GPS/IMU and LiDAR integration to aerial photogrammetry: Development and practical experiences with Helimap System ®

The arrival during the 90’s of the direct georeferencing technologies (GPS-inertial) and laser altimetry, combined to the emerging high resolution digital imagery, changes drastically the world of aerial photogrammetry. Those techniques widened the application field of airborne mapping: remote area mapping, corridor mapping, fast delivery mapping. Nevertheless, the commercial “high-tech” products, available on the market, focused essentially on large area mapping and nothing was designed for “local” applications and especially for complex terrain or vertical objects. Generally operated from fixed-wing aircrafts, looking downward mapping systems are not suitable for cliffs or steep slopes for example. It is now a fact; there is a huge demand for high precision mapping of small surface, where expensive commercial mapping system does not suit either in terms of economical or practical point of view. Development of Helimap System® started in 1999 in the photogrammetric and geodetic laboratories of EPFL, to address needs for natural hazard mapping (Vallet 2002) in collaboration with the Swiss federal institute For Snow and Avalanche Research of Davos. The requirements were accurate data ( 2-3 post/m 2 ) digital terrain/surface model (DTM/DSM) and high resolution orthoimagery (GSD<0.1m) either in “standard” nadir or oblique configuration. Since 2005, Helimap system® is commercially exploited by UW+R SA in all types of land management project (power lines, cliffs, avalanche, roads, railroads, landslide…), but always focused on small area (<2000ha). All those practical experiences lead in the past 3 years gave an important feedback and allowed fine tuning of the system either in operational phase or processing phase. Direct georeferencing and LiDAR measurements change drastically the airborne mapping process in positive aspects automation, high resolution but also add complexity and gaps to avoid. Thus, those several flights pinpoint the importance of the calibration step and especially the digital camera calibration stability, the procedure of alignment, and the necessary periodical ground control. The last important issue of our practice is that the imagery should never be neglected! It is the backup for everything.