selectrandomquad: Draws rectangular quadrats in a plot at random.
In forestgeo/ctfs: Tools for the Analysis of Forest Dynamics

Description Usage Arguments Details Functions Arguments details

Draw rectangular quadrats in a plot at random. Returns a dataframe of coordinates defining the corners of all the quadrats.

selectrandomquad(size, shape, rep, plotdim = c(1000, 500),
  graphit = FALSE)

selectrandomquad2(size, rep, plotdim = c(1000, 500), graphit = FALSE)

selectrandomquad3(size, shape, rep, plotdim = c(1000, 500),
  graphit = FALSE)

`size`	A vector of quadrat sizes, referring to the x-dimension of a rectangular quadrat.
`shape`	The ratio of y to x dimensions of the rectangles; `rectdim = 1` (the default) for squares.
`rep`	The number of replicated random quadrats (per dimension submitted) to create.
`plotdim`	The x and y dimensions of the plot.
`graphit`	Logical; if `TRUE` adds plot output.

Quadrats are chosen simply: a point is chosen by randomly drawing one x and one y coordinates to serve as the lower-left corner. The x is chosen from a uniform distribution between 0 and plotdim[1]-size; y is chosen similarly.

All 3 algorithms (selectrandomquad(), selectrandomquad2() and selectrandomquad3()) under sample the plot corners. That bias, selectrandomquad2() and selectrandomquad3() intended to overcome; but neither does.

selectrandomquad2: Its algorithm aims at capturing corners. The result, however, is not to capture corners any better than selectrandomquad() does.

Imagine a line running vertically at x = 0 from y = 0 to y = plotdim[2] - size, then continues at x = 1 from 0 to plotdim[2] - size, etc. It's wrapping analogous to the way quadrat indices wrap (see ?gxgy.to.index). This line has length (plotdim[1] - size) * (plotdim[2] - size). Draw a random number on that line, and place the lower left corner of random square at that point. A position x on the line is converted to plot coordinates gx, gy using function index.to.gxgy() with grid = 1.
selectrandomquad3: The lower left x, y coordinates are drawn at random from a range extending outside the plot dimensions, then quadrats are used only if all 4 corners fall inside the plot.