estimateAnisotropy {intamap}

estimateAnisotropy

Description

This function estimates geometric anisotropy parameters for 2-D scattered data using the CTI method.

Usage

estimateAnisotropy(object,depVar, formulaString)

Arguments

object

(i) An Intamap type object (see intamap-package) containing one SpatialPointsDataFrame data frame named observations which includes the observed values (ii) or a SpatialPointsDataFrame which includes both coordinates and observations.

depVar

name of the dependent variable; this is used only in case (ii).

formulaString

formula that defines the dependent variable as a linear model of independent variables, only used for case (ii); suppose the dependent variable has name z, for ordinary and simple kriging use the formula z~1; for universal kriging, suppose z is linearly dependent on x and y, use the formula z~x+y. The formulaString defaults to "value~1" if value is a part of the data set. If not, the first column of the data set is used.

Details

Given the input object that defines N coordinate pairs (x,y) and observed values (z), this method estimates of the geometric anisotropy parameters. Geometric anisotropy is a statistical property, which implies that the iso-level contours of the covariance function are elliptical. In this case the anisotropy is determined from the anisotropic ratio (R) and the orientation angle (\theta) of the ellipse.

Assuming a Cartesian coordinate system of axes x and y, \theta represents the angle between the horizontal axis and PA1, where PA1 is one of the principal axes of the ellipse, arbitrarily selected (PA2 will denote the other axis). R represents the ratio of the correlation along PA1 divided by the correlation length PA2. Note that the returned value of R is always greater than one (see value below.)

The estimation is based on the Covariance Tensor Identity (CTI) method. In CTI, the Hessian matrix of the covariance function is estimated from sample derivatives. The anisotropy parameters are estimated by explicit solutions of nonlinear equations that link (R,\theta) with ratios of the covariance Hessian matrix elements.

To estimate the sample derivatives from scattered data, a background square lattice is used. The lattice extends in the horizontal direction from x.min to x.max where x.min (x.max) is equal to the minimum (maximum) x-coordinate of the data, and similarly in the vertical direction. The cell step in each direction is equal to the length of the lattice to the respective direction divided by the square root of N.

BiLinear interpolation, as implemented in akima package, is used to interpolate the field's z values at the nodes of the lattice.

The CTI method is described in detail in (Chorti and Hristopulos, 2008).

Note that to be compatible with gstat the returned estimate of the anisotropy ratio is always greater than 1.

For observations assumed to have a trend, the trend is first subtracted from the data using universal kriging. This is an approximation, as the trend subtraction does not take anisotropy into account.

Value

(i) If the input is an Intamap object, the value is a modification of the input object, containing a list element anisPar with the estimated anisotropy parameters. (ii)if the input is a SpatialPointsDataFrame, then only the list anisPar is returned. The list anisPar contains the following elements:

ratio

The estimate of the anisotropy ratio parameter. Using the degeneracy of the anisotropy under simultaneous ratio inversion and axis rotation transformations, the returned value of the ratio is always greater than 1.

direction

The estimate of the anisotropy orientation angle. It returns the angle between the major anisotropy axis and the horizontal axis, and its value is in the interval (-90,90) degrees.

Q

A 3x1 array containing the sample estimates of the diagonal and off-diagonal elements (Q11,Q22,Q12) of the covariance Hessian matrix evaluated at zero lag.

doRotation

Boolean value indicating if the estimated anisotropy is statistically significant. This value is based on a statistical test of the isotropic (R= 1) hypothesis using a non-parametric approximation for the 95 percent confidence interval for R. This approximation leads to conservative (wider than the true) estimates of the confidence interval. If doRotation==TRUE then an isotropy restoring transformation (rotation and rescaling) is performed on the coordinates. If doRotation==FALSE no action is taken.

Note

This function uses akima package to perform "bilinear" interpolation. The source code also allows other interpolation methods, but this option is not available when the function is called from within INTAMAP.

In the gstat package, the anisotropy ratio is defined in the interval (0,1) and the orientation angle is the angle between the vertical axis and the major anisotropy axis, measured in the clockwise direction. If one wants to use ordinary kriging inside INTAMAP the necessary transformations are performed in the function estimateParameters.automap. If one wants to use ordinary kriging in the gstat package (but outside INTAMAP) the required transformations can be found in the source code of the estimateParameters.automap function.

Author(s)

A.Chorti, D.T.Hristopulos,G. Spiliopoulos

References

[1] Pebesma, E., Cornford, D., Dubois, G., Heuvelink, G.B.M., Hristopulos, D., Pilz, J., Stohlker, U., Morin, G., Skoien, J.O. INTAMAP: The design and implementation f an interoperable automated interpolation Web Service. Computers and Geosciences 37 (3), 2011.

[2] A. Chorti and D. T. Hristopulos (2008). Non-parametric Identification of Anisotropic (Elliptic) Correlations in Spatially Distributed Data Sets, IEEE Transactions on Signal Processing, 56(10), 4738-4751 (2008).

[3] Em.Petrakis and D. T. Hristopulos (2009). A non-parametric test of statistical isotropy for Differentiable Spatial Random Fields in Two Dimensions. Work in progress. email: [e-mail]

Examples

    library(gstat)
    data(sic2004)
    coordinates(sic.val)=~x+y
    sic.val$value=sic.val$dayx

    params=NULL
    
    estimateAnisotropy(sic.val,depVar = "joker")