|
Photogrammetry and airborne laser scanning are basic data acquisition techniques for generation of digital terrain models used for GIS analysis. This study focuses on the question of the suitability of these DTM creation methods for construction design purposes. According to the technical regulations such models have to be highly precise and should be characterized by economical and at the same time terrain representative sampling. Comparative study has been conducted between photogrammetry and airborne laser scanning. Test area is stretched along the typical rail route with slopes covered with dense vegetation and the agricultural or wooded surroundings. Both the photogrammetric and ALS data were very good quality. The photographs were taken with a digital camera with a pixel size of 5 cm, scanning data were acquired by full-waveform laser scanner with point density of 11 pkt/m2. Automatic matching was used for 3D photogrammetric measurements followed by necessary manual checks and final points adjustment as well as additional stereoscopic point-line measurement. ALS data were filtered based on active TIN model algorithm, which resulted with a subset of the point cloud representing the topographic surface area. Studies have confirmed better quality of the ALS-based DTMs in comparison with the photogrammetry-based digital terrain models. This superiority refers to both the accuracy of modes and the measurement ability in areas covered with vegetation. ALS method provides DTM data within required accuracy of 10 cm, whereas photogrammetric method failed to deliver that level of accuracy. On the other hand it is very difficult to generate structural lines based on the ALS data, these lines are essential for accurate but not overly dense representation of the terrain. Additionally the study incorporated the impact of full-waveform scanning method on density of the terrain points in areas with varied vegetation height cover. Only 50% decrease in the point density on bushed areas was observed in comparison with opened regions. In the areas beneath the trees smaller amount of point samples were surveyed, however achieved density was at least as good as in direct measurements. Conducted studies have shown that currently neither photogrammetry nor ALS techniques are suitable for map creations needed in construction design phase. While ALS measurements itself are precise and efficient, the lack of automatic detection of structural lines from point clouds prevents the accurate representation of the surveyed areas.
|