Trajic: An Effective Compression System for Trajectory Data

Abstract

The need to store vast amounts of trajectory data becomes more problematic as GPS-based tracking devices become increasingly prevalent. There are two commonly used approaches for compressing trajectory data. The first is the line generalisation approach which aims to fit the trajectory using a series of line segments. The second is to store the initial data point and then store the remaining data points as a sequence of successive deltas. The line generalisation approach is only effective when given a large error margin, and existing delta compression algorithms do not permit lossy compression. Consequently there is an uncovered gap in which users expect a good compression ratio by giving away only a small error margin. This paper fills this gap by extending the delta compression approach to allow users to trade a small maximum error margin for large improvements to the compression ratio. In addition, alternative techniques are extensively studied for the following two key components of any delta-based approach: predicting the value of the next data point and encoding leading zeros. We propose a new trajectory compression system called Trajic based on the results of the study. Experimental results show that Trajic produces 1.5 times smaller compressed data than a straight-forward delta compression algorithm for lossless compression and produces 9.4 times smaller compressed data than a state-of-the-art line generalisation algorithm when using a small maximum error bound of 1 meter.