|
Development of a simulator for airborne altimetric LiDAR
Nishank Agrawal, Nitish Kumar Agrawal, Bharat Lohani Department of Civil Engineering Indian Institute of Technology Kanpur, Kanpur 208016 (India) Tel: 259 7623, Fax: 259 7395 Email: blohani@iitk.ac.in 1. Introduction: Laser altimetry is a direct method to efficiently map the earth’s surface in three spatial dimensions. Topographic mapping with LiDAR seems on the verge of supplanting traditional photogrammetry in the near future for detailed terrain mapping. This technique has a large and growing number of actual and anticipated applications in disciplines ranging from urban analysis to natural resources. Initially the focus of research in this field was on how to obtain terrain geography and objects such as buildings, trees etc. Furthermore, several other researchers were interested in identifying the error involved in LiDAR data. In all these studies raw LiDAR data were employed and attempts were made to compare results with ground truth. For a detailed review of these efforts please refer Lohani (1999) and special LiDAR issue of ISPRS Journal (ISPRS, 1999). However, the literature clearly indicates that a limited success has been achieved in these efforts and a lot needs to be done to make full utilization of this technology. The basic drawback of methods relying only on field data is that, it is not always possible to collect true ground truth for validation of these approaches. In view of the above it is of utmost importance that research be carried out to develop theoretical equivalents of LiDAR data collection process. Realizing this, recently the research related to simulation of LiDAR data has started taking place. The purpose of these researches should be to study the effect of various flight parameters (flying height, direction of flight etc.), LiDAR instrument parameters (firing frequency, scanning angle, number of scan lines), wrong instrument calibration and environment on LiDAR data generated for different types of objects and terrains being scanned. Beinat and Crosilla (2002) created a virtual scenery with four distinct buildings using MatlabTM and then produced some LiDAR images by randomly sampling the surface points of the model with an average resolution of 0.5 points/m2 and a random error of ±10 cm. In this way the effect of different weighting models was studied. Holmgren et al. (2003) described the effect of LiDAR scanning angle for estimation of mean tree height and canopy closure. For this a computer modeled forest was simulated for different scanning angles. Most of these researchers have studied the effect of one parameter on one object only, and it was assumed that either the effect of other parameters is negligible or independent of the effect of parameters being studied. However, in real world all parameters affect together and have varying degree of effect on different terrain conditions. Furthermore, as in case of Beinat and Crosilla (2002) the data points were assumed to be randomly distributed which does not reflect the way LiDAR collects data. 2. Outline of present work: The aim of present work is to develop a simulator which will be helpful in simulation of more than one parameter on surfaces which will resemble terrain in real. A simulator is a computer programme that creates the model of existing external conditions and generates output. In the present case external conditions are LiDAR data collection system, and output generation is data points. The simulation is realized using a set of mathematical equations and coding them in C programming language. With the help of simulator at any instant the location of aircraft and laser vector angles with three axes are obtained, thus equation of laser vector is known. Surfaces with known mathematical equations are chosen to represent ground. More than one mathematical surfaces are stitched together to resemble complex terrain. Data points are obtained by the intersection of laser vector and the surface (Figure 1). Nature of surfaces is chosen with the aim to model the possible ground types and the objects such as buildings, trees etc. The data points thus obtained can be plotted in suitable software (Matlab here) to visualize the output of simulator. Various such displays can be obtained to study the effect of various parameters on different terrain surfaces.  Figure 1: Working of LIDAR simulator
Page 1 of 8
| Next | |