基于等高线的表面估计滤波方法

Filtering method of LIDAR data based on contours

提出一种基于等高线的滤波方法,它先由LIDAR数据生成数字表面模型,并内插出等高线,再根据DSM等高线的特征,如闭合性、首尾点距离、等高线的长度及等高线间距离等,通过设定阈值自动提取出属于自然地面的等高线线段,以获得初始的自然地面点,然后内插生成初始数字地面模型,最后使用迭代逼近法生成最终的(精确的)数字地面模型,即比较初始DTM与DSM,差值小于预设阈值的点视为DTM点,而差值大于预设阈值的点则标记为无数据点,最后,这些无数据点由选择的DTM点内插出。通过与现有表面估计的滤波方法的对比实验以及所提取地物轮廓线与航片的叠加对比试验,证明新方法可适用于地表起伏较大的地形,地物提取精度高、计算量小、效率高。

Filtering is an important method to extract digital terrain model（DTM） form Light Detection and Ranging（LIDAR） data and has made some good effects in application, but existing filtering methods are deficiency in certain applications, especially unsuitable for undulated terrain. In this paper, a new filtering method which is based on contours is developed. The method mainly consists of three steps ： Firstly, digital surface model （DSM） is generated from raw LIDAR data by surface interpolation methods, such as the nearest neighbor interpolation method, and then contours of DSM are generated. Secondly, these DSM contours are separated into three groups： ground contour, which only consists of points on ground; object contour, which only consists of points off-ground, such as on trees or roofs; and mixing contour, which consists of points on ground or off-ground. According to some properties of contours, including closeness, start-to-end distance, length of curve, denseness, ground contours and ground contour section of mixing contours are extracted and then some ground points are obtained, finally an initial approximation of the DTM is determined by interpolating those ground points. Then a refined DTM is obtained by an iterafive method： comparing the approximate DTM with the original DSM and if the difference is below a predefined threshold the original DSM points are selected and made into DTM data points. Otherwise, the location is marked as no data; and then the no data points are interpolated using the selected DSM data. Comparisons between initial and refined DTM obtained by methods proposed previously is done. To calculate the accurate probability, the objects obtained by new filter is compared with standard objects obtained from aerial images manually. All the experimental results show that the proposed method is a simple and practical one needing little calculation, and can be used in undulating terrain area.