Restitution of airborne laser scanner data in wooded regions

    Karl Kraus

    Vienna University of Technology
    Institute for Photogrammetry and Remote Sensing
    Gusshausstrasse, 27-29
    A-1040, Austria

    Tel: +43-1-58801-3812, Fax: + 43-1-505 6268

    The area managed by the Municipality of Vienna amounts to 415 km^2. Of this area, 70 km^2 are covered by woods, and a further 27 km^2 are parks and private gardens. Up to this point, wooded areas have been surveyed mainly by employing photogrammetry thus achieving accuracy hardly better than +/- 2 m. This accuracy is not sufficient for the increasing requirements of special applications in hydrology, forestry, nor for designing national parks or urban settlements.

    For this reason, an area of 91 km^2 in the Vienna Woods (Wienerwald) has been scanned by an airborne rotating laser scanner resulting 9.25 million points records. Some 52 % of this amount has been eliminated by a specially constructed filter to exclude early reflections, in the first line by the foliage. Based on the remaining 4.5 million points, a digital terrain model has been computed with a 3.13 m grid width.

    Detailed investigations into the accuracy of this model depending on the terrain slope resulted in standard deviations between +/- 28 cm (flat terrain) and +/- 89 cm (terrain with 70 % slope). The dependence of the accuracy from the coverage amounts to approximately 25 %.

    The interpolation with filtering which is used at the moment for the surface modelling does not distinguish between "positive" and "negative" residuals. But the data of the laser scanning has "unbalanced" residuals; namely it is looked for this surface which approximates to the "lowest" base points. The higher located base points may be treated with a lower weight than the lower located base points. A provisional interpolation with base points of equal rank makes possible the selection of weights; the indicated iteration converges to the expected result. A new algorithm for the interpolation of laser scanner data will be presented.

    After the interpolation of a dense grid a geomorphological revision is necessary. Such a procedure must be able to detect and to remove many pits. The improved terrain model is then used for obtaining river networks and delineating basins.