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Contour Lines Kriging Problem

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    Kari Dickenson

    Hi Alice,

    This problem is usually a result of the grid resolution and/or gridding methods.

    The most common cause of contours not honoring data points is insufficient resolution in the grid (GRD) file. The contours are based on the grid nodes in the grid file. If the grid nodes are too far apart, Surfer will use all the data points that lie in between and around the nodes to interpolate that node's Z value - so it can overgeneralize some areas with tightly spaced data points.

    To resolve the problem, grid the data with a small enough grid line spacing to match the distance between closest data points. For example, from the image, it looks like these two 0 points are pretty close together.

    If the grid node spacing is father apart than the distance between these two points, then the grid nodes can't accurately reflect that data since it will be interpolated using the 10 data points as well. The closer the grid nodes are to the data points, the more likely they'll match the actual data point values. When you are in the Grid Data dialog, increase the # of Nodes (lower right) or decrease the grid Spacing value.

    If that doesn't help, there are a few more suggestions:

    1. Use a different gridding method. The default Kriging gridding method creates trends in your data and can extrapolate the Z values in the grid file to extend past the limits of the Z data in your original file. It also creates a best fit grid and does not always exactly honor each data point from the original file. Try another gridding method, such as Natural Neighbor, Radial Basis Function, Triangulation with Linear Interpolation, or Inverse Distance to a Power.
    2. Add more data points. Sometimes, this problem is because Surfer simply can’t create the contours with the current data points and accurately keep each value. More data points would help. If you do not have more data points, you might be able to add a few dummy points to your data file to help constrain the contours to the shape that you want.

     

    I hope that helps!

    Thanks,

    Kari

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