[R-sig-Geo] Generating Random Transects of Same Length

[Marcelino de la Cruz]

As an spatstat aficionado, I would use the following code:

Enjoy!

Marcelino


library(spatstat)

# Define polygon, length of transect and number of (points)transects
data(letterR)
mywindow <- letterR
ltransect <- 0.3
npoints <- 100
s<- 1:npoints

# Generate random origin points
cosa <- runifpoint(npoints, w=mywindow)
plot(cosa)


cosaxy <- data.frame(cosa$x,cosa$y)

#compute a circle around each point
cosadisc<- apply(cosaxy,1, function(x) disc(r=ltransect, x))

# Test if every circle point is inside polygonal boundary
cosadisc.df <- lapply(cosadisc, function(W){
                          inside.owin(W$bdry[[1]]$x,W$bdry[[1]]$y 
,w=mywindow)})

#function to sample circle points within the window
samplea2 <- function(cosaxy, l1=cosadisc, l2=cosadisc.df){
result<-c(0,0)
for (i in 1:length(l1)){
                  truinside<-sum(l2[[i]])
                  inside 
<-cbind(l1[[i]]$bdry[[1]]$x,l1[[i]]$bdry[[1]]$y)[l2[[i]],]
                  result<-rbind(result,  inside[sample(1:truinside, size=1),])
}
result<-result[-1,]
result<-cbind(cosaxy,result)
return(result)
}

#the result is a matrix with x0,y0, x1, y1 for each transect
#Plot the random transects:

segmentos<-samplea2(cosaxy)
segments(segmentos[,1][s], segmentos[,2][s],segmentos[,3][s], segmentos[,4][s])




At 19:33 12/02/2009, Barry Rowlingson wrote:
>2009/2/12 Pierre Racine <Pierre.Racine at sbf.ulaval.ca>:
> > Hi,
> >
> > Anybody knows a tool or algorythm (R, GRASS or ArcGIS Script, anything!)
> > to generate a number of random same length transects (e.g. 1000) within
> > the extent of a given polygon or raster?
> >
> > I can't just create random point in the extent and joint hem two by two.
> > This would result in different length transects.
> >
> > A simple algorythm would be:
> >
> > -create a transect of a certain length from a random point and a random
> > angle
> > -reject any transect having an end falling outside the given extent
> >
> > But there is chances that this takes years to run...
>
>  What do you mean by 'random' transects?
>
>  You could define a buffer line of distance d around the inside your
>polygon. This divides space into three regions - the outside area, the
>buffer zone area, and the inside area. The buffer zone area and the
>inside area make up your original polygon.
>
>  Then choose your first point from a spatial uniform distribution on
>the inside area. Then pick another point at a random U(0,2*pi) angle
>and distance d. This is guaranteed to be in the original polygon, so
>there's no rejection step. However, you'll never get transects with
>start and end both within the buffer zone. But they would still, in
>some sense of the word, be 'random'.
>
>  If this isn't clear then I think a picture would make it so! I think
>you can do buffer operations in R... Grass certainly can!
>
>Barry
>
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________________________________

Marcelino de la Cruz Rot

Departamento de  Biología Vegetal
E.U.T.I. Agrícola
Universidad Politécnica de Madrid
28040-Madrid
Tel.: 91 336 54 35
Fax: 91 336 56 56
marcelino.delacruz at upm.es