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. Click File | Open in Worksheet, specify the DAT, CSV, XLS, or other file name, and click Open.
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 A1 is left-justified due to a space between the decimal point and the "3". 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 the X, Y and Z units are the same and linear (i.e. feet, meters, etc.) for volume calculations (or you can convert the Z units to the XY units using a simple conversion Z Scale Factor). This is useful if your XY units are in meters, but your Z values are in feet.
As mentioned, the units need to be linear. It is recommended not to use lat/lon for the XY when calculating volume. This is because when the XY units are decimal degrees of longitude and latitude, and the Z units are always linear unit such as meters, the volume and area calculations units are meaningless (lon*lat*meters). The coordinate system of grid can be changed using the Grid | Edit | Project command.
If you aren't sure what coordinate system or units your data is using, go back to your original data source for additional information.
Save the changes, and change to a plot window by clicking the File | New | Plot Document 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, enable the Assign NoData outside convex hull of data option in the Grid Data dialog, choose a different gridding method like Natural Neighbor or Triangulation with Linear Interplation.
Natural Neighbor gridding automatically assigns NoData to the area outside the convex boundary surrounding the data. It is a good method if there are no significant concave or convex shapes in the boundary. For example, notice the concave shape in the lower right of the map below.
Natural Neighbor gridding eliminates most
of the area outside the boundary, but not all.
Alternatively, you can assign NoData (blank/erase) the areas outside any polygon boundary. The easiest way is to simply draw the boundary as a polygon over the map. To do this, right-click over the map and click Add to Map | Empty Base. Then in the Contents window, click the new Base(vector) layer and Home | Insert | Polygon and draw the polygon boundary you want.
Draw a polygon boundary defining the desired data limits in an empty base layer.
To assign NoData outside the boundary, click Grids | Edit | Assign NoData, enter the input GRD file, the base layer, select to assign NoData inside or outside the polygon, and specify a new output GRD file name. You can choose to add a new contour layer to the map from the blanked grid.
The blanked GRD file conforms to the BLN boundary.
Calculate the volume
Now that the GRD file is in good shape, click Grids | Calculate | Volume.
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 Cancel. Click the Grids | Info | Grid Info command, choose the grid file, and click Open to view the minimum Z value.
View the GRD Z Minimum by clicking the information icon.
Change the Constant Z = value to 420 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.