GMS - contours and 3D iso-surfaces can be used to display the variation of input data or computed results
Contour Interval
Four options are provided for defining contour intervals in the Contour Options dialog. The options are as follows:
Number of Contours
With the Number of Contours option, an integer is entered representing the total number of contours. The contour interval is adjusted based on the current active data set so that the contours are evenly spaced and the number of contours correspond exactly to the specified value.
Specified Interval
With the Specified Interval option, the contour interval (5, 10, 20, etc.) is entered directly by the user.
Specified Values
The Specified Values option allows the user to enter a list specific contour values. Contours are only generated at these values.
Log Interval
When contouring chemical data, it is common to have a small "hot spot" somewhere in the interior of the grid where the concentrations are several orders of magnitude higher than the rest of the grid. In such cases, the contours are all biased to the high concentration zones and no contours are drawn in the low concentration zones. A common approach for dealing with such situations is to select a set of contour values corresponding to multiples of ten (0.01, 0.1, 1.0 10.0, etc.). This can be accomplished by explicitly assigning a set of contour intervals using the Specified Values option. The problem with this approach is that the colors assigned to the contours are still based on a linear interpolation of the data values, thus skewing the colors to the higher concentration zones.
If the Log Interval option is selected, GMS automatically assigns contour intervals as multiples of 10. Furthermore, if the color-fill or color ramp option is being used, the colors assigned to the contours are distributed in a logarithmic fashion rather than linearly interpolated from the low to high values. A sample plot using logarithmic contour intervals is shown below:
If the Log interval option is selected, all contour intervals will be positive. No contours are drawn in areas where the data values are zero or negative. Furthermore, a "Starting value" must be entered. This value represents the minimum contour value.
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