1. Assign a NoData value to grid nodes outside the outline

Create a vector boundary file (BLN, SHP, DXF, etc.) or draw a polygon defining the boundary you want the data limited to, and assign NoData to the grid file outside the boundaries defined in the vector file or drawn polygon.

To draw a polygon boundary, use the steps below:

1. Select the Map object containing the grid file you would like to assign NoData to in the Contents. 
2. Click Home | Add to Map | Layer | Base | Empty Base. This essentially creates a blank canvas. 
3. Select the Base layer in the Contents and click Features | Group | Start Editing. 
4. You may want to zoom in a few times so you can see the map clearly.
5. Click Features | Insert | Polygon and draw your polygon over the map. You can use the scroll bars on the sides of the window, or press the center wheel on your mouse when drawing the polygon.
6. Double click when you are finished to end the polygon. Now the base map layer has one polygon object in it. Draw as many polygons as needed.
7. Press the ESC key on the keyboard to exit drawing mode.
8. Click Features | Group | Stop Editing.

Once the boundary is drawn, or if you are using an existing vector file, assign NoData using the below steps:

1. Click Grids | Edit | Assign NoData.
2. Click Browse next to the Input Grid section, select the grid file, and click Open.
3. In the Blanking Polygon Boundary section, either select the Map containing the polygon from the dropdown, or, click Browse and select the vector file, and click Open.
4. Toggle NoData Outside.
5. Enter a name to save the new grid file as and click Save.
6. Create the map from this new blanked grid file.

2. Use a fault file to define your outline.

Create a BLN boundary file that defines the outline you want and use the BLN file as a fault file during gridding.

1. Click Grids | New Grid | Grid Data, select your data file and click Open. 
2. In the Grid Data dialog, select a gridding method that supports faults. Only some gridding methods support faults (Inverse Distance to a Power, Minimum Curvature, Nearest Neighbor, and Data Metrics). Kriging does not support faults, but Minimum Curvature creates a result that is very similar to Kriging.
3. Click on the Advanced Options button.
4. Click the Breaklines and Faults tab.
5. Click the Change Filename folder icon under File Containing Fault Traces.
6. Specify the BLN and click Open.
7. Click OK and OK to create the grid file.
8. Create the map from this grid file.

3. Use a breakline file and also blank the grid

This is most useful when trying to create bathymetry contours for a lake or other closed area. Create a BLN boundary file defining the outline of your data limits (ie. the lake edge), including a third column of Z values in the BLN file for the breakline (for a lake, this could be 0 for the lake edge).

1. Click Grids | New Grid | Grid Data, select your data file and click Open.
2. In the Grid Data dialog, select the Kriging gridding method.
3. Click the Advanced Options button.
4. Click the Breaklines tab.
5. Click the Change Filename folder icon under File Containing Breaklines.
6. Specify the BLN and click Open.
7. Click OK and OK to create the grid file.
8. Click Grids | Edit | Assign NoData.
9. Click Browse in the Input Grid section, select the grid file, and click Open.
10. Click Browse in the NoData Polygon Boundary section, select the BLN file, and click Open.
11. Toggle NoData Outside.
12. Enter a name to save the new grid file as and click Save.
13. Create the map from this new blanked grid file.

You might want to create the BLN file and try each of the above methods to see which results best suit your needs.

See Also:

How can I create a BLN file in Surfer?

How can I grid only the area inside my data limits using Surfer?

How can I calculate the volume or area inside an irregular polygon in Surfer?

Updated October 23, 2018