gstat (version 0.9-11)

krige.cv: kriging cross validation, n-fold or leave-one-out

Description

Cross validation function for simple, ordinary or universal point kriging, kriging in a local neighbourhood.

Usage

krige.cv(formula, locations, data, model = NULL, beta = NULL, nmax = Inf, 
	nmin = 0, maxdist = Inf, nfold = nrow(data), verbose = TRUE, ...)

Arguments

formula

formula that defines the dependent variable as a linear model of independent variables; suppose the dependent variable has name z, for ordinary and simple kriging use the formula z~1; for simple kriging also define be

locations

formula with only independent variables that define the spatial data locations (coordinates), e.g. ~x+y; if data is of class spatial.data.frame, this argument may be ignored, as it can be derived from the data

data

data frame; should contain the dependent variable, independent variables, and coordinates.

model

variogram model of dependent variable (or its residuals), defined by a call to vgm or fit.variogram

beta

only for simple kriging (and simulation based on simple kriging); vector with the trend coefficients (including intercept); if no independent variables are defined the model only contains an intercept and this should be the simple kriging mean

nmax

for local kriging: the number of nearest observations that should be used for a kriging prediction or simulation, where nearest is defined in terms of the space of the spatial locations. By default, all observations are used

nmin

for local kriging: if the number of nearest observations within distance maxdist is less than nmin, a missing value will be generated; see maxdist

maxdist

for local kriging: only observations within a distance of maxdist from the prediction location are used for prediction or simulation; if combined with nmax, both criteria apply

nfold

apply n-fold cross validation; if nfold is set to nrow(data) (the default), leave-one-out cross validation is done; if set to e.g. 5, five-fold cross validation is done

verbose

logical; if TRUE, progress is printed

...

other arguments that will be passed to gstat

Value

• a data frame containing the coordinates of newdata, and columns of prediction and prediction variance of cross validated data points, observed values, residuals, zscore (residual divided by kriging standard error), and fold.

Details

Leave-one-out cross validation (LOOCV) visits a data point, and predicts the value at that location by leaving out the observed value, and proceeds with the next data point. (The observed value is left out because kriging would otherwise predict the value itself.) N-fold cross validation makes a partitions the data set in N parts. For all observation in a part, predictions are made based on the remaining N-1 parts; this is repeated for each of the N parts. N-fold cross validation is much faster than LOOCV.

References

http://www.gstat.org/

See Also

krige, gstat, predict.gstat

Examples

data(meuse)
m <- vgm(.59, "Sph", 874, .04)
# five-fold cross validation:
x <- krige.cv(log(zinc)~1, ~x+y, model = m, data = meuse, nmax = 40, nfold=5)
bubble(x, z = "residual", main = "log(zinc): 5-fold CV residuals")