R/getSpeed.R

In ferrettigs/OIB: Ocean Intelligence Bureau Toolbox

#' Get Speed from Satellite Track
#'
#' 
#' @param dat a data.frame with a timeseries of location points from PAT tags or AIS track. It has to have coordinates (lat and lon) and timestamps (datetime) in GMT
#' @param track whether it is a PAT track or AIS
#' @export 
getSpeed = function(dat, track = "PAT"){
	if (track=="AIS") {
			names(dat)[2] = "datetime"
			dat = unique(dat)
			}
			
	dat = dat[order(dat$datetime),]
	if (track=="PAT") dat = dat[dat$lc %in% c("3","G"),] 
	dat$distance = (geosphere::distm(x = dat[,c("lon","lat")])/1000)[1,] # distance in kilometers
	dat$reldist = abs(c(0,diff(dat$distance))) # distance cannot be negative. Sometimes is negative because the next point may be closer to the first because the drifter is changing course and getting closer to the initial point.
	#dat$reltime = c(0, diff(dat$datetime)/60) # time diffe in hours, decimal
	dat$reltime = c(0, as.numeric(diff(dat[,grep("timestamp|datetime", names(dat))]), units = "hours"))
	dat$speed = with(dat, reldist/reltime)
	dat
}