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Contours and gridding methods



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    Kari Dickenson


    The Z range calculated by the gridding algorithm in the grid file may or may not match the Z range in the original raw data. This is caused by the gridding algorithm itself and how it interpolates the Z values of the grid nodes from the original data points. For example, if you only have one data point that has Z=9, and the grid nodes are around that data point but not on it, then the grid nodes around it won't have a Z value of 9, but something a little smaller (if it's going downhill from that point).

    There are a few things you can try changing in the Grid Data dialog when gridding the data:

    1. If you are using Surfer 13, you can set the grid Z limits using the Grid Z Limits section in the Grid Data dialog.
    2. Use a different gridding method. The default Kriging 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. Some gridding methods do not extrapolate the Z range in the grid beyond the range of the data. These methods include: Inverse Distance to a Power, Natural Neighbor, Nearest Neighbor, and Triangulation with Linear Interpolation.
    3. Try increasing the # of Nodes in the Grid Data dialog.


    You might want to check out the KB article: My grid file has negative Z values, or a Z range that goes beyond the Z limits in my data file. How do I change this?

    I don't quite understand what you mean by each line representing 1 object, or what the objects are, or why you'd only have 8 lines instead of 9. Note that to have a contour line, you must have data above and below the value of the contour line. So if you want a contour line at Z=9, you need to be sure you have data above 9 so the line can be drawn. If you'd like, you are also welcome to email me a SRF file showing the contour map you have (a post layer of the original data points on top of the contours would also help), and explain what you are looking for.


    Kari kari@goldensoftware.com

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    Dirk Müller
    Hello, your description sound like that you defined a nugget effect in your variogram estimation. You can easily switch it off (if that was the reason for your problem). Regards
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