poly.image {fields} | R Documentation

## Image plot for cells that are irregular quadrilaterals.

### Description

Creates an image using polygon filling based on a grid of irregular quadrilaterals. This function is useful for a regular grid that has been transformed to another nonlinear or rotated coordinate system. This situation comes up in lon-lat grids created under different map projections. Unlike the usual image format this function requires the grid to be specified as two matrices x and y that given the grid x and y coordinates explicitly for every grid point.

### Usage

    poly.image(x, y, z, col = tim.colors(64), transparent.color = "white",
     midpoint = FALSE, zlim = range(z, na.rm = TRUE),
     xlim = range(x), ylim = range(y), add = FALSE, border=NA,...)

    poly.image.regrid(x)

### Arguments

`x` | A matrix of the x locations of the grid.
`y` | A matrix of the y locations of the grid.
`z` | Values for each grid cell. Can either be the value at the grid points or interpreted as the midpoint of the grid cell.
`col` | Color scale for plotting.
`transparent.color` | Color to plot cells that are outside the range specified in the function call.
`midpoint` | Only relevant if the dimensions of x,y, and z are the same. If TRUE the z values will be averaged and then used as the cell midpoints. If FALSE the x/y grid will be expanded and shifted to represent grid cells corners. (See poly.image.regrid.)
`zlim` | Plotting limits for z.
`xlim` | Plotting limits for x.
`ylim` | Plotting limits for y.
`add` | If TRUE will add image onto current plot.
`border` | Color of the edges of the quadrilaterals, the default is no color.
`...` | If add is FALSE, additional graphical arguments that will be supplied to the plot function.

### Details

This function is straightforward except in the case when the dimensions of x,y, and z are equal. In this case the relationship of the values to the grid cells is ambigious and the switch midpoint gives two possible solutions. The z values at 4 neighboring grid cells can be averaged to estimate a new value interpreted to be at the center of the grid. This is done when midpoint is TRUE. Alternatively the full set of z values can be retained by redefining the grid. This is accomplisehd by finding the midpoints of x and y grid points and adding two outside rows and cols to complete the grid. The new result is a new grid that is is (M+1)X (N+1) if z is MXN. These new grid points define cells that contain each of the original grid points as their midpoints. Of course the advantage of this alternative is that the values of z are preserved in the image plot; a feature that may be important for some uses.

The function image.plot uses this function internally when image information is passed in this format and can add a legend. In most cases just use image.plot.

The function `poly.image.regrid` does a simple averaging and extrapolation of the grid locations to shift from midpoints to corners. In the interior grid corners are found by the average of the 4 closest midpoints. For the edges the corners are just extrapolated based on the separation of nieghboring grid cells.

### Author(s)

Doug Nychka

### See Also

image.plot

### Examples

    data(RCMexample)
    set.panel( 1,2)
    par(pty="s")
    # plot with grid modified
    poly.image( RCMexample$x, RCMexample$y, RCMexample$z[,,1])

    # use midpoints of z
    poly.image( RCMexample$x, RCMexample$y, RCMexample$z[,,1],midpoint=TRUE)

    # images are very similar.
    set.panel()
    # Regridding of x and y
    l1<- poly.image.regrid( RCMexample$x)
    l2<- poly.image.regrid( RCMexample$y)

    # test that this works
    i<- 1:10
    plot( l1[i,i], l2[i,i])
    points( RCMexample$x[i,i], RCMexample$y[i,i],col="red")

[Package fields version 6.01 Index]