3D-VISUALIZATION OF AIRBORNE LASER DERIVED ELEVATION MODEL FOR MOBILE DEVICES
U. Pyysalo, T. Sarjakoski, L.T. Sarjakoski
Finnish Geodetic Institute, Department of Geoinformatics and Cartography, Masala, Finland
Airborne laser scanning has become an efficient method for terrain measurement. It enables measurement of large areas, since scanning is carried out from the air, and it simultaneously provides 3D co-ordinates directly from the surface reflecting laser pulses. Generation of digital elevation model is one of the most common application areas of airborne laser measurement data. The objective of the presented study was to visualize a laser derived elevation model from determined data structure and combine it with vector data from other geospatial databases. Special attention was paid to the visualization on mobile devices, which typically have limited display sizes. As mobile users are moving outdoors, the light conditions are also critical for the display colours. Additionally, the orientation of mobile maps is a challenge for visualization of DEMs.
There are several alternatives to visualize 3D terrain: contour lines, horizontal profiles, hill shading, perspective views and combinations of these. Visualizations are produced based on the data structure including the digital model. The detailed nature of our laser data having point density more than one square meter, offers either a triangulated irregular network or a regular grid to be used for elevation model data structure. The TIN approach is natural, since the laser measurements are originally points that have been calculated based on the laser range measurement, its orientation and the scanner’s position. However, due to the technical solution of the laser, points are distributed over the area by a scanning mechanism instead of well determined locations, in which breaklines take place or the elevation changes. Filtering is required for this data structure to reduce the redundancy of the points in flat areas. The grid approach is more efficient in data storage point of view, but grid resolution affects the vertical approximation errors. Various levels-of-detail representations and visualizations of the elevation model on a mobile display requires generalisation.
In the case study airborne laser scanning campaign was carried out in Nuuksio test area that is a recreation area located in southern Finland. The area is challenging due to the large number of lakes, steep slopes with elevation range from 20-120 m and thick forest canopy. The paper shows a set of mobile map visualizations of Nuuksio elevation model combined with geospatial database utilizing 3D visualization methods. Finally, advantages and disadvantages of the visualizations are discussed.