Laser altimetry is a new measurement technique. Altimeter receives the return signals after launching laser pulses to the
ground. Three-dimensional information on the ground is obtained by analyzing the corresponding relationship between
return signal features and topographical features. But there is an apparent deficiency when using the data points to
establish the three-dimensional DSM. That is the accurate time information of the return signal cannot be obtained when
positioning the edge lines of the ladder terrain. In this article, the distribution of the echo waveform over the ladder
terrain of city buildings is simulated by a self-designed altimeter waveform simulator under different direction angles
and parameters of the terrain. The best extraction method of the return time information at the edge line is identified by
analyzing the waveform characteristics at the edge line of this terrain, which enhances the accuracy of applications of the
ladder terrain modeling. The time information of the return signal is extracted under various parameters by applying five
methods which are accepted internationally, namely Peak point, Mean point, 50% Rise time point, Center of Area point,
and Midpoint. Then the experiment values are compared to the actual values. According to the results of the simulations,
the difference between the experiment value of the 50% Rise time point and the actual value is minimal. Its resolution
ratio is about ten time units maintaining in a narrow range. It proves that this method can effectively improve the
accuracy of the extracted time information of the return signal at the edge line over ladder terrain. And it provides a valid
data source for establishing a more accurate DSM model.
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