Issue 58

View Your Data before Calculating Volumes

Surfer has the ability to quickly calculate the volume between two GRD files, or between a GRD file and a horizontal plane. It’s easy to pick the defaults and get a volume calculation, but are the defaults the best choices for your data set? View your data before calculating the volume to help you choose the best settings for your data.

View your data to help you choose the right settings for the volume calculation.

Know your data file
The first step to getting to know your data is to open the data file in the Surfer worksheet. Choose File | Open, specify the DAT, CSV, XLS, or other file name, and click Open. Data files are opened in the worksheet automatically.

Take a look at the worksheet and verify that the data are in the expected columns. Surfer expects the X, Y, and Z data to be in columns A, B, and C, by default, but you can change these settings if you know the columns containing each variable.

View the data file in the Surfer worksheet.
Check for typos by noting the right- and left-justification for text and numbers.

Verify that the data are in a numeric format. Surfer displays numeric data as right-justified and text data is left-justified by default. Note that the value in cell A2 is left-justified due to a space between the decimal point and the "9". Click on the cell, then click on the cell edit box above the column letters and below the tool bars to delete the space.

Surfer requires that all units are the same for volume calculations, so check this in the worksheet by looking at the data values. For example, if the XY units are decimal degrees of longitude and latitude, and the Z units are meters, the volume and area calculations will be meaningless. If you can't be sure, go back to your original data source for information about the units.

Save the changes, and change to a plot window by closing the worksheet window, or choosing the Window | Plot 1menu command.

Display the data points
Next, display the data points graphically using a post map or classed post map. The classed post map has the added feature of displaying the points with different colors based on the data values.

Classed post map of data file. Note existence of outlier points
beyond the roughly circular area of interest.

In this file, the data of interest describe a roughly circular mound of material, and the intent is to calculate the volume of the mound. The classed post map shows that there are data outside the area of interest, which can be deleted in the worksheet to focus on the mound.

Classed post map with outliers deleted from data file in worksheet.

Grid the data
The default gridding method is kriging, which grids the entire bounding box surrounding the data points. The default for all gridding methods produces 100 grid lines in the longest direction. Is this the best method? Let's try it and see.

Contour map of data gridded with kriging defaults.

Note that the contours extend beyond the data limits, so that area would be included in the volume calculation. To exclude the area from gridding, choose a different gridding method like Natural Neighbor or Triangulation, or create a BLN file of the data boundary and blank the area outside the BLN boundary.

Natural Neighbor gridding automatically blanks the area outside the convex boundary surrounding the data. It is a good method if there are no "reentrants" or concave indentations in the boundary.

Natural Neighbor gridding eliminates most
of the area outside the boundary, but not all.

The best way is to create a BLN boundary surrounding the data set. Right-click on the map, choose Digitize, and click on the points to add them to the boundary. Save the results to a BLN file extension. Open the BLN file in the Surfer worksheet, change cell A2 to 0 to blank outside the boundary, save the changes, and close the worksheet.

Digitize a BLN boundary defining the data limits.
Here the boundary is loaded as a base map and shown in red.

To blank the data outside the boundary, choose Grid | Blank, enter the input GRD file, the BLN file, and a new output GRD file name. Substitute the new GRD file in the contour map.

The blanked GRD file conforms to the BLN boundary.

Calculate the volume
Now that the GRD file is in good shape, take a look at the Grid | Volume settings.

Default Grid Volume settings can be changed.

Note that the default Lower Surface uses a Constant Z = 0. To find out if this is the best value for the base of the mound, take a look at the GRD file. Click the information icon  in the Grid Volume dialog and view the minimum Z value.

View the GRD Z Minimum by clicking the information icon.

Change the Constant Z = value to 687 for better results.

Choosing the default value for the lower surface
results in a much larger volume than expected.

Changing the lower surface to match the GRD Z minimum
produces a better volume calculation.