[R-sig-eco] Strange calculation of dt in adehabitat
Mathieu Basille
basille at ase-research.org
Thu Jul 15 20:51:03 CEST 2010
Anders,
I run into the exact same problem because of DST. As you said that it
was GPS data, it is safe to explicitly set up the timezone to GMT (or to
any other timezone *without* DST that better suits your needs) at the
importation step. For example, try:
as.POSIXct(strptime(points$LMT_Date, format = "%Y-%m-%d %H:%M:%S", tz =
"GMT")) ## Check out the exact 'format' here
And then have a look again at your results. The problem comes from
as.POSIXct which assumes, if not stated otherwise, that times are in
your locale (i.e. LC_TIME=Swedish_Sweden.1252, I guess with DST). And
the output of the trajectory does not tell you the actual timezone. To
be sure of this, try:
points$LMT_Date[6]
points$LMT_Date[7]
The second time should be with DST. Alternatively, you can use the
modified function attached, which shows you, in a ltraj summary, the
timezone of the different bursts.
Hope this helps,
Mathieu.
Anders Kagervall a écrit :
> Hi Matt,
>
>> From: Matt Beard [mailto:biozealot21 at gmail.com]
>> Sent: den 10 juli 2010 04:35
>> To: Anders Kagervall
>> Cc: r-sig-ecology at r-project.org
>> Subject: Re: [R-sig-eco] Strange calculation of dt in adehabitat
>>
>> Anders,
>
>> I was not able to recreate your problem with the demo you provided. What are the details of your platform?
>
> Here is my sessionInfo()
> R version 2.10.1 (2009-12-14)
> i386-pc-mingw32
>
> locale:
> [1] LC_COLLATE=Swedish_Sweden.1252 LC_CTYPE=Swedish_Sweden.1252
> [3] LC_MONETARY=Swedish_Sweden.1252 LC_NUMERIC=C
> [5] LC_TIME=Swedish_Sweden.1252
>
> attached base packages:
> [1] stats graphics grDevices utils datasets methods base
>
> other attached packages:
> [1] gpclib_1.5-1 adehabitat_1.8.3 ade4_1.4-14
>
> I also ran the test with Linux and R 2.11.1 and got the same results.
> However, your comment made me think a little further and I'm now pretty sure that the problem comes from the fact that 2008-10-26 was a date when we changed from daylight savings time to normal time. I tested by replacing the month from 10 to 09 in the data file and got results without any missing points.
> I'm still not sure if the problem I see is in how we record time in our database or how adehabitat calculates the time interval. I will re-run the analyze using GMT-datetime, that we also have in the database, and see if I can get rid of the problem.
>
> ps
> Your assumption about the precision of the X and Y coordinates was correct. I have now attached the actual data file I used.
> ds
>
>
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--
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Mathieu Basille, Post-Doc
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Laboratoire d'Écologie Comportementale et de Conservation de la Faune
+ Centre d'Étude de la Forêt
Département de Biologie
Université Laval, Québec
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http://ase-research.org/basille
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``If you can't win by reason, go for volume.''
Calvin, by Bill Watterson.
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