Computing Trend Surface Residuals

RockWorks | Utilities | Grid |
Trend-Surface Analysis | Residuals

Use this program to determine regional trend for selected data in the main data sheet. This can help you to determine what trend order to select when creating a Trend Surface grid model, and it can help you to isolate regional anomalies by computing trend residuals. These tools are offered:

• Use the Trend Surface Analysis | Residuals command to perform a trend surface analysis on a selected z-value column and compute the residuals for a selected polynomial order, representing the difference between observed z-values and trend z-values. The residuals are stored in a data sheet column of the user’s choice. 
• Use the Trend Surface Analysis | Report command to perform a trend surface analysis on a selected Z-value column in the data sheet and to list the correlation and residuals for first through sixth order polynomials in a report. The completed report is loaded into a RockWare text window. You may save the report text to disk, print the report, or copy its contents to the Clipboard for pasting into other applications.  See Creating a Trend Surface Report for details.

Menu Options

• Input Columns:  Select the input data in the left pane of the window.
  • X (Easting), Y (Northing): Select the columns in the data file that contain the X and Y coordinates for the sample sites.
  • Z (elevation): Select the column that contains the data to be analyzed.
• Output Column: Select the output data column in the middle-left pane.
  • Residuals: Select the column into which the computed residuals will be recorded.  Any existing data will be overwritten.
• Polynomial Order:  Choose the order for the polynomial for which the residuals are to be computed by clicking in the appropriate radio button.  The best way to know which one to select is to refer to a Trend Surface Report, which lists the correlation coefficient and residuals for each order. 

Step-by-Step Summary

1. Access the RockWorks Utilities program tab.
2. Enter/import your data into the datasheet.  This tool requires that you have a data set displayed in the main RockWorks data sheet, containing multiple sample sites with X and Y coordinates and a Z value of some kind for which the trend surface and residuals will be computed. You also need to have an available data column into which the computed residuals will be recorded.
3. Select the Grid | Trend Surface Analysis | Residuals menu option.
4. Enter the requested menu settings, as described above.
5. Click the Process button to continue.

The program will determine the difference between each Z-value in your data sheet and the computed Z-value at the same location based on the polynomial surface that was created. This difference value will be stored in the column you selected, for each sample or row in the data sheet.  

Use these residuals to determine where localized differences from the regional background may be. 

See also

• Trend Surface Gridding for information about creating trend surface maps, and examples of different polynomials. 
  

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