Merge a list of SpatialLines

Question

I started off with a SpatialPointsDataFrame, containing locations from animals wearing GPS loggers. Aside from the coordinates, the data frame also contains the animals' ID and a timestamp.

From this SpatialPointsDataFrame, I created lines that show the trajectory of each animal over the tracking period. I've used the following code:

spdf4tri#the SpatialPointsDataFrame

ids=unique(spdf4tri$id)
trajectory <- list()
 for (i in ids){ 
   spdf4bird<-subset(spdf4tri, id==i)
   birdtrajectory<-SpatialLines(list(Lines(list(Line(spdf4bird)), "id")))
   trajectory[[i]]<-birdtrajectory
   print(i)
 }

Now, I have a problem, because trajectory is of the class 'list'. I cannot figure out how to unlist it.

Answer 1

As I mentioned in my comment, you can use do.call for this. Here is an example.

First, add sp library and create a list with three SpatialLines objects (would work with Lines object as well).

library(sp)
sp.lines <- list()
  sp.lines[[1]] <- SpatialLines(list(Lines(list(Line(cbind(c(1,2,3),c(3,2,2)))),ID="a"))) 
  sp.lines[[2]] <- SpatialLines(list(Lines(list(Line(cbind(c(1,2,3)+0.5,c(3,2,2)+0.5))),ID="b"))) 
  sp.lines[[3]] <- SpatialLines(list(Lines(list(Line(cbind(c(1,2,3),c(1,1.5,1)))),ID="c"))) 

Now, use do.call with rbind to merge the lines into a single SpatialLines object.

merged.lines <- do.call(rbind, sp.lines)
  class(merged.lines)
  length(merged.lines)
  plot(merged.lines, col=1:3)

Answer 2

The clue is building the SpatialLines object later, once you have a list of Lines objects. The ID MUST be unique, otherwise all your Lines objects will have the same ID, and SpatialLines will throw an error.

I prepared this example code to illustrate:

 library(sp)
 library(raster) # need a raster to generate fake data
 library(dismo)  # I'm using the "randomPoints" function from dismo

 # first we need a canvas
 # we can start with an arbitrary extent
 ext <- extent(-20,10,-10,0)
 r <- raster(ext) # this is our raster object
 r[] <- 1 # let's fill it up with 1's

 indiv <- LETTERS[1:5] # let's say we have 5 individuals with data
 df <- data.frame()
 # we are generating 10 random points per "individual"
 for (i in 1:5)
      df <- rbind(df,randomPoints(r, 10))
 # you will get a warning about the projection, ignore that.

 # with this we can generate a SpatialPointsDataFrame
 spdf <- SpatialPointsDataFrame(
              coords=as.matrix(df), 
              data=data.frame(ind=sort(rep(indiv,10))))

 # now we loop for each individual 
 # and store "Lines" objects in a list
 trajectories <- list()
 for (i in 1:length(indiv))
         trajectories[[i]] <- Lines(list(Line(subset(spdf, ind == indiv[i]))), 
                                    ID=paste(i))
 # pay close attention on how the Lines object must be structured:
 # Lines(list(Line(spdf)), ID="unique string")

 # then we simply transform our trajectories list to SpatialLines
 splo <- SpatialLines(trajectories)

You can then visualize the data like this:

 image(r)
 plot(splo, add=T, col=rainbow(5))
 plot(spdf, add=T, pch=16, col=rainbow(5)[ spdf$ind ])

Answer 3

This is a list because your asked it to be a list. If your question is to have all your trajectories inside an object that is a SpatialLines, you should try this:

ids=unique(spdf4tri$id)
trajectory <- list()
 for (i in ids){ 
   spdf4bird <- subset(spdf4tri, id == i)
   trajectory[[i]] <- Lines(list(Line(spdf4bird)), "id")
   print(i)
 }
trajectory.sp <- SpatialLines(trajectory)

You have then one Lines for each animal. You can also add a dataframe if you have more information on the animals, for instance, with their id again:

trajectory.sp.data <- SpatialLinesDataFrame(trajectory.sp, 
    data = data.frame(id = ids))

Then you can plot with colour depending on id:

plot(trajectory.sp.data, col = rainbow(length(ids))[trajectory.sp.data$id])

I cannot test the script without your data, but this should work.
Is this what you were looking for ?

Answer 4

As an alternative: the rgeos package has a function gLineMerge to directly merge multiple lines from a SpatialLinesDataFrame to a single line:

library("rgeos")
merged_lines <- gLineMerge(your_spatial_data_frame)