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You can constrain the gridded area (and hence the contours) to the area inside your data boundaries by either of the methods below. To add your vote to the request on the suggestion file to only grid inside the data limits, please email surfersupport@goldensoftware.com.

1.     Use a fault file to define your data limits.
Create a BLN boundary file that defines the outline of your data points.

To create a BLN file, please see: http://www.goldensoftware.com/activekb/questions/674. Or, if you have our MapViewer or Didger software packages, you can easily do this by importing your data points, selecting them and using the Convex Hull command to create a polygon of the convex hull around the data points, and then use File | Export to export the polygon to a BLN file.

After the BLN file is created, use the BLN file as a fault file during gridding. Go to Grid | Data, select your data file and click Open. In the Grid Data dialog box, click on the Advanced Options button, go to the Breaklines and Faults tab, and specify the BLN as a fault file. Only some gridding methods support faults. Kriging does not, but Minimum Curvature (very similar to Kriging) does.

2.     Blank the grid outside the data limits.
Grid your data as normal by going to Grid | Data, selecting the data file and clicking Open, specifying any gridding parameters you wish and clicking OK (it will be a rectangular grid). Create a BLN boundary file defining the outline of your data point. See above for information on creating a BLN file. After the BLN file is created, go to Grid | Blank, select the grid file and click Open, select the BLN file and click Open, and save the blanked grid to a new name.  Create maps from the blanked grid file.

 3.     Use a different gridding method.
Use a different gridding method that doesn’t create data in areas where there were no data to begin with (ie. Triangulation or Natural Neighbor). The areas outside your data limits will be automatically blanked.

4.     Grid your data twice and use Grid Math.

If you prefer to grid with a different method like Kriging, grid the data again with Triangulation with Linear Interpolation, and then use the Grid | Math menu command to blank the Kriging grid with the Triangulation grid. Use the formula C = A + B - B, where C is the output GRD, A is the Kriging GRD, and B is the Triangulation GRD. The output GRD will be blanked where blanked in the Triangulation GRD, and have values in the non-blanked area from the Kriging GRD.

5.     Grid the data and specify a reduced search radius.
Click on the Advanced Options button in the Grid Data dialog, then click on the Search tab. Remove the check mark from the No Search (use all of the data) if necessary, and specify the Radius 1 and Radius 2 values.

The default Kriging method offers the Search option. Other methods that offer it include Inverse Distance, Modified Shepard's Method, Nearest Neighbor, Radial Basis, Moving Average, Data Metrics, and Local Polynomial.

Some gridding methods do not have this option, including Minimum Curvature, Natural Neighbor, Polynomial Regression, and Triangulation.

Attached is an example file showing the results of methods 1-3.

See Also:

How can I create a BLN file?

How can I create a map inside an irregular boundary?