[R] Duplicated date values aren't duplicates

bhradsky bhradsky at student.unimelb.edu.au
Thu Jan 8 00:51:38 CET 2015

I had the same error "Error in as.ltraj(xy, id, date = da) : non unique dates
for a given burst", but from a different cause, that I'd like to share in
case others have the same issue.

My data came from GPS collars that collected dates and time in Greenwich
Mean Time (GMT).  However, the package automatically uses time appropriate
for your local area.  My data had been collected during October when
Australia switches from standard time to summer time.  Thus the times 0200h
and 0230h on 6 October did not exist and were perceived as 'non unique
dates' between 0100h AEST and 0300h AEDT.  This can easily be fixed by
specifying the time zone when extracting the dates using as.POSIXct

GPSdata$datetime <- (strptime(paste(GPSdata$Date, GPSdata$Time), "%Y %m %d
GPSdata$datetime <- as.character(GPSdata$datetime)

when <- as.POSIXct(GPSdata$datetime, tz="GMT")

foxdata <- as.ltraj(xy = GPSdata[,c("North.", "East.")], date = when, id =

Bronwyn Hradsky
Department of Forest & Ecosystem Science
University of Melbourne

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