When photogrammetric techniques are used to map surface topography both high-resolution digital elevation models (DEM) and ortho-images can be produced simultaneously (Figure 1). In ortho-images, displacements resulting from the interplay between the surface relief, imaging geometry, and the exterior orientation of the imaging system have been eliminated by differential rectification, making use of the relief information of the DEM. Ortho-images thus provide the metric properties of a map and complement the continuous description of surface topography represented in the DEM.
Airborne photogrammetry is a powerful tool for monitoring mountain areas characterized by topography-related natural hazards such as landslides, avalanches, and lava flows on volcanoes. However, its use has been limited in the past due to the intensive data acquisition and processing needed to achieve high accuracy. This situation has been considerably improved by applying (1) a digital, high-resolution sensor system for photogrammetric data acquisition in combination with (2) direct sensor orientation techniques, replacing the use of abundant ground control points (GCP) with direct measurements of the sensor's position and attitude. Knowledge of the sensor orientation for the time of exposure is a prerequisite for point determination, since stereophotogrammetry derives the position of visible surface points in a three-dimensional reference system by using the metric properties of overlapping images acquired from different viewpoints.