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 Post subject: problem applying RBF example rbf_d_ml_simple example
PostPosted: Thu May 02, 2013 6:53 am 

Joined: Tue Apr 23, 2013 3:18 am
Posts: 1
It maybe silly but I read over the example "rbf_d_ml_simple example" and found me not pro enough at statistics to realize the algorithm of RBF code T_T

I intended to write a method which could convert grid data into different intervals for GIS performance with RBF functions. But tested with ecmwf temperature grid data?value of which is between -20 to 40?I got values larger than 100...

Were the parameters set wrong especially 'lambda' of rbfsetalgomultilayer?

The function is written as below:

int interpolationmetod1()
    // This example shows how to build models with RBF-ML algorithm. Below
    // we assume that you already know basic concepts shown in the example
    // on RBF-QNN algorithm.
    // RBF-ML is a multilayer RBF algorithm, which fits a sequence of models
    // with decreasing radii. Each model is fitted with fixed number of
    // iterations of linear solver. First layers give only inexact approximation
    // of the target function, because RBF problems with large radii are
    // ill-conditioned. However, as we add more and more layers with smaller
    // and smaller radii, we get better conditioned systems - and more precise models.
    rbfmodel model;
    rbfreport rep;
    double v;
    int lon=361,lat=281;

    // We have 2-dimensional space and very simple interpolation problem - all
    // points are distinct and located at straight line. We want to solve it
    // with RBF-ML algorithm. This problem is very simple, and RBF-QNN will
    // solve it too, but we want to evaluate RBF-ML and to start from the simple
    // problem.
    //     X        Y
    //     -2       1
    //     -1       0
    //      0       1
    //     +1      -1
    //     +2       1
    rbfcreate(2, 1, model);
    real_2d_array xy0;
    qDebug()<<"matrix done";
    //read data of diamond4 type
    QString str,filename="E:\\13042420.003";
    QFile in(filename);
    if (!
        qDebug()<<filename<<" not exists";
        return 0;
    QTextStream file(&in);
    QFile out("e:\\interpolation.txt");;
    QTextStream ts(&out);

    int steplon=0, steplat=0;
        //ts<<"("<<steplat<<","<<steplon<<") "<<str<<"="<<xy0(steplat,steplon)<<" the same?"<<endl;

    rbfsetpoints(model, xy0);

    // First, we try to use R=5.0 with single layer (NLayers=1) and moderate amount
    // of regularization.... but results are disappointing: Model(x=0,y=0)=-0.02,
    // and we need 1.0 at (x,y)=(0,0). Why?
    // Because first layer gives very smooth and imprecise approximation of the
    // function. Average distance between points is 1.0, and R=5.0 is too large
    // to give us flexible model. It can give smoothness, but can't give precision.
    // So we need more layers with smaller radii.
    //rbfsetalgomultilayer(model, 5.0, 1, 1.0e-3);
    //rbfbuildmodel(model, rep);
    //printf("%d\n", int(rep.terminationtype)); // EXPECTED: 1
    //v = rbfcalc2(model, 0.0, 0.0);
    //printf("%.2f\n", double(v)); // EXPECTED: -0.021690

    // Now we know that single layer is not enough. We still want to start with
    // R=5.0 because it has good smoothness properties, but we will add more layers,
    // each with R[i+1]=R[i]/2. We think that 4 layers is enough, because last layer
    // will have R = 5.0/2^3 = 5/8 ~ 0.63, which is smaller than the average distance
    // between points. And it works!
    rbfsetalgomultilayer(model, 5.0, 4, 1.0e-2);
    rbfbuildmodel(model, rep);
    printf("%d\n", int(rep.terminationtype)); // EXPECTED: 1
    ts<<"now start interpolation"<<endl;
    for(double stepy=0; stepy<lat;stepy+=0.2)
        for(double stepx=0; stepx<lon;stepx+=0.2)
            //ts<<xy0(stepy,stepx)<<" ";
            v = rbfcalc2(model, stepy, stepx);
            ts <<"("<<stepy<<","<<stepx<<") = "<< v<<" ";
            ts <<endl;
    //printf("%.2f\n", double(v)); // EXPECTED: 1.000000

    // BTW, if you look at v, you will see that it is equal to 0.9999999997, not to 1.
    // This small error can be fixed by adding one more layer.
    return 0;

Hope someone could help me, thanks.

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