Gathering data for digital elevation models

The following article describes how "in-Terra GmbH" uses the AscTec Falcon 8 in their daily operation to gather data for their digital elevation models.

Introduction
Photogrammetry is a procedure to evaluate geometric properties of objects from photographic images. This technique is widely used to generate elevation models from vertically taken aerial images. Until now, these images were taken from airplanes or helicopters, which has made this procedure expensive and only attractive to map relatively large areas. By using the AscTec Falcon 8, other areas become feasible, for example the documentation of large construction sites or stockpiles. Only surfaces with very little contrast like snow covered or dark shadowed areas provide insufficient data for photogrammetric post-processing of the images.

Flight planning
A carefully planned flight is the key factor to success. Choose your flight path so that the flight system is always visible. With an image overlap of 60-80 % the stitching of the images becomes easy and only the central areas of the images are used, which result in top image quality. The flight level has to be adapted to the scale and desired resolution of the orthophoto or digital elevation model. Usual flight levels are between 50-150 m. A day with light cloud cover provides balanced illumination to the images with soft shadows.

The mission
After the planning, the mission can be started. The mission can be paused when the battery is empty. The AscTec Falcon 8 can then be directed by the operator to the ‘home' location to change batteries. After the restart, the mission can be resumed. The usual total flight time for one mission is 15-40 minutes, which can require up to two breaks. After the flight, the positions of landmarks that are clearly visible in the images are determined by precision GPS or tachymeter for later geo-referencing.

Post-processing of the images
The photogrammetric data is post-processed with proprietary software, which produces orthophotos and digital elevation models. In practice, elevation models with pixel resolutions of 0.5-2.0 m, and position height accuracy of 10-20 cm are required. Orthophotos gathered with the AscTec Falcon 8 usually have a pixel resolution of 2-5 cm. The whole process requires 2-5 working days from mission planning to the completed three dimensional elevation models. Depending on the available software infrastructure, overlays, visualization of planned civil engineering projects or geo-data for GIS or CAD programs can be generated.

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