Speed implied by consecutive position fixes

The distance from each record to the next, divided by the time between them. Computed from what the receiver logged rather than from any speed column, so it is available on any file carrying positions and a clock.

Usage

track_speed(dat, by = NULL, max_gap = 300)

Arguments

dat

A data frame with LATITUDE, LONGITUDE and TIME, in survey order.

by

Columns identifying a stretch to compute within, so speed is never taken across a break. NULL (default) uses LEGNO3 when present, else DATE and FILEID.

max_gap

Ignore intervals longer than this many seconds. Default 300. A long gap is a break in the record rather than slow travel, and including it drags the speed down.

Value

A numeric vector of knots, one per row: the speed from that record to the next. NA at the end of each stretch and wherever the interval is unusable.

Why this exists

Nothing in a NARWC file reliably says what the platform was. PLATFORM is optional, has no code book, and is empty on real extracts; LEGTYPE codes 5 and 6 mark shipboard records but a mixed file may use the aerial codes throughout. Speed cannot be mistaken for something else: a survey aircraft flies at 90-120 knots and a vessel surveys at about 10, and no arrangement of the other columns makes one look like the other.

See also

classify_platform(), which turns this into a platform label.

Examples

path <- system.file("extdata", "narwc-example.csv", package = "narwcr")
dat <- make_leg_id(read_narwc(path, quiet = TRUE), quiet = TRUE)
summary(track_speed(dat))
#>    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.     NAs 
#>    0.00   86.46   86.46   81.50   86.46  106.74      19