smooth.map {maps} | R Documentation

## Smooth out aggregated data

### Description

Increases the resolution of data aggregated over map regions, by either smoothing or interpolation. Also fills in missing values.

### Usage

    smooth.map(m, z, res = 50, span = 1/10, averages = FALSE, type = c("smooth",
    "interp"), merge = FALSE)

### Arguments

`m` | a map object
`z` | a named vector
`res` | a vector of length two, specifying the resolution of the sampling grid in each dimension. If a single number, it is taken as the vertical resolution, with double taken as the horizontal resolution.
`span` | kernel parameter (larger = smoother). `span = Inf` is a special case which invokes the cubic spline kernel. `span` is automatically scaled by the map size, and is independent of `res`.
`averages` | If `TRUE`, the values in `z` are interpreted as averages over the regions. Otherwise they are interpreted as totals.
`type` | see details.
`merge` | If `TRUE`, a region named in `z` includes all matching regions in the map (according to `match.map`). If `FALSE`, a region named in `z` is assumed to refer to exactly one region on the map.

### Details

For `type = "smooth"`, the region totals are first converted into point measurements on the sampling grid, by dividing the total for a region among all sample points inside it. Then it is a regular kernel smoothing problem. Note that the region totals are not preserved.

The prediction zo for location xo (a vector) is the average of `z` for nearby sample points:

zo = (sum_x k(x, xo) z(x))/(sum_x k(x, xo))

k(x, xo) = exp(-lambda ||x - xo||^2)

lambda is determined from `span`. Note that xo is over the same sampling grid as x, but zo is not necessarily the same as z(xo).

For `type = "interp"`, the region totals are preserved by the higher-resolution function. The function is assumed to come from a Gaussian process with kernel k. The measurement `z[r]` is assumed to be the sum of the function over the discrete sample points inside region `r`. This leads to a simple formula for the covariance matrix of `z` and the cross-covariance between `zo` and `z`. The prediction is the cross-covariance times the inverse covariance times `z`. Unlike Tobler's method, the predictions are not constrained to live within the original data range, so there tends to be "ringing" effects.

See the references for more details.

### Value

A data frame with columns `x`, `y`, and `z` giving the smoothed value `z` for locations (x, y). Currently the (x, y) values form a grid, but this is not guaranteed in the future.

### Author(s)

Tom Minka

### References

W.F. Eddy and A. Mockus. An example of the estimation and display of a smoothly varying function of time and space - the incidence of disease mumps. Journal of the American Society for Information Science, 45(9):686-693, 1994. http://web.eecs.utk.edu/~audris/papers/jasis.pdf

W. R. Tobler. Smooth pycnophylactic interpolation for geographical regions. Journal of the American Statistical Association 74:519-530, 1979.

### Examples

    # compare to the example for match.map
    data(state, package = "datasets")
    data(votes.repub)
    z = votes.repub[, "1900"]
    m = map("state", fill = TRUE, plot = FALSE)
    # use a small span to fill in, but not smooth, the data
    # increase the resolution to get better results
    fit = smooth.map(m, z, span = 1/100, merge = TRUE, ave = TRUE)
    mat = tapply(fit$z, fit[1:2], mean)
    gray.colors <- function(n) gray(rev(0:(n - 1))/n)
    par(bg = "blue")
    filled.contour(mat, color.palette = gray.colors, nlev = 32, asp = 1)
    # another way to visualize:
    image(mat, col = gray.colors(100))

    # for a higher degree of smoothing:
    # fit = smooth.map(m, z, merge = TRUE, ave = TRUE)
    # interpolation, state averages are preserved:
    # fit = smooth.map(m, z, merge = TRUE, ave = TRUE, type = "interp")

[Package maps version 3.2.0 Index]