Issue 51

Calculate the Z Value At Any XY Point in a Surfer Grid File

Introduction

After calculating a grid from XYZ data in Surfer, you may want to calculate the Z value of additional XY points within the map. Surfer provides several tools for this task.

• Grid | Residuals interactive method.
• Grid.Interpolation in automation.
• The Grid Node Editor.

Calculate the Z value of any XY data point.

Work Interactively with Grid | Residuals

The Grid | Residuals menu command provides a way to calculate the difference between the grid value and data value at any XY location. This procedure is designed to measure how well a grid matches a data set, or to calculate the difference between a trend surface grid and the raw data.  It can be used to return the grid value for XY points that have an unknown Z value. Surfer uses a bilinear interpolation method to calculate Z values at points that do not coincide with grid nodes.

In the Surfer worksheet, create or import a list of XY data points. Fill a blank column with 0's using theData | Transform menu command, using the formula

C = 0

where C is the blank column letter. Save the changes, close the worksheet window, and open or switch to a plot window. Choose the Grid | Residuals menu command, specify the GRD file, the data file, and the column to contain the residual values. Click OK, and the worksheet displays the data file with the residuals column.

Specify XYZ and residual columns.

The Residuals command calculates the residuals by subtracting the grid value from the data value, so the negative of the grid value is returned when the values in the data column are 0. To reverse the sign of the values, select the Residuals column, and choose the Data | Transform menu commands and enter the formula

D = -D

where D is the Residuals column. Save the changes with the worksheet  File | Save command.

Automate Z Calculations with Grid.Interpolate

The Surfer automation interface provides another way to return the Z value in a grid file. The Interpolate method of the Grid object returns the Z value at the specified XY location. It uses bilinear interpolation to return Z values at points that do not coincide with grid nodes.

zvalue = grid1.Interpolate(x:=2.2, y:=3.4)

The grid1 object can be a GRD file or the grid information in a SRF file containing a map based on a GRD file.

The GridInterpolate.bas script on our ftp site runs in the Surfer Scripter program and calculates the Z values at the XY locations specified in a data file.

Z Value at Grid Nodes

For the special case of XY data points that coincide with the grid nodes, there are interactive and file-based methods to return the Z values.

• Open a GRD file with the File | Open or Grid | Grid Node Editor menu commands. Click within the map or scroll with the arrow keys to display the X, Y, and Z values. In addition, Z values can be modified and saved to a file.

Use the Grid Node Editor to view Z values coincident with the grid nodes.

• Choose the Grid | Convert menu commands to convert any existing GRD file into a GS ASCII GRD file or an ASCII XYZ DAT file. Open the GRD file in a text editor to view the values. The DAT file can be opened in a text editor or in the Surfer worksheet.

Summary

Surfer offers many useful tools for querying the grid file for Z values, including interactive methods via the user interface, and automated methods via the scripting language.

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