speedGPS.R
speedGPS <- function(xyz, time, rOut = c(.25, .75), naValue = .25) {
speed <- trackSpeed(xyz, time)
rIn <- range(speed, na.rm = TRUE)
value <- diff(rOut) * (speed - rIn[1])/diff(rIn) + rOut[1]
ifelse(is.na(value), rep(naValue, length(value)), value)
}