An Approach to Semantic Processing of GPS Traces

Abstract

Recording the movement of objects or persons with GPS-technology has been widely adopted. So called GPS traces are used for enriching travel diaries (Wolf et al. 2001), for learning significant places (Ashbrook and Starner 2003), for monitoring animal movement (Steiner et al. 2000) or for building road maps (Cao and Krumm 2009). While all of the mentioned applications rely on GPS traces, the methods for collecting and processing the data differ. One of the basic goals in processing GPS raw data is to extract re-occurring motion patterns (Laube et al. 2005, Zheng et al. 2008). In a current research project called HOTSPOT, we try to answer the question, which knowledge about an object or person's movement can be extracted from a single GPS trace. One of the unique aspects of the project is that our approach to knowledge extraction only relies on GPS raw data and explicitly avoids any kind of map matching or usage of additional data sources. In this position paper we tackle aspects of semantic data processing. With semantic data processing we refer to methods attaching meaning to sub-sequences of GPS traces. We propose a method for semantic processing of GPS traces, resulting in basic motion and course change activities. The method can be applied to any kind of raw GPS data and prepares the data for further analysis of motion patterns.

1. Introduction

2. GPS traces

For testing the algorithms for semantic data processing the project team collected a reference data set of about 400 GPS traces. Traces were collected in a representative manner, considering different kinds of GPS receivers, different modes of transport (on foot, running, hiking, bicycle, bus, tram, train, car and combined modes), different geographic regions (intensively built areas, rural landscapes, tracks with tunnels, forests), different road types, different days with different satellite constellation, different daytimes as well as different weather conditions. It was necessary to collect our own GPS traces, since the automatic recognition of motion patterns can only be validated by comparing the patterns to real motion behaviour. However, a carefully annotated reference set of GPS tracks would be a great help in validating the algorithms.

From the collected raw GPS data only lat/lon coordinates, elevation and timestamps were used. Other parameters like velocity, acceleration and course changes were calculated from subsequent positions and timestamps. We found it a good practice to post-calculate all relevant parameters from the basic positions, since velocities, accelerations and courses vary between receivers. For recording GPS traces we used a sampling rate of 1 Hz, although a sampling rate up to 20Hz with high-end receivers and a sampling rate of 5Hz with semi-professional receivers (e.g. QStarz BT-Q1000EX) would be feasible. Some of the receivers (e.g. the Garmin Forerunner series) optimize the amount of logging data by providing adaptive logging (e.g. logging a position every 1-5 seconds depending on the travelled distance or absolute course change).

In a pre-processing step we detected and removed severe GPS errors (sudden drifts, unrealistic values in velocity and course changes).

3. Semantic processing

Basically, a GPS trace can be interpreted as a discrete capture (according to the chosen sampling rate) of the motion of objects over time. One of the goals of semantic processing is to abstract point data to higher level motion patterns. Before starting with the processing it is worth to think about the basic motion patterns an object or person can accomplish while moving. Certainly, each of the patterns is depending on physics of the moving object (e.g. vehicle) or the physiology of a person and the underlying surface (e.g. uphill or downhill, road infrastructure used for movement). The basic parameters to express motion in space and time are velocity and course (Zheng et. al., 2008) . Both parameters can be computed from two sequent GPS measurements. By describing changes of these basic parameters over time, a set of six basic motion patterns can be defined (Table 1).

Rules velocity < 1 m/s velocity > 1 m/s & acceleration < +0.3 m/s2 and acceleration > -0.3m/s2 velocity > 1 m/s & acceleration > +0.3 m/s2 velocity > 1 m/s & acceleration < -0.3m/s2

Course

Positive course change Negative course change course change > +0.4°/s course change < -0.4°/s

3.1 Semantic classification of motion

are not the expected behaviour of a train. Since the interruptions are located at the beginning of a longer phase of slowdown, values slightly below or above the threshold cause varying classifications. A solution to this problem could be to detect longer subsequences and subsume short sub-sequences (e.g. shorter than 3 seconds) in the longer ones.

3.2 Semantic classification of course changes

As reported by Zheng et al. (2008) , the rate of course changes varies significantly with the mode of transportation. Moving with higher velocity allows a lower rate of course changes. Moreover, absolute course changes within short time periods may be significantly higher when moving with lower velocity, e.g. a pedestrian may accomplish 90 degree turns within a short time period (within 1 or 2 seconds). A train however is bound to the railway infrastructure and is limited in its course changes. For detecting course changes we use the mean course change rate in one time period (+°/s). Figure 3 shows course changes in a GPS trace collected during a train ride. For semantic classification of course changes, we again use a fuzzy rule set. When the course change rate within one time period (the change of degrees within one second) is above or below a threshold value, the GPS point is matched either to the semantic class positive course change or negative course change. In comparison to the motion detection, for the course change detection we use velocity-dependent thresholds. Velocity-dependent thresholds means, that for trace segments with low velocity, the threshold for course changes is set significantly higher compared to trace segments with higher velocity. In other words, as higher the velocity gets, as lower the threshold for course changes is set. The empirically estimated range of thresholds are +-5°/s (pedestrians moving at 1.5m/s), +-1°/s (car moving at 20m/s), +-0.4°/s (high speed train moving at 55m/s). The relatively high threshold for slow movement is necessary to deal with inaccuracies of GPS positioning. While moving slowly, minimal inaccuracy of positions has higher impact on the course change rate compared to moving at a higher speed (since the distance between two measurement is longer at higher speed and occurring positions errors have relatively less impact on calculated velocity and course). Increasing the threshold to +-5°/s allows detection of significant course changes only (Figure 4).

4 Conclusions and open issues

In the paper we introduced an approach to semantic processing of GPS traces by classifying sequences of GPS points with motion and course change patterns. Although the reported approach works basically well, a number of open issues remain. Firstly, the threshold-based classification of motion only reveals basic, coarse-grained motion patterns. A more fine-grained classification of steady motion as well as acceleration is expected. For further processing we find it useful not to rely on fixed thresholds, but to apply cluster analysis e.g. to reveal fine-grained clusters of steady motion. In first tests we found density-based clustering (e.g. ST-DBSCAN) (Birant and Kut 2007, Tietbohl et al. 2008) to be a well suited method for sub- classification. At time of writing we are not able to provide final results, but promising preliminary results. The same method can be applied to cluster acceleration (mean acceleration), standstills (using time spans) and course changes (using mean change rates). A second open issue concerns the semantic classification of multimodal traces (traces including motion with more than one means of transport). The first open question is the automatic detection of transport modes and change points. Although some authors tackled this problem (e.g. Zheng et al. 2008) we could not find robust methods for automatically detecting any transportation mode. Authors either rely on map matching (e.g. by matching standstills to bus stops and thus deriving the transport mode bus) or only focus on some modes, but miss others. Currently we are working on robust algorithms, using case-based reasoning as well as cluster analysis. Another open question is how to deal with severe errors of GPS positioning during walks as well as at interchange points. Since interchange points are typically buildings with underground passages, a high number of GPS errors occur. Although we apply methods for error correction, the semantic processing of such trace parts is a challenging question (Figure 5).

Acknowledgements

The project HOTSPOT is funded by the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT) within the programme "IV2Splus – intelligent transport systems and services".

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