[R-sig-Geo] [R-sig-eco] How to calculate time spent on a world grid with circumpolar trips

[Michael Sumner]

Sorry to have missed this.

There's really no way to do this in longitude / latitude without a lot
of manual handling. You could split into two trips, one using the
eastern hemisphere, the other the western and use a shared [-180,180]
grid that you sum together.

What to do at the split coordinate depends on your actual data, there
are many options.

If you are still looking for an answer I could have a closer look at
your example code, but real data would be more helpful if you can
share it. There are a number of problems in the code you posted that
prevent it from being easily run.

Cheers, Mike.




On Fri, Aug 6, 2010 at 8:10 PM, Pinaud David <pinaud at cebc.cnrs.fr> wrote:
> Dear all,
> I want to calculate a grid of time spent per cell with albatross data. The
> goal is then to relate this to fisheries data set on a 1x1° grid.
> I use the trip package with tripGrid(), but have some problems with birds
> crossing the date line.
> Usually I use a projection to do that but here I need to keep the 1x1° grid
> in order to relate density tu some data (fisheries...).
> I've tried with recenter() and nowrapSpatialLines(), without success.
> Here a code:
>
> library(trip)
> library(maps)   # to draw world maps
>
> d <- data.frame(x=c(seq(50, 180, 10), seq(-170, 50, 10)), y=rnorm(n=37,
> m=-50, sd=3), tms=Sys.time()-(37:1*60*60*24), id=rep(1, 37))  # an example
> coordinates(d) <- ~x+y
> tr <- trip(d, c("tms", "id"))
> proj4string(at) <- CRS("+proj=longlat +ellps=WGS84")
>
> map('world')
> plot(tr, add=T)
> points(coordinates(tr), t="l")
>
> # grid
> grid.base <- expand.grid(Long=seq(-180, 180, by=1), Lat=seq(-70, -15, by=1))
>   # tab coords
> coordinates(grid.base) <- ~ Long + Lat
> proj4string(grid.base) <- CRS("+proj=longlat +ellps=WGS84")
> gt <- makeGridTopology(grid.base, cellsize = c(1, 1))  # def grid topo
>
> # time spent per cell
> tppc <- tripGrid(tr, grid = gt)
> tppc01 <- tppc               # a copy for pres/abs
> tppc01 at data$z <- ifelse(tppc at data$z > 0, 1, 0)
>
> image(tppc, col=rev(heat.colors(500)))  # seems ok
> map('world', add=T)
> points(coordinates(tr), t="l")
>
> # but with presence/absence:
> image(tppc01, col=c("white", "green"))
> map('world', add=T)
> points(coordinates(tr), t="l", col="blue")
>
> Some ideas ?
>
>
> Many thanks for your time.
> David
>
> --
> ***************************************************
> David PINAUD
> Ingénieur de Recherche "Analyses spatiales"
>
> Centre d'Etudes Biologiques de Chizé - CNRS UPR1934
> 79360 Villiers-en-Bois, France poste 485
> Tel: +33 (0)5.49.09.35.58
> Fax: +33 (0)5.49.09.65.26
> http://www.cebc.cnrs.fr/
>
> ***************************************************
>
>
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-- 
Michael Sumner
Institute for Marine and Antarctic Studies, University of Tasmania
Hobart, Australia
e-mail: mdsumner at gmail.com