# Stop identification with ST-DBSCAN

    library(stdbscan)
    library(readr)
    library(lubridate)
    library(ggplot2)
    library(plotly)

## Presentation

This vignette briefly demonstrates how to perform stop identification in a GPS track using ST-DBSCAN, which is a classic application of this algorithm.

## Dataset

The GeoLife GPS Trajectories dataset is used for this demonstration. The GPS trajectories are located in Beijing. We previously converted the pings to a metric coordinate reference system (EPSG:4586) and selected only the relevant variables.

    head(geolife_traj)
    #>         date     time        x       y
    #> 1 2008-10-23 02:53:04 441782.8 4428131
    #> 2 2008-10-23 02:53:10 441785.6 4428129
    #> 3 2008-10-23 02:53:15 441782.8 4428129
    #> 4 2008-10-23 02:53:20 441780.1 4428130
    #> 5 2008-10-23 02:53:25 441769.6 4428126
    #> 6 2008-10-23 02:53:30 441749.3 4428121

    ggplot() +
      geom_path(data = geolife_traj, aes(x, y)) +
      labs(x = "", y = "",
        title = "GPS track analyzed in this vignette",
        caption = "Data: GeoLife GPS Trajectories (Microsoft, 2012). Author: Antoine Le Doeuff, 2026",
      ) +
      coord_equal() +
      theme_minimal() +
      theme(plot.title = element_text(size = 16, face = "bold"))

## Preprocessing

For `stdbscan` to work, the time variable must be numeric. We therefore convert it to seconds since the beginning of the track.

    geolife_traj$date_time <- as_datetime(
      paste(geolife_traj$date, geolife_traj$time), tz = "GMT"
    )
    geolife_traj$t <- as.numeric(
      geolife_traj$date_time - min(geolife_traj$date_time)
    )

## Run ST-DBSCAN

We can then run ST-DBSCAN using `st_dbscan()`. We set a spatial neighborhood of 3 meters, a temporal neighborhood of 30 seconds, and require a minimum of 3 pings to form a cluster. Note that these parameters are used only for demonstration purposes; in practice, a grid search (or similar tuning strategy) should be used to determine optimal values.

    clusters <- st_dbscan(
      x = geolife_traj$x,
      y = geolife_traj$y,
      t = geolife_traj$t,
      eps_spatial = 3, # meters
      eps_temporal = 30, # seconds
      min_pts = 3
    )
    geolife_traj$clust <- as.factor(clusters)

## Check result

We can check the number of pings in each cluster using `table()`.

    table(geolife_traj$clust)
    #>
    #>  -1   1   2   3   4   5
    #> 420   4   5  12  12  15

Clusters can be plotted directly using `ggplot2` :

    # Extract stops and movements
    geolife_traj_mvt <- geolife_traj[geolife_traj$clust == "-1", ]
    geolife_traj_stop <- geolife_traj[geolife_traj$clust != "-1", ]

    # Plot
    ggplot() +
      geom_path(data = geolife_traj_mvt, aes(x, y)) +
      geom_point(data = geolife_traj_stop, aes(x, y, color = clust), size = 4) +
      labs(x = "", y = "", color = "stop ID",
        title = "ST-DBSCAN stop identification",
        subtitle = "eps_spatial = 3 m, eps_temporal = 30 s and min_pts = 3",
        caption = "Data: GeoLife GPS Trajectories (Microsoft, 2012). Author: Antoine Le Doeuff, 2026",
      ) +
      scale_color_manual(values = MetBrewer::met.brewer("Isfahan2", 5)) +
      coord_equal() +
      theme_minimal() +
      theme(
        legend.position = "bottom",
        plot.title = element_text(size = 16, face = "bold"),
      )

Clusters can be visualized in 3D using `plotly` :

    # Zoom on stop 4
    geolife_traj_f <- geolife_traj[
      geolife_traj$x > 441060 & geolife_traj$x < 441100,
    ]
    geolife_traj_f <- geolife_traj_f[
      geolife_traj_f$y > 4428780 & geolife_traj_f$y < 4428820,
    ]

    # Extract stop
    geolife_traj_f_stop <- geolife_traj_f[geolife_traj_f$clust != "-1", ]

    # Plotly figure
    fig <- plot_ly(
      data = geolife_traj_f,
      x = ~x,
      y = ~y,
      z = ~t,
      type = "scatter3d", mode = "lines+markers",
      line = list(wigeolife_trajh = 4, color = "grey"),
      marker = list(size = 3, color = "grey")
    )
    fig |>
      add_markers(
        x = ~geolife_traj_f_stop$x,
        y = ~geolife_traj_f_stop$y,
        z = ~geolife_traj_f_stop$t,
        marker = list(size = 4, color = 'red'),
        name = 'Stop'
      ) |>
      layout(
        scene = list(
          xaxis = list(title = "x"),
          yaxis = list(title = "y"),
          zaxis = list(title = "t")
      )
    )