Quality Checking of Als Projects Using Statistics of Strip Differences

We present an automatic method for checking the geometric accuracy of ALS points. First for each strip a DEM is interpolated using the moving planes method. The accuracy of the interpolated height at each grid point and the distance between each grid point and the centre of gravity of the original ALS points used for interpolation are used to derive a smoothness mask for each strip. Afterwards for pairs of overlapping strips the difference of their DEMs is computed. Grid points, which are inside the smoothness mask of both strips, are compared with a given threshold for the height differences. The percentage of the grid points exceeding this threshold is a first quality measure for the given ALS data. If this percentage is larger than a given threshold, then we perform another analysis, which is based on LSM. LSM computes the 3D shift between both overlapping strips in many locations of a window which slides from one end of the strip overlap to the other. By comparing the X, Y, Z components of these shifts with predefined tolerances for the accuracy in planimetry and height the location and size of regions hurting these tolerances can be spotted.