. With regard to the high demands on the accuracy, the indirect geo-referenced aerial images were used for a photogrammetric evaluation of ground objects and compared to a conventional terrestrial survey that was carried out by the company Senftner Vermessung ZT GmbH. The comparison revealed that the evaluation of railway lines is possible with standard deviations of 1-2cm. As a by-product of the photogrammetric evaluation, a high resolution orthophoto suited for documentation or planning purposes could be derived. After a short introduction, infrastructure objects are described in general terms before focussing on railway lines in more details due to the cooperation with the company ÖBB (Austrian Federal Railways). Chapter two gives an overview on UAVs in general, the associated regulatory framework and deals with the description of the applied UAV in form of a hexacopter. In the third chapter, the focus is laid on direct geo-referencing, integrated navigation systems and the employment of Kalman filtering in this context. Data synchronization, camera calibration and first test-scenarios are depicted in chapter four, followed by a description of the surveyarea, flight planning and the flight mission in chapter five. In chapter six, the evaluation of the measurement data and the quality assessment of direct geo-referencing are presented. A complete summary of the thesis can be found in chapter seven.