Surfer 12 New Feature Highlight: Grid Logarithmic Data

A great new feature has been added in Surfer 12! There is now the ability to grid or display logarithmic data. This feature can be used to display your data in a way that accurately conveys the logarithmic information in an easy to interpret display.

First, it would be useful to ask a few simple questions about why you would want to use this feature.

Why would I grid my data as log save as linear over log save as log?

This option is a good choice if your data spans several orders of magnitude and you want the resulting Z value in the grid file to be in the same units as your original data. However, the areas with the very low data values and the areas with the very high data values are smoothed over.

Why should I set the Level method for a contour map using my log save as log file to Logarithmic?

When you choose the Log, save as log option, log-distributed data is displayed much more accurately than the linear contour map, as it more closely honors the very low and very high data points.

Why should I set the colormap for that same contour map to Logarithmically-scaled?

It is important that the colormap and the contour map display using the same scaling. If the contours are displayed with a linear colormap, a logarithmic color scale bar would have a meaningless correlation to the map.

Below I have included a simple illustration from the Logarithmic sample file that comes with Surfer 12. The example shows how the same dataset is displayed when:

A. Gridding data as linear, displaying with a linear levels and colormap.
B. Gridding data as logarithmic, displaying with linear levels and colormap.
C. Gridding data as logarithmic, displaying with logarithmic levels and colormap.

Surfer 12 Example File: Logarthmic.srf
(C:\Program Files\Golden Software\Surfer 12\Samples\Logarithmic.srf)

You can see from the color scale in example C that the concentration (ppm) values range over 5 orders of magnitude, from 0.001 to 4000. Displaying a contour map of this data using a linear scale generates lines of equal concentration that lose much of the variability in the data. Displaying the contours using a logarithmic scale provides a much better indication of the distribution of the concentration throughout the map.

To put a real-world spin on this topic, consider that the above maps represent the plume of a ¹DNAPL contaminant, such as ²PCE. For this type of contaminant, even low-level contamination can present a significant human health risk when dissolved in groundwater. Using the grid from Example A, you might be tempted to underestimate the risk associated with the contamination which has migrated away from the source zone. Examples B and C better illustrate the extent and direction of contamination. They show isolated regions of high contamination concentration that do not stand out significantly in Example A. The logarithmically contoured grids show a higher degree of plume delineation than the linear grid does.

¹DNAPL: Dense Non-Aqueous Phase Liquid.
²PCE: Tetrachloroethene or Perchloroethene.

To create these maps for yourself, follow the example I have provided to make use of the new functionality in Surfer 12.