Trajectory Data Analyses for Pedestrian Space-time Activity Study

Podsumowanie

A suite of spatiotemporal processing methods are presented to analyze human trajectory data, such as that collected using a GPS device, for the purpose of modeling pedestrian space-time activities.

Streszczenie

It is well recognized that human movement in the spatial and temporal dimensions has direct influence on disease transmission^1-3. An infectious disease typically spreads via contact between infected and susceptible individuals in their overlapped activity spaces. Therefore, daily mobility-activity information can be used as an indicator to measure exposures to risk factors of infection. However, a major difficulty and thus the reason for paucity of studies of infectious disease transmission at the micro scale arise from the lack of detailed individual mobility data. Previously in transportation and tourism research detailed space-time activity data often relied on the time-space diary technique, which requires subjects to actively record their activities in time and space. This is highly demanding for the participants and collaboration from the participants greatly affects the quality of data⁴.

Modern technologies such as GPS and mobile communications have made possible the automatic collection of trajectory data. The data collected, however, is not ideal for modeling human space-time activities, limited by the accuracies of existing devices. There is also no readily available tool for efficient processing of the data for human behavior study. We present here a suite of methods and an integrated ArcGIS desktop-based visual interface for the pre-processing and spatiotemporal analyses of trajectory data. We provide examples of how such processing may be used to model human space-time activities, especially with error-rich pedestrian trajectory data, that could be useful in public health studies such as infectious disease transmission modeling.

The procedure presented includes pre-processing, trajectory segmentation, activity space characterization, density estimation and visualization, and a few other exploratory analysis methods. Pre-processing is the cleaning of noisy raw trajectory data. We introduce an interactive visual pre-processing interface as well as an automatic module. Trajectory segmentation⁵ involves the identification of indoor and outdoor parts from pre-processed space-time tracks. Again, both interactive visual segmentation and automatic segmentation are supported. Segmented space-time tracks are then analyzed to derive characteristics of one's activity space such as activity radius etc. Density estimation and visualization are used to examine large amount of trajectory data to model hot spots and interactions. We demonstrate both density surface mapping⁶ and density volume rendering⁷. We also include a couple of other exploratory data analyses (EDA) and visualizations tools, such as Google Earth animation support and connection analysis. The suite of analytical as well as visual methods presented in this paper may be applied to any trajectory data for space-time activity studies.

Protokół

1. Getting Data

1. Trajectory data can be collected with handheld GPS units, GPS-enabled smart phone tracking applications, as well as A-GPS (assisted GPS) devices such as the one employed in our study, a commercial child tracker device.
2. Trajectory data is usually saved in terms of time-latitude-longitude records. A desired time interval should be set based on application needs. Often the most frequent interval is desired for space-time activity studies.
3. Convert the data to comma-separated values, or .csv files with separate columns for record id, latitude, longitude, and time, respectively. Then convert the .csv files into commonly-used Geographic Information Systems (GIS) file format (i.e. ESRI shapefile⁸).
4. Load in a shapefile of building polygons and another of the boundary of the study area with the trajectory analyzer. Set the "extrusion" of the buildings properly for a 3D display and set the "extrusion" and "transparency" of the boundary layer properly to display a space-time cube^{6, 9} with the x,y dimensions representing space and the z dimension representing time.

2. Pre-processing

1. Two options are available for pre-processing the noisy raw trajectory data. One may choose from the drop down list of the pre-processing menu.
2. If 'Interactive' is chosen, a 2D projection of the 3D trajectory is created for easy viewing and selection. Manipulate the 3D display to examine the raw trajectory in space and time. Identify errors in the data based on the shape, speed and/or topology of track segments. Usually track points (vertices) with unrealistic high speed or abrupt direction change signify errors. Select and remove them from the original trajectories. Select and remove them from either the 3D trajectory or its 2D projection.
3. A cluster of track points with spiky shapes (Figure 1) spatially and a long duration temporally signify errors that are most possibly caused by indoor locations where GPS signal is weak. If a group of these points is selected, the program can calculate the spatiotemporal centroid of the selected points and adjust the track to go through the centroid.
4. Alternatively, if 'Automatic' is chosen from the pre-processing menu, set the input and output locations as well as empirical parameters that determine the abnormal high speed and abrupt turning of points. The program searches through the loaded trajectory data and runs automatically based on an algorithm that mimics the visual error detection approach.

3. Trajectory Segmentation & Activity Space Characterization

1. Trajectory segmentation requires the building layer, so ensure the building shape file is loaded.
2. Click the segmentation tool in the toolbar to start the function. Set the input and output and located the building shape file as the reference layer. Use the building names to label the segmented trajectory. The algorithm identifies indoor segments based on set or default criteria such as speed, duration, etc. of track points, as well as the spatial topology with relation to buildings.
3. Click the activity space summarization tool to load in segmented trajectories and calculate selected summary attributes to characterize one's activity space, such as total activity radius, radius at a certain time period, ratio of total time spent indoors vs. outdoors, and so on.
4. The attributes can be exported to a spreadsheet for quantitative modeling uses.

4. Density Surface Mapping

1. Density surface shows the density of activities in space with the temporal dimension collapsed. Three options are available from the drop-down list of the density surface mapping menu.
2. If the 'Track point density' option is selected, fill in the dialog box with input and output information and choose to display in either 3D or 2D. All vertices from the trajectory data are used to calculate kernel densities of the points. Figure 2 shows a density surface.
3. If 'Track path density' is selected, the algorithm calculates and displays density of individual paths traveled (Figure 3).
4. If the 'Re-sampled point density' option is selected, the algorithm re-samples the trajectory data using a set time interval and maps the densities of points spread evenly in time. This option is designed for tracking devices that collect tracking points in irregular time intervals due to varying sensitivity of the devices under various physical conditions or segmented trajectories. Figure 4 shows the 2D and 3D density surfaces of segmented trajectories.
5. If 'Temporal focusing' is selected for any of the above options, temporal focusing¹⁰ can be performed to examine activity patterns at different time periods. For example, activity density surfaces at different times in a day may be visualized for easy identification of hot spots across time (Figure 5).

5. Density Volume Estimation and Volume Rendering

1. Density volume visualization uses the notion of a space-time cube as in the visualization of trajectories. The core of such visualization is the disaggregation of space into voxels¹¹. Our approach to visualizing density volume first estimates density volume in individual voxels by counting the number of space-time tracks that intersect with the voxels. One may click 'Density volume calculation' under the density volume visualization menu for this step.
2. The same three options are available for density volume visualization as for density surface visualization.
3. Next click 'Volume rendering' to launch the 3D volume visualization interface for interactive volume rendering¹². By setting the number of divisions along each axis, one may examine clusters at different scales. A z-factor is used to set the vertical exaggeration for better visualization. A reference layer such as the buildings can be loaded to aid visualization as well. The results of volume rendering can be interactively adjusted by manipulating the transfer function that controls the mapping from density to color. (Figure 6).

6. Other Exploratory Data Analyses (EDA) and Visualizations

1. A procedure is available to create animated series to be displayed in Google Earth. Under 'Other', click 'Export to KML for EDA' to access this procedure. It creates a kml¹³ file that opens in Google Earth for interactive animation of the trajectory.
2. One may follow the trajectory to travel the environment in time by scrolling along the timeline in Google Earth.
3. A procedure is available to visualize connections among places of interest through 'Connection analysis'. For example, connections among different buildings on a University campus are derived from segmented trajectory data that were collected by students (Figure 7).
4. Based on the derived connections, hotspots such as those buildings with the most outbound or inbound traffic and hubs that connect the most trafficked places may be identified.

Wyniki

Trajectory data was collected by volunteering undergraduate students from Kean University (NJ, USA) in spring 2010. The purpose was to study activity patterns of students who caught influenza (diagnosed by doctor or self-diagnosed) in comparison to those who did not. In order to illustrate the methods and procedure presented in this paper we took the trajectories collected within the suburban campus area to generate representative results. Trajectories within the campus area are mostly pedestrian trajectories, wit...

Dyskusje

We used add-in mechanism of ArcGIS to develop the interface. All the interactive operations were implemented using C++. All the automatic processing and analysis functions were developed using Python.

AGPS data, or GPS data collected by pedestrian presents unique challenge in preprocessing as the errors can be massive due to adjacency to buildings and frequent indoor stops. Moreover, the focus of preprocessing should not be data reduction as what is usually done for vehicle GPS trajector...

Materiały

Name	Company	Catalog Number	Comments
WorldTracker GPRS	Tracking The World
A personal computer for running the analysis
ArcGIS software	ESRI
Trajectory Analyzer Extension