R: Are points inside boundary?

inSide {mgcv} | R Documentation

## Are points inside boundary?

### Description

Assesses whether points are inside a boundary. The boundary must enclose the domain, but may include islands.

### Usage

    inSide(bnd,x,y)

### Arguments

`bnd` | This should have two equal length columns with names matching whatever is supplied in `x` and `y`. This may contain several sections of boundary separated by `NA`. Alternatively `bnd` may be a list, each element of which contains 2 columns named as above. See below for details.
`x` | x co-ordinates of points to be tested.
`y` | y co-ordinates of points to be tested.

### Details

Segments of boundary are separated by `NA`s, or are in separate list elements. The boundary co-ordinates are taken to define nodes which are joined by straight line segments in order to create the boundary. Each segment is assumed to define a closed loop, and the last point in a segment will be assumed to be joined to the first. Loops must not intersect (no test is made for this).

The method used is to count how many times a line, in the y-direction from a point, crosses a boundary segment. An odd number of crossings defines an interior point. Hence in geographic applications it would be usual to have an outer boundary loop, possibly with some inner ‘islands’ completely enclosed in the outer loop.

The routine calls compiled C code and operates by an exhaustive search for each point in `x, y`.

### Value

The function returns a logical array of the same dimension as `x` and `y`. `TRUE` indicates that the corresponding `x, y` point lies inside the boundary.

### Author(s)

Simon N. Wood simon.wood@r-project.org

### References

http://www.maths.bath.ac.uk/~sw283/

### Examples

    require(mgcv)
    m <- 300;n <- 150
    xm <- seq(-1,4,length=m);yn<-seq(-1,1,length=n)
    x <- rep(xm,n);y<-rep(yn,rep(m,n))
    er <- matrix(fs.test(x,y),m,n)
    bnd <- fs.boundary()
    in.bnd <- inSide(bnd,x,y)
    plot(x,y,col=as.numeric(in.bnd)+1,pch=".")
    lines(bnd$x,bnd$y,col=3)
    points(x,y,col=as.numeric(in.bnd)+1,pch=".")
    ## check boundary details ...
    plot(x,y,col=as.numeric(in.bnd)+1,pch=".",ylim=c(-1,0),xlim=c(3,3.5))
    lines(bnd$x,bnd$y,col=3)
    points(x,y,col=as.numeric(in.bnd)+1,pch=".")

[Package mgcv version 1.8-3 Index]