[R-sig-Geo] Trouble with first passage time analysis

[Clement Calenge]

> I posted this question a few days ago on a different forum, and was directed
> here. Hopefully someone will be able to help me out. I've managed to figure
> out just about everything I need, I'm just left with one unresolved issue.
> I'm fine up to the point of creating the ltraj object from a dataset.
>
> /test<-read.table("~/Desktop/R_Directory/test.txt",header=TRUE)
> attach(test)
> library(adehabitatLT)
> xy<-test[,c("x","y")]
> date1<-as.character(date)
> date<-as.POSIXct(date1)
> fit1<-as.ltraj(xy,date,id=test$name,burst=name,typeII=TRUE)/
>
> from here things start to get fuzzy...
>
>
> Instead of following the basic expression,
>
> /fpt(lt, radii, units = c("seconds", "hours", "days"))/
>
>
> the sample code on the tutorial expressed fpt as,
>
> /fpt(lt, seq(300,1000, length=30))/
>
>
> But as far as I can tell this code does not set a specific radius distance
> (e.g., 100 m), but rather defines how many radii are to be distributed
> across the dataset.

radii is a vector containing the radii of the circle. Actually:

 > seq(300,1000, length=30)
  [1]  300.0000  324.1379  348.2759  372.4138  396.5517  420.6897  444.8276
  [8]  468.9655  493.1034  517.2414  541.3793  565.5172  589.6552  613.7931
[15]  637.9310  662.0690  686.2069  710.3448  734.4828  758.6207  782.7586
[22]  806.8966  831.0345  855.1724  879.3103  903.4483  927.5862  951.7241
[29]  975.8621 1000.0000

Therefore, the fpt will be calculated for a circle with a radius 300 m, 
for another circle with radius 324 m, etc. The use of the function seq() 
is just a way to generate quickly a vector of values for radii. But you 
can generate a vector of radii yourself, e.g. with:

fpt(lt, radii=300, units = c("seconds", "hours", "days"))

To calculate the fpt for a radius equal to 300 m. Or,

fpt(lt, radii=c(300, 500), units = c("seconds", "hours", "days"))

To calculate the fpt for a radius equal to 300 m, and for a radius equal 
to 500 m.

HTH,

Clément Calenge

-- 
Clément CALENGE
Cellule d'appui à l'analyse de données
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