改进的Sliding Window在线船舶AIS轨迹数据压缩算法
Online compression algorithm of AIS trajectory data based on improved sliding window

分析了船舶AIS数据的时间序列特征与船舶操纵特性,提出了改进的Sliding Window在线压缩算法; 计算了277艘船舶总计1 026 408个坐标点的AIS轨迹数据,确定了合适的压缩阈值,分析了距离阈值与角度阈值对算法压缩率的敏感程度; 根据压缩率图像的阶跃点,推荐了高、中、低3个档位的距离阈值和1个角度阈值,对比了Douglas-Peucker算法和改进Sliding Window算法的压缩率与压缩效率。试验结果表明:随着压缩率的提高,压缩后所剩下的点越来越少,数据所保留下来的有用信息也越来越少; 压缩率与距离阈值、角度阈值均呈正比; 经量纲为1化处理的高、中、低档位压缩距离阈值分别为43%、38%、33%船长; 距离阈值为130 m时,角度阈值超过9°后压缩率平稳,所以推荐角度阈值为9°,与《海港总体设计规范》(JTS 165―2013)中风流压差角8°相接近; 随着距离阈值的增大,Douglas-Peucker算法和改进Sliding Window算法压缩率趋于相近,当距离阈值为120 m时,Douglas-Peucker算法压缩率仅比改进Sliding Window算法高1.74%; 在5种距离阈值的情况下,Douglas-Peucker算法运行所用的平均时间是改进Sliding Window算法的5.39倍; 随着数据量的增大,2种算法压缩效率的差距更加明显。可见,改进的Sliding Window算法能在降低压缩风险的同时大幅提高压缩效率,可以在数据持续更新的状态下一直保持压缩状态,与普通压缩模式相比,系统所占用的资源更少,处理效率更高,可用于船舶轨迹数据处理、电子海图显示与对船舶关键行为特征提取等方面。
The time series characteristics of ship AIS data and ship maneuvering characteristics were analyzed. An improved sliding window online compression algorithm was proposed. The AIS trajectory data from 277 ships with a total of 1 026 408 coordinate points were calculated, and the appropriate compression thresholds were determined. The sensitivities of distance and angle thresholds to the compression rate of the algorithm were analyzed. According to the step points of the compression rate image, three distance thresholds of high, middle, and low levels, and one angle threshold were recommended. The compression rate and efficiency of the Douglas-Peucker algorithm and improved sliding window algorithm were compared. Experimental result shows that with the improvement of the compression rate, the remaining points after compression and the useful information retained by the data become less. The compression rate is positively proportional to the distance threshold and angle threshold. The dimensionless compression distance thresholds of high, middle, and low levels are 43%,38% and 33% of ship length, respectively. When the distance threshold is 130 m and the angle threshold is more than 9°, the compression rate becomes stable, so the recommend angle threshold is 9°, which is similar to 8° that is the pressure difference angle in Design Code of General Layout for Sea Ports(JTS 165―2013). With an increase in the distance threshold, the compression rates of the Douglas-Peucker and improved sliding window algorithms tend to be similar. When the distance threshold is 120 m, the compression rate of the Douglas-Peucker algorithm is only 1.74% higher than that of the improved sliding window algorithm. In the case of five distance thresholds, the average operating time of the Douglas-Peucker algorithm is 5.39 times that of the improved sliding window