Choose the right gridding method in Surfer

Updated

Accurate data interpolation (gridding) is vital to the creation of accurate maps. This guide is designed to help you select and validate the gridding method best suited for your data and desired visual effect. The process involves narrowing your choice of gridding methods, creating test grid files, validating the accuracy of the test grids, and then creating a final high quality grid from your data that you can have complete confidence in.

To learn more about the individual gridding methods available in Surfer, check out the article Understanding Surfer gridding methods.

 

Process overview:

Also included:

Narrow your gridding algorithm choices

To narrow down the gridding method options in Surfer from over a dozen to just two or three likely candidates, start by asking these three questions:

1. Do I have faults or breaklines to model?

If you have faults to model, the minimum curvature, inverse distance to a power, and nearest neighbor gridding algorithms support faults. 

If you have breaklines, continue to question 2.

2. How is my data distributed?
If you have evenly spaced or very dense data, the triangulation with linear interpolation or nearest neighbor gridding algorithms will likely perform best.

If you have clustered or irregular data, continue to question 3.

3. What visual effect am I looking for?
For smooth contours, the kriging, minimum curvature, and natural neighbor gridding methods are best. For a cellular or polygonal effect, use the nearest neighbor algorithm. And for concentric or circular results, use inverse distance, modified Shepard’s hook, or radial basis function.

Create test grids in Surfer

Test grids allow you to visually examine the impact different gridding methods have on the final output without using large amounts of hard drive space or processing power. You can create test grids manually or you can use one of the attached scripts to grid and contour your data using multiple gridding methods.

To create test grids manually:

  1. Click Home | Grid Data | Grid Data
  2. Click Browse and select your input data file.
  3. Verify the X, Y, and Z columns are assigned correctly.
  4. Click the desired gridding method.
  5. Click Skip to End.
  6. Adjust the name in the Output Grid field.
  7. Check the box next to Add grid as layer to and verify the New layer field is set to Contour.
  8. Click Finish.

To run a GridDataComparison script:

  1. Download the attached TXT file.
  2. Right click on the file and select Rename.
  3. Delete .txt from the end of the file name and press ENTER.
  4. Click Yes, on the warning message about changing the file extension.
  5. In Windows Explorer, navigate to C:\Program Files\Golden Software\Surfer\Scripter.
  6. Double click on Scripter.exe to open the Scripter program.
  7. Click File | Open, choose the downloaded BAS file, and click Open.
  8. Click Script | Run.
  9. When prompted, choose your data file, x,y,z columns, and BLN file (if you're using the fault or breakline script).

McWhirter Image1.png

With the test grids created and contoured you can choose which gridding methods look best to test for accuracy.

Check your test grid accuracy

Surfer’s grid residuals feature calculates the difference between the value of a grid and a data file at the same X,Y coordinates.  We can use this feature to calculate an average difference between the grid file and the input values which will give us an approximate goodness-of-fit.

To calculate the sum of residuals:

  1. Click Grids | Calculate | Residuals.
  2. In the Grid Residuals dialog,
    1. In the Input Grid field, choose your grid file
    2. In the XYZ Data field, choose your data file from the drop down or click Browse to find it
    3. Assign the X, Y, and Z data columns.
    4. In the Store residuals in column field, select an empty column for the calculated residuals.
    5. Click OK.
  3. A new Worksheet window will open showing your data and the calculated residuals.

The residuals values may be positive or negative because they are the z value in the data file minus the z value in the grid file for the same x,y pair. Since we just want an indication of how far the grid values are from the original values, we need the absolute value of the residuals:

  1. Click Data | Data | Transform.
  2. In the Transform dialog:
    1. Verify the Transform with field is set to Column variables (e.g., C = A + B).
    2. In the Transform equation box, type D = FABS(D) (where D is the column storing the residual values).
    3. Verify the First row is 2 and the Last row is the last row containing residuals.
    4. Click OK.
  3. If you have additional test grids to test, repeat steps 1-5 for the other grids.

Finally we'll review statistics for each column of residuals to find the best fit.

  1. Select all of the residuals values in the worksheet.
  2. Click Data | Data | Statistics.
  3. In the Statistics dialog, uncheck all boxes except Sum.
  4. Click Copy to worksheet in the Results section.
  5. Change the Starting in cell to C49 or the first empty row of the column before the first residuals column.
  6. Click OK.

McWhirter Image3.png

The gridding method with the smallest sum of residuals is the one that best fits the data and should be used to create your final grid file.

Create your final grid file

Now that you know which gridding method best fits your data you can use best practices to create your final high quality grid file.  Some best practices to be aware of include:

  • Grid Resolution - The higher the grid resolution (number of nodes) the better small features are expressed in the results. Use your knowledge of the model area to determine the minimum grid node spacing to ensure all important features are accurately represented.
  • Search & Anisotropy - XYZC data is often more dense in the Z direction than the X and Y directions.  Adjust the search ellipse to extend farther in the X and Y direction than the Z direction to ensure accurate representation across the entire area.
  • Data Filters - Dense datasets will often contain overlapping points.  Use data filters to tell Surfer which value to import and use during interpolation.
  • Z Transform & Limits - Use Z limits to define any known limits to the range in Z values and apply a Z transform to reduce the impact of data spikes (both common in concentration data)

To create your final grid file:

  1. Click Home | Grid Data | Grid Data
  2. Click the Data Type you are working with.
  3. Click Browse and select your input data file.
  4. Verify the X, Y, and Z columns are assigned correctly.
  5. Click the desired Gridding Method.
  6. Click Next.
  7. Click Next on the Grid Data - Kriging - Variogram page (if presented).  We do not recommend changing these settings in most cases.
  8. On the Grid Data - Options page, adjust the search parameters and load fault and breakline files as needed and then click Next.
  9. Click Next on the Grid Data - Cross Validation page.
  10. On the Grid Data - Output page, adjust the grid node Spacing or # of Nodes, Grid Z Limits, Z Transform, and NoData settings as desired.
  11. Adjust the grid file name and location in the Output Grid field.
  12. Check the box next to Add grid as layer to and adjust the New layer field as desired.
  13. Click Finish.

Additional Resources

Attachments

Was this article helpful?
...

Related articles

Comments

0 comments

Please sign in to leave a comment.