vgram {gear} | R Documentation |
vgram
calculates an empirical variogram. Note that, by convention,
the empirical variogram actually estimates the semivariogram, not the
theoretical variogram (which is twice the semivariogram).
vgram(
formula,
data,
coordnames = NULL,
nbins = 10,
maxd = NULL,
angle = 0,
ndir = 1,
type = "standard",
npmin = 2,
longlat = FALSE,
verbose = TRUE,
coords = NULL
)
formula |
A formula describing the relationship between the response and any covariates of interest, e.g., response ~ 1. The variogram is computed for the residuals of the linear model |
data |
A |
coordnames |
The columns of |
nbins |
The number of bins (tolerance regions) to use when estimating the sample semivariogram. |
maxd |
The maximum distance used when calculating the semivariogram. Default is NULL, in which case half the maximum distance between coordinates is used. |
angle |
A single value (in degrees) indicating the starting direction for a directional variogram. The default is 0. |
ndir |
The number of directions for which to calculate a sample semivariogram. The default is 1, meaning calculate an omnidirection semivariogram. |
type |
The name of the estimator to use in the estimation process. The default is "standard", the typical method-of-moments estimator. Other options include "cressie" for the robust Cressie-Hawkins estimator, and "cloud" for a semivariogram cloud based on the standard estimator. If "cloud" is specified, the |
npmin |
The minimum number of pairs of points to use in the semivariogram estimator. For any bins with fewer points, the estimate for that bin is dropped. |
longlat |
A logical indicating whether Euclidean ( |
verbose |
Print computation information. Default is |
coords |
A deprecated argument. |
Note that the directions may be different from other packages
(e.g., gstat
or geoR
packages) because those
packages calculate angles clockwise from the y-axis, which is
a convention frequently seen in geostatistics
(e.g., the GSLIB software library).
Additionally, note that calculating the empirical variogram for the residuals of lm(response ~ 1) will produce identical results to simply computing the sample semivariogram from the original response. However, if a trend is specified (the righthand side of ~ has non-trival covariates), then the empirical variogram of the residuals will differ from that of the original response. A trend should be specified when the mean is non-stationary over the spatial domain.
Returns an evgram
object with components:
Joshua French
data(co)
v = vgram(Al ~ 1, co, ~ easting + northing)
plot(v)
v2 = vgram(Al ~ 1, co, c("easting", "northing"), angle = 22.5, ndir = 4)
plot(v2)