You cannot take the log of a negative number or 0. The number must be positive and non-zero. This is not Surfer's rule, but a mathematical one. The log of (X) becomes asymptotic as X approaches 0.

If you already have a grid file with Z values that are negative or 0 and you create a contour map and try to set the *Level method* to *Logarithmic*, then you will get an error that the minimum contour level must be greater than 0.0. Click* OK* in the error. You will need to change the *Minimum* contour value to be >0.0, and then you can change the *Level method* to *Logarithmic.*

If you already have a grid file with negative or 0 values for some grid nodes, try to change the colormap to use logarithmic scaling, then you will get an error that logarithmic scaling requires positive data. Click *OK* in the error. You will need to change the *Minimum* and/or *Maximum* data value for the colormap so that they are both >0.0.

*Updated **October 25, 2018*

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