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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.

Image1Surfer 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.

Use the File | Open command to open the data file VOC_Concentrations.xlsx.

Use the Data | Statistics command to generate the following statistical table of the X, Y, and Z (ppm) data in the dataset:

 

X

Y

Z (ppm)

Number of values

49.00

49.00

49.00

Sum

165330.81

314649.53

12321.71

Minimum

443.22

4681.29

0.01

Maximum

4625.39

8556.32

5600.00

Range

4182.17

3875.03

5599.99

Mean

3374.10

6421.42

251.46

Standard deviation

1232.72

817.72

877.89

 
1.Create a Grid File from an XYZ Data File

  1. Use the Grid | Data command to begin the process of gridding your XYC data.
  2. In the Open | Data dialog, select the data file: VOC_Concentration.xlsx and click Open.
  3. In the Grid Data dialog:
    1. Set the X, Y, and Z Data Columns to Column A: X, Column B: Y, and Column C: ppm from your dataset.
    2. Select the Gridding Method that you would like to use. I have left this to the default method, Kriging.

      Image2a.png
    3. Specify the Z Transform you would like to use for the grid. The available options are:
      1. Linear – uses the Z values in the worksheet for gridding. No transformation is applied to the Z values. The Linear method is a good option for data that gradually increases over space.
      2.  Log, save as log – takes the log (base 10) of the Z values and uses the log value for gridding. The grid is then saved with the log (base 10) values
      3. Log, save as linear –takes the log (base 10) of the Z values and uses the log value for gridding. The grid is then converted back to the linear Z values by taking the antilog of the gridded results.

        Image3a.pngSpecify the Z Transform to create linear or log grid files of our data.

    4. Uncheck the box next to Blank grid outside convex hull of data so the contours will extend all the way from the minimum data limits to the maximum data limits, filling the entire rectangular map frame.
    5. Click OK to generate your grid file.
  4. If you checked the box next to Grid Report in the Grid Data dialog, a text report of the grid parameters and grid statistics will be generated. I have included the grid reports for each of the grid files created in this example and listed below:
    1.  Linear – GridDataReport-VOC_Concentration_Linear.rtf
    2.  Log, save as log – GridDataReport-VOC_Concentration_LogAsLinear.rtf
    3.  Log, save as linear – GridDataReport-VOC_Concentration_LogAsLog.rtf

2. Display your grid file using a contour map or 3D surface map

a.       To create a contour map, click Map | New | Contour Map. To create a 3D Surface, Click Map | New | 3D Surface.

b.      In the Open Grid dialog, select the grid file you just created and click Open.

3.    Apply Logarithmic scaling your colormap

If you have created a contour map using logarithmic distributed data, then the color scale will automatically adjust to a logarithmic color interval. In the example below, the image on the right is displayed with a logarithmic colormap, while the image on the left is display with a linear colormap.

 

 

Image4
Left: Log data displayed with a
Linear colormap and Simple Level Method. Right: Log data displayed with a Logarithmic colormap and Logarithmic Level Method

You can specify whether you want to display the Level method for the contours as linear (Simple and Advanced options) or Logarithmic.  To change this:

1.       In the Object Manager, select a Contours layer.

2.       In the Levels tab of the Property Manager, expand the + next to General.

3.       Next to Level method, select Simple, Logarithmic, or Advanced.

You can also specify the Minimum contour and Maximum contour for your Logarithmic color scale (outlined in green below).

Image5a
Specify the Level method for your Contour layer and set custom Minimum and Maximum contour levels to display on your contour map and in the color scale.

Your Contour map has now been created using Logarithmic scaling for both the contour interval and the color scale.

I hope that this article has been helpful in displaying your logarithmic data using a contour map in Surfer 12. It might also be useful for you review one of our recent webinars that discussed the New Features of Surfer 12. In this webinar, each of the Z Transform methods are described. You can view the webinar below.

New Features of Surfer 12



1.       Gridding and displaying logarithmically distributed data (0:00 to 11:36)

2.       Downloading base maps from online servers (11:37 to 22:04)

3.       Gridding and displaying data in date/time format (22:05 to 23:38)

4.       Post map enhancements (28:39 to 37:37)

5.       Quick and easy new features (37:38 to 44:19) 

Length: 44:20

If you need additional assistance to create or setup your map using these instructions, please do not hesitate to contact Golden Software Support. Below are the different support resources that are offered by Golden Software:

Phone Support
8:00 am to 5:00 pm Mountain Time
Monday through Friday (excluding US holidays)
+1 303 279 1021
+1 800 972 1021

LiveChat (available through the Golden Software  website)
8:00 am to 4:00 pm Mountain Time
Monday through Friday (excluding US holidays)

 

 

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Wednesday, 22 November 2017

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