Computing Variograms for XYZ Data

RockWorks | Utilities | Stats | Variography

Use this program to read XYZ data from the RockWorks datasheet and generate variogram reports and graphs, showing the variability of the Z values for point pairs as a function of the distance between the points. RockWorks creates observed variograms of your data, and then finds the variogram model that offers the best fit - thus defining the distance and directional relationships in your data.

These tools are the same as those offered with the Kriging gridding method.

Feature Level: RockWorks Basic and higher

See also: What is a Variogram, Kriging

Menu Options
Step-by-Step Summary

Data Columns: Use these prompts to define the columns in the RockWorks datasheet that contain the X, Y, and Z data to be analyzed.
- X (Easting): Select the column in the datasheet that contains the X or Easting coordinates to be analyzed.
  The X and Y coordinates can be Eastings in meters or feet, decimal longitudes, etc. See Defining your Datasheet Coordinates for more information.
- Y (Northing): Select the column that contains the Y or Northing coordinates.
- Z (Elevation): Select the column that contains the Z values to be analyzed for variability. The Z values can be any measured values, such as elevation, quantitative values (geochemistry), etc.
Options: These settings determine whether you want the program to do most of the heavy-lifting ("Automatic") or whether you want full control ("Manual"). Many users find it helpful to run the variogram analysis automatically, with a variety of reports, and then refine the analysis manually.
- Sampling Method
  - Automatic: Click in this radio button if you want the program to set the variogram settings automatically for you (probably the best place to start). When set to Automatic, the program will determine the best-fit variogram type, based on the type with the highest correlation with your data. It will search the data using a variety of spoke directions and variogram types.
  - Manual: Click this button if you prefer to set the variogram variables manually.
    - Spoke Spacing: Since variograms represent point-to-point variability, the program creates a set of point pairs to work with. Rather than analyzing all possible pairings between all data points (a huge number), the program will sample point pairs along specific directional bearings. These bearing lines can be conceptualized as bi-directional "spokes" running through the data points; you define the spacing in degrees between sampling spokes. For example, a 45 degree spoke spacing will generate 4 bi-directional spokes (180 / 45 = 4), and a 30-degree spoke spacing will generate 6 spokes.
    - Spoke Tolerance: Since it is unlikely that many data points will lie along the narrow directional lines established above, you need to establish a "tolerance" or zone alongside each bearing line to include in the data point search. The default is 22.5 degrees.
    - Distance Increments: In addition to the program searching for data points in a directional manner, it also searches in an orderly, step-wise manner, along each bearing line at certain distance increments. These are expressed in map units. The default is 250.
    - Distance Tolerance: It is unlikely that data points lying within the directional search zone will fall right on one of the distance increments, so it is necessary to establish a tolerance zone around each increment to widen the search area. This is also expressed in map units. The default is 125.
    - Maximum Distance: This variable establishes the maximum distance from each data point that the data search will be conducted, in map units.
- Other Options
  - Pre-Grid Points for Variogram: This setting tells RockWorks to first create a regular grid of points (using Inverse-Distance squared) which will then be used for generating the variogram. Keeping this turned on can create better variograms for small data sets. Turning this off can help create a better match between Automatic and Manual settings.
  - Interactively Edit / Examine Variogram: Check this box to see the interactive Variogram Editor.
  - Create Report: Check this box to generate a textual report that lists the detailed variogram results.
  - Create Variogram Matrix: Check this box to generate a detailed diagram that depicts all of the variograms, and a large number of other statistics. (See 2D Variogram Matrix for details.)
    - Items: Use these check-boxes to select which variogram models you wish to include in the matrix diagram.
    - Variograms per Row: Defines the maximum number of variograms to be plotted per row, for each activated variogram type. Default = 10.
      (Typically you won't have that many. For a 90 degree spoke spacing, you'd have two variograms per model. For a 45 degree spoke spacing, you'd have 4.)
Other 2D Diagram Options
Use these checkboxes to append other layers to your diagram.
Click each tab to set up the 2D diagram layers.
- Other 2D Files: Append other saved Rockplot2D diagrams
- Peripherals: Add a logo, title block, or other peripheral items to your diagram.
Output Options: Use these settings to define whether the output graphic is to be saved (or displayed as "untitled"), how the file should be named, and whether it is to be displayed after it is created. It also offers export options. (More info)

Access the RockWorks Datasheet program tab.
Enter/import your XYZ data into the datasheet.
Click on the Stats menu and choose the Variography option.
Enter the requested menu settings, described above.
Click the Continue button to proceed.
If you've selected Automatic variography, the program will compute the average minimum and maximum spacing of your control points, to suggest default sampling distance increments and total distance. It will search for point pairs at 90 degree spoke increments using these lag bins, and then at successively smaller spoke increments, pitching bins without a minimum number of samples. It will compute the observed variograms for all spoke samples, and will determine the variogram model that has the best correlation.

If you've selected Manual variography, the program will search for point pairs along the spoke and distance increments you've specified, out to the maximum distance, and will compute the observed variograms for these bearings. It will fit the selected variogram model to the data.

If you have requested to Edit / Examine Variogram, the best-fit variogram model will be displayed, along with a reference range plot. Adjust this as you wish, and click OK. (See Examining and Editing Variograms for details.)

If you've requested the text report or 2D variogram matrix, they will be displayed in a text tab and/or a RockPlot2D tab in the Options window.
You can adjust the variogram settings in the main Options tab to the left and click the Continue button again to re-generate the output.

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