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Data Compression of Airborne Laser Scanner Data
2-3. Calculation of the target compression ratio The compression ratio needed to transmit data to the ground was calculated by converting from the amount of pulse data and GPS/IMU data acquired by the airborne at specified time. The compression ration needed to transmit the data by the effective transmission ratio is less than 20 %, and this figure is the target of the compression ratio in this study. 3. Analysis of airborne laser data 3-1. Characteristics of the laser pulse data The pulse data acquired from laser scanner consisted of the 3 kinds of binary data, the laser launch time, the mirror angle, and the return pulse time (Fig. 2). The most suitable compression method was selected by the data characteristics. The continuity of the laser launch time, the mirror angle, and the return pulse time is shown in Fig. 3, 4, and 5, respectively. The result shows that the shape of the laser launch time is the straight line to the line number, the mirror angle is the sine curve, and the return pulse time reflects the structure of the land surface on the sine curve including the noise. The compression method consists of two processes, changing the most suitable format and coding the data. The compression method was examined by the amount of data and the compression ratio derived from the entropy effect for 3 kinds of data. Table 3 shows the comparison of 4 kinds of compression methods considered from the mirror angle data. The mirror angle data determine the laser launch direction, and it is recorded by the same cycle of the launch. The quadratic difference method was selected as a most suitable compression method from the mean amount of the data and the effect of the entropy.  Fig. 2. The obtained laser data and system structure.  Fig. 3. The laser launch time data.  Fig. 4. The mirror angle data.  Fig. 5. The return pulse time data.
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