Question : Why the first value of dt is NA and not the last one?
VVanlan opened this issue · 2 comments
Hi,
I have a question about how dt (in GetSpeed) is calculated. In other package dealing with animal movement such as adehabtiatLT and amt, dt is calculated as the time between the relocation i and the relocation i+1.
Example for one individual
dt = c(difftime( time[-1], time[-nrow(time)], NA)
Loc | dt |
---|---|
1 | t2-t1 |
2 | t3-t2 |
3 | t4-t3 |
4 | NA-t4 |
The last value is NA, because no observed step is 'started' at the last point.
Your code :
dt = c(NA, difftime(Time[-1], Time[-length(Time)], units = "hours"))
Loc | dt |
---|---|
1 | NA |
2 | t2-t1 |
3 | t3-t2 |
4 | t4-t3 |
Is there a reason for that?
Thanks!
Thanks for pointing that out ... I (for one) don't usually use movement data structure from those other packages.
The choice is mainly arbitrary - and is just how I have always done it since I was making tables like this (now many years ago) - but my choice has always been somewhat philosophically motivated by the fact that you can say where someone came from but should avoid predicting where someone is going to. Thus, at
The counter-argument is that maybe your goal is to predict something about a movement based on attributes of a given location, for example. In that case, it makes more sense the other way.
Again - this is mainly my (our) preference and what we're used to. The important thing is to be mindful of what you're looking at (and looking for!)
Thanks for the anwers!
I really appreciate it. I never thought about it like that because i am used to predict movement. It makes sense now.