How do I create a contour map within an irregular boundary?

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There are multiple options to create a contour map with an irregular or non-rectangular boundary.

Option 1: Use a [.BLN] file to blank the [.GRD] file.
Grid the data over the entire rectangular bounding box and blank the area inside or outside a polygon defined in an ASCII [.BLN] file.

Create a blanking file [.BLN] to define the irregular boundary. Then use the Grid | Blank command to blank the area inside or outside the boundary. This produces a new blanked grid file.

The [.BLN] file format consists of a header with the number of vertices, and a number indicating the option to blank outside (0) or inside (1). The header is followed by a list of the X, Y coordinates of the vertices.

To create a [.BLN] file, digitize the boundary on the map, enter the coordinates in the Surfer worksheet, or save an existing [.DXF] or other file in the [.BLN] format.

To digitize:

Right-click on a map and choose Digitize.
Left click on the map to save the coordinate to the digit.bln dialog.
Copy and paste the first value as the last value. The first and last values need to be identical to create a closed polygon.
Save the results to a [.BLN] file.
Open the [.BLN] file in the worksheet to edit the header to a 0 for blanking outside if desired.

If you know the coordinates, you can create a [.BLN] file in the worksheet using the proper [.BLN] file format.

To save an existing [.DXF] or other file in the [.BLN] format:

Load the file as a base map.
Turn off the axes in the Object Manager.
Choose File | Export to save the [.BLN] file.
Open the [.BLN] file in the worksheet to edit the header to a 0 for blanking outside if desired.

Option 2: Grid the data and specify a reduced search radius.
Grid the data with a method that supports Search Radii. The grid is blanked when data are not within the search radii

Go to Grid | Data.
In the Grid Data dialog, click on the Advanced Options button, 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.
Not all gridding methods have the Search option

Gridding methods that do offer the Search option:

The default gridding method Kriging, Inverse Distance, Modified Shepard's Method, Nearest Neighbor, Radial Basis, Moving Average, Data Metrics, and Local Polynomial.

Gridding methods do not have this option:

Minimum Curvature, Natural Neighbor, Polynomial Regression, and Triangulation.

Option 3: Choose a gridding method that limits interpolation to the convex boundary around the data

Choose Grid | Data.
Specify the Gridding Method as Triangulation or Natural Neighbor.

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