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, create an image map (or other map type) and 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 20, 2017*

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