Geo-referencing establishes a link between flight supported sensors and a local or global co-ordinate system, making direct three dimensional restitution of aerial imagery possible. Two different approaches can be used for this process:

1. Direct Geo-Referencing

In direct geo-referencing the imagery sensors (aerial camera, laser scanner, thermal scanner) are spatially orientated by means of differential GPS and a high accuracy inertial navigation system. This process considerably reduces flight time and work on the ground and in ideal cases makes pass points unnecessary while cross strips could also possibly be dispensed with. This cuts time and costs.

The Inertial Measurement Unit (IMU) linked to the imagery sensor registers at the time of image capture the acceleration and angle of the system in space. In this context the operation of line scanners is practical because every line can be referenced.

For flights at altitudes of 2,000 metres positional accuracy is around 20 cm at ground level.

2. Indirect Geo-Referencing

Indirect geo-referencing – aerial triangulation – is used for the imaging sensors (aerial cameras). Calculation of the orientation of the aerial photo strips is carried out in accordance with clear and scientifically proven methods using the products PATB-GPS and MATCH-AT.

For the calculation co-ordinates of clearly defined points are measured in the overlap area of each photo. The co-ordinates are determined by the tying together of these points in several photos, together with camera calibration parameters, the GPS recordings and flight data sensor information at the time of shutter release. The co-ordinates are then used to geo-reference and orientate the photos to one another. The relating of local and global co-ordinate systems is established through pass points which must be systematically ordered in relation to the achieved accuracy.

Conventional aerial triangulation with its visual selection of points, separation of point measurement and adjustment by means of PATB-GPS is almost completely replaced by the digitial aerial triangulation program MATCH-AT. With this processing time from inner orientation through determination of tie points and measurement of pass points to block adjustment with orientation parameters is substantially automated and speeded up.