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Correct GPS errors in AIS tracks

Source: R/AIScorrect_speed.R
AIScorrect_speed.Rd

Detects and corrects GPS errors and delayed AIS messages that generate unrealistic vessel speeds, travelled distances, and travel times. Note: For consecutive GPS errors, only the first point is removed to avoid overcorrection.

Usage

AIScorrect_speed(
  ais_data,
  crs_meters = 3035,
  threshold_speed_to_correct = 100,
  threshold_speed_to_correct_function = NULL,
  nb_cores = 1,
  outfile = tempfile()
)

Arguments

ais_data

AIS data frame containing timestamp, lon, lat, and mmsi. timestamp must be Unix time (seconds since 1970-01-01), while lon and lat must be numeric. Another vessel identifier may be used if the column is named mmsi.

crs_meters

CRS (metres) used to calculate distances in the study area (defaults to EPSG:3035, Europe). Tip: use suggest_crs function (crsuggest package) to find a suitable CRS for your study area.

threshold_speed_to_correct

Speed threshold (km/h) above which observations are corrected.

threshold_speed_to_correct_function

a function to estimate vessel-specific speed thresholds. The function can use all columns of ais_data as argument (such as speed_kmh, distance_travelled or time_travelled). If set to NULL, an internal function is used (see details).

nb_cores

Number of CPU cores used.

outfile

File used to save logs.

Value

The input data with corrected travel metrics and the following columns:

  • speed_kmh_corrected: Whether the speed was corrected.

  • time_travelled: Travel time (s).

  • distance_travelled: Travelled distance (m).

  • speed_kmh: Vessel speed (km/h).

Details

When threshold_speed_to_correct_function is set to NULL, a vessel-specific speed threshold is estimated from the observed vessel speed_kmh. The threshold is calculated as:

$$ T = 15 + \operatorname{median}(v_{>1}) + 5 \times \operatorname{SD}(v_{1 < v < Q_{0.75}}) $$

where \(v_{>1}\) represents vessel speeds greater than 1 km/h, and \(v_{1 < v < Q_{0.75}}\) represents vessel speeds between 1 km/h and the 75th percentile. The equation is designed to capture the typical travelling speed of the vessel while accounting for variation in its observed travelling speeds.

Examples

data("ais")

# use only a sample for the example:
ais <- ais[ais$mmsi %in% ais$mmsi[1:5], ]

# Define the Unix time (seconds since 1970-01-01)
ais$timestamp <- as.numeric(lubridate::ymd_hms(ais$datetime))

# calculate the travelled distance, time, and speed:
ais <- AIStravel(ais_data = ais, crs_meters = 3035)

# Correct speed:
out <- AIScorrect_speed(ais_data = ais,
                        crs_meters = 3035)