NAG FL Interface
g05zqf (field_​2d_​user_​setup)

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1 Purpose

g05zqf performs the setup required in order to simulate stationary Gaussian random fields in two dimensions, for a user-defined variogram, using the circulant embedding method. Specifically, the eigenvalues of the extended covariance matrix (or embedding matrix) are calculated, and their square roots output, for use by g05zsf, which simulates the random field.

2 Specification

Fortran Interface
Subroutine g05zqf ( ns, xmin, xmax, ymin, ymax, maxm, var, cov2, even, pad, icorr, lam, xx, yy, m, approx, rho, icount, eig, iuser, ruser, ifail)
Integer, Intent (In) :: ns(2), maxm(2), even, pad, icorr
Integer, Intent (Inout) :: iuser(*), ifail
Integer, Intent (Out) :: m(2), approx, icount
Real (Kind=nag_wp), Intent (In) :: xmin, xmax, ymin, ymax, var
Real (Kind=nag_wp), Intent (Inout) :: ruser(*)
Real (Kind=nag_wp), Intent (Out) :: lam(MAXM(1)*MAXM(2)), xx(NS(1)), yy(NS(2)), rho, eig(3)
External :: cov2
C Header Interface
#include <nag.h>
void  g05zqf_ (const Integer ns[], const double *xmin, const double *xmax, const double *ymin, const double *ymax, const Integer maxm[], const double *var,
void (NAG_CALL *cov2)(const double *x, const double *y, double *gamma, Integer iuser[], double ruser[]),
const Integer *even, const Integer *pad, const Integer *icorr, double lam[], double xx[], double yy[], Integer m[], Integer *approx, double *rho, Integer *icount, double eig[], Integer iuser[], double ruser[], Integer *ifail)
The routine may be called by the names g05zqf or nagf_rand_field_2d_user_setup.

3 Description

A two-dimensional random field Z(x) in 2 is a function which is random at every point x2, so Z(x) is a random variable for each x. The random field has a mean function μ(x)=𝔼[Z(x)] and a symmetric positive semidefinite covariance function C(x,y)=𝔼[(Z(x)-μ(x))(Z(y)-μ(y))]. Z(x) is a Gaussian random field if for any choice of n and x1,,xn2, the random vector [Z(x1),,Z(xn)]T follows a multivariate Normal distribution, which would have a mean vector μ~ with entries μ~i=μ(xi) and a covariance matrix C~ with entries C~ij=C(xi,xj). A Gaussian random field Z(x) is stationary if μ(x) is constant for all x2 and C(x,y)=C(x+a,y+a) for all x,y,a2 and hence we can express the covariance function C(x,y) as a function γ of one variable: C(x,y)=γ(x-y). γ is known as a variogram (or more correctly, a semivariogram) and includes the multiplicative factor σ2 representing the variance such that γ(0)=σ2.
The routines g05zqf and g05zsf are used to simulate a two-dimensional stationary Gaussian random field, with mean function zero and variogram γ(x), over a domain [xmin,xmax]×[ymin,ymax], using an equally spaced set of N1×N2 points; N1 points in the x-direction and N2 points in the y-direction. The problem reduces to sampling a Normal random vector X of size N1×N2, with mean vector zero and a symmetric covariance matrix A, which is an N2×N2 block Toeplitz matrix with Toeplitz blocks of size N1×N1. Since A is in general expensive to factorize, a technique known as the circulant embedding method is used. A is embedded into a larger, symmetric matrix B, which is an M2×M2 block circulant matrix with circulant blocks of size M1×M1, where M12(N1-1) and M22(N2-1). B can now be factorized as B=WΛW*=R*R, where W is the two-dimensional Fourier matrix (W* is the complex conjugate of W), Λ is the diagonal matrix containing the eigenvalues of B and R=Λ12W*. B is known as the embedding matrix. The eigenvalues can be calculated by performing a discrete Fourier transform of the first row (or column) of B and multiplying by M1×M2, and so only the first row (or column) of B is needed – the whole matrix does not need to be formed.
The symmetry of A as a block matrix, and the symmetry of each block of A, depends on whether the variogram γ is even or not. γ is even in its first coordinate if γ([-x1,x2]T)=γ([x1,x2]T), even in its second coordinate if γ([x1,-x2]T)=γ([x1,x2]T), and even if it is even in both coordinates (in two dimensions it is impossible for γ to be even in one coordinate and uneven in the other). If γ is even then A is a symmetric block matrix and has symmetric blocks; if γ is uneven then A is not a symmetric block matrix and has non-symmetric blocks. In the uneven case, M1 and M2 are set to be odd in order to guarantee symmetry in B.
As long as all of the values of Λ are non-negative (i.e., B is positive semidefinite), B is a covariance matrix for a random vector Y which has M2 blocks of size M1. Two samples of Y can now be simulated from the real and imaginary parts of R*(U+iV), where U and V have elements from the standard Normal distribution. Since R*(U+iV)=WΛ12(U+iV), this calculation can be done using a discrete Fourier transform of the vector Λ12(U+iV). Two samples of the random vector X can now be recovered by taking the first N1 elements of the first N2 blocks of each sample of Y – because the original covariance matrix A is embedded in B, X will have the correct distribution.
If B is not positive semidefinite, larger embedding matrices B can be tried; however if the size of the matrix would have to be larger than maxm, an approximation procedure is used. We write Λ=Λ++Λ-, where Λ+ and Λ- contain the non-negative and negative eigenvalues of B respectively. Then B is replaced by ρB+ where B+=WΛ+W* and ρ(0,1] is a scaling factor. The error ε in approximating the distribution of the random field is given by
ε= (1-ρ) 2 traceΛ + ρ2 traceΛ- M .  
Three choices for ρ are available, and are determined by the input argument icorr:
g05zqf finds a suitable positive semidefinite embedding matrix B and outputs its sizes in the vector m and the square roots of its eigenvalues in lam. If approximation is used, information regarding the accuracy of the approximation is output. Note that only the first row (or column) of B is actually formed and stored.

4 References

Dietrich C R and Newsam G N (1997) Fast and exact simulation of stationary Gaussian processes through circulant embedding of the covariance matrix SIAM J. Sci. Comput. 18 1088–1107
Schlather M (1999) Introduction to positive definite functions and to unconditional simulation of random fields Technical Report ST 99–10 Lancaster University
Wood A T A and Chan G (1994) Simulation of stationary Gaussian processes in [0,1]d Journal of Computational and Graphical Statistics 3(4) 409–432

5 Arguments

1: ns(2) Integer array Input
On entry: the number of sample points to use in each direction, with ns(1) sample points in the x-direction, N1 and ns(2) sample points in the y-direction, N2. The total number of sample points on the grid is, therefore, ns(1)×ns(2).
Constraints:
  • ns(1)1;
  • ns(2)1.
2: xmin Real (Kind=nag_wp) Input
On entry: the lower bound for the x-coordinate, for the region in which the random field is to be simulated.
Constraint: xmin<xmax.
3: xmax Real (Kind=nag_wp) Input
On entry: the upper bound for the x-coordinate, for the region in which the random field is to be simulated.
Constraint: xmin<xmax.
4: ymin Real (Kind=nag_wp) Input
On entry: the lower bound for the y-coordinate, for the region in which the random field is to be simulated.
Constraint: ymin<ymax.
5: ymax Real (Kind=nag_wp) Input
On entry: the upper bound for the y-coordinate, for the region in which the random field is to be simulated.
Constraint: ymin<ymax.
6: maxm(2) Integer array Input
On entry: determines the maximum size of the circulant matrix to use – a maximum of maxm(1) elements in the x-direction, and a maximum of maxm(2) elements in the y-direction. The maximum size of the circulant matrix is thus maxm(1)×maxm(2).
Constraints:
  • if even=1, maxm(i) 2 k , where k is the smallest integer satisfying 2 k 2 (ns(i)-1) , for i=1,2;
  • if even=0, maxm(i) 3 k , where k is the smallest integer satisfying 3 k 2 (ns(i)-1) , for i=1,2.
7: var Real (Kind=nag_wp) Input
On entry: the multiplicative factor σ2 of the variogram γ(x).
Constraint: var0.0.
8: cov2 Subroutine, supplied by the user. External Procedure
cov2 must evaluate the variogram γ(x) for all x if even=0, and for all x with non-negative entries if even=1. The value returned in gamma is multiplied internally by var.
The specification of cov2 is:
Fortran Interface
Subroutine cov2 ( x, y, gamma, iuser, ruser)
Integer, Intent (Inout) :: iuser(*)
Real (Kind=nag_wp), Intent (In) :: x, y
Real (Kind=nag_wp), Intent (Inout) :: ruser(*)
Real (Kind=nag_wp), Intent (Out) :: gamma
C Header Interface
void  cov2 (const double *x, const double *y, double *gamma, Integer iuser[], double ruser[])
1: x Real (Kind=nag_wp) Input
On entry: the coordinate x at which the variogram γ(x) is to be evaluated.
2: y Real (Kind=nag_wp) Input
On entry: the coordinate y at which the variogram γ(x) is to be evaluated.
3: gamma Real (Kind=nag_wp) Output
On exit: the value of the variogram γ(x).
4: iuser(*) Integer array User Workspace
5: ruser(*) Real (Kind=nag_wp) array User Workspace
cov2 is called with the arguments iuser and ruser as supplied to g05zqf. You should use the arrays iuser and ruser to supply information to cov2.
cov2 must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which g05zqf is called. Arguments denoted as Input must not be changed by this procedure.
Note: cov2 should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by g05zqf. If your code inadvertently does return any NaNs or infinities, g05zqf is likely to produce unexpected results.
9: even Integer Input
On entry: indicates whether the covariance function supplied is even or uneven.
even=0
The covariance function is uneven.
even=1
The covariance function is even.
Constraint: even=0 or 1.
10: pad Integer Input
On entry: determines whether the embedding matrix is padded with zeros, or padded with values of the variogram. The choice of padding may affect how big the embedding matrix must be in order to be positive semidefinite.
pad=0
The embedding matrix is padded with zeros.
pad=1
The embedding matrix is padded with values of the variogram.
Suggested value: pad=1.
Constraint: pad=0 or 1.
11: icorr Integer Input
On entry: determines which approximation to implement if required, as described in Section 3.
Suggested value: icorr=0.
Constraint: icorr=0, 1 or 2.
12: lam(maxm(1)×maxm(2)) Real (Kind=nag_wp) array Output
On exit: contains the square roots of the eigenvalues of the embedding matrix.
13: xx(ns(1)) Real (Kind=nag_wp) array Output
On exit: the points of the x-coordinates at which values of the random field will be output.
14: yy(ns(2)) Real (Kind=nag_wp) array Output
On exit: the points of the y-coordinates at which values of the random field will be output.
15: m(2) Integer array Output
On exit: m(1) contains M1, the size of the circulant blocks and m(2) contains M2, the number of blocks, resulting in a final square matrix of size M1×M2.
16: approx Integer Output
On exit: indicates whether approximation was used.
approx=0
No approximation was used.
approx=1
Approximation was used.
17: rho Real (Kind=nag_wp) Output
On exit: indicates the scaling of the covariance matrix. rho=1.0 unless approximation was used with icorr=0 or 1.
18: icount Integer Output
On exit: indicates the number of negative eigenvalues in the embedding matrix which have had to be set to zero.
19: eig(3) Real (Kind=nag_wp) array Output
On exit: indicates information about the negative eigenvalues in the embedding matrix which have had to be set to zero. eig(1) contains the smallest eigenvalue, eig(2) contains the sum of the squares of the negative eigenvalues, and eig(3) contains the sum of the absolute values of the negative eigenvalues.
20: iuser(*) Integer array User Workspace
21: ruser(*) Real (Kind=nag_wp) array User Workspace
iuser and ruser are not used by g05zqf, but are passed directly to cov2 and may be used to pass information to this routine.
22: ifail Integer Input/Output
On entry: ifail must be set to 0, −1 or 1 to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of 0 causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of −1 means that an error message is printed while a value of 1 means that it is not.
If halting is not appropriate, the value −1 or 1 is recommended. If message printing is undesirable, then the value 1 is recommended. Otherwise, the value 0 is recommended. When the value -1 or 1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry ifail=0 or −1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
ifail=1
On entry, ns=[value,value].
Constraint: ns(1)1, ns(2)1.
ifail=2
On entry, xmin=value and xmax=value.
Constraint: xmin<xmax.
ifail=4
On entry, ymin=value and ymax=value.
Constraint: ymin<ymax.
ifail=6
On entry, maxm=[value,value].
Constraint: the minima for maxm are [value,value].
Where, if even=1, the minimum calculated value of maxm(i) is given by 2 k , where k is the smallest integer satisfying 2 k 2 (ns(i)-1) , and if even=0, the minimum calculated value of maxm(i) is given by 3 k , where k is the smallest integer satisfying 3 k 2(ns(i)-1) , for i=1,2.
ifail=7
On entry, var=value.
Constraint: var0.0.
ifail=9
On entry, even=value.
Constraint: even=0 or 1.
ifail=10
On entry, pad=value.
Constraint: pad=0 or 1.
ifail=11
On entry, icorr=value.
Constraint: icorr=0, 1 or 2.
ifail=-99
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
ifail=-399
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
ifail=-999
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

If on exit approx=1, see the comments in Section 3 regarding the quality of approximation; increase the values in maxm to attempt to avoid approximation.

8 Parallelism and Performance

g05zqf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
g05zqf makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

None.

10 Example

This example calls g05zqf to calculate the eigenvalues of the embedding matrix for 25 sample points on a 5×5 grid of a two-dimensional random field characterized by the symmetric stable variogram:
γ(x) = σ2 exp(- (x) ν ) ,  
where x=|x1+y2|, and 1, 2 and ν are parameters.
It should be noted that the symmetric stable variogram is one of the pre-defined variograms available in g05zrf. It is used here purely for illustrative purposes.

10.1 Program Text

Program Text (g05zqfe.f90)

10.2 Program Data

Program Data (g05zqfe.d)

10.3 Program Results

Program Results (g05zqfe.r)