[R-sig-eco] Strange calculation of dt in adehabitat

[Anders Kagervall]

Hi,
I have just started to use adehabitat have maybe stumbled on bug in the calculation of dt in as.ltraj I have GPS data sampled in reasonable regular interval. In this case the interval should be near 1800 seconds.
What I don't understand is how dt for relocation number 6 can be 5471 seconds. Is it a bug or is there something that I don't understand here? Any input appreciated.
Demo data and code: 

> library(adehabitat)
> points <- read.csv("points.txt",header=T) points$LMT_Date <- 
> as.POSIXct(points$LMT_Date) points
      Object_ID       X       Y            LMT_Date
1  aa_ac_05_115 1498706 7387365 2008-10-26 00:00:41
2  aa_ac_05_115 1498711 7387367 2008-10-26 00:30:12
3  aa_ac_05_115 1498708 7387367 2008-10-26 01:00:48
4  aa_ac_05_115 1498707 7387366 2008-10-26 01:30:12
5  aa_ac_05_115 1498707 7387365 2008-10-26 02:00:16
6  aa_ac_05_115 1498708 7387367 2008-10-26 02:30:12
7  aa_ac_05_115 1498711 7387376 2008-10-26 03:01:23
8  aa_ac_05_115 1498712 7387419 2008-10-26 03:30:14
9  aa_ac_05_115 1498724 7387618 2008-10-26 04:00:17 10 aa_ac_05_115 1498723 7387624 2008-10-26 04:30:23
> tr <- as.ltraj( points[,c("X","Y")], date = points$LMT_Date, id = 
> points$Object_ID) tr

*********** List of class ltraj ***********

Type of the traject: Type II (time recorded) Irregular traject. Variable time lag between two locs

Characteristics of the bursts:
            id        burst nb.reloc NAs          date.begin            date.end
1 aa_ac_05_115 aa_ac_05_115       10   0 2008-10-26 00:00:41 2008-10-26 04:30:23

> tr[[1]]
         x       y                date            dx           dy       dist   dt          R2n  abs.angle   rel.angle
1  1498706 7387365 2008-10-26 00:00:41  4.9607648100   2.20537155   5.428890 1771     0.000000  0.4183232          NA
2  1498711 7387367 2008-10-26 00:30:12 -3.5757074798  -0.09854094   3.577065 1836    29.472851 -3.1140412  2.75082096
3  1498708 7387367 2008-10-26 01:00:48 -0.9629397001  -0.54593210   1.106930 1764     6.357119 -2.6258344  0.48820682
4  1498707 7387366 2008-10-26 01:30:12 -0.0005537099  -1.19318821   1.193188 1804     2.614587 -1.5712604  1.05457398
5  1498707 7387365 2008-10-26 02:00:16  0.9633071600   1.33767369   1.648433 1796     0.312927  0.9466836  2.51794401
6  1498708 7387367 2008-10-26 02:30:12  3.2656030301   9.80078719  10.330518 5471     4.826202  1.2491675  0.30248392
7  1498711 7387376 2008-10-26 03:01:23  1.2939203100  42.65423967  42.673861 1731   154.018885  1.5404705  0.29130299
8  1498712 7387419 2008-10-26 03:30:14 12.1896293599 198.91718789 199.290328 1803  2968.685928  1.5095929 -0.03087759
9  1498724 7387618 2008-10-26 04:00:17 -1.3766610399   6.23432703   6.384515 1806 64377.113802  1.7881282  0.27853523
10 1498723 7387624 2008-10-26 04:30:23            NA           NA         NA   NA 67523.484052         NA          NA
> 
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