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

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 boundary.

To draw a polygon boundary:

1. Select the Map object containing the grid file you would like to assign NoData to in the Contents window.
2. Click Home | Add to Map | Layer | Empty Base.
3. Select the Base(vector) layer in the Contents window and click Features | Group | Start Editing.
4. Click Features | Insert | Polygon and draw your polygon over the map. Double click when you are finished to end the polygon.
5. Press the ESC key on the keyboard to exit drawing mode.
6. Click Features | Group | Stop Editing.

Once the boundary is drawn, or if you are using an existing vector file, assign NoData values to the grid nodes outside of the boundary:

1. Click Grids | Edit | Assign NoData.
2. In the Input Grid section, select your grid-based map layer from the list, or click Browse, select the grid file, and click Open.
3. In the NoData Polygon Boundary section, select your base map layer from the list, or click Browse, select the vector file, and click Open.
4. Toggle NoData Outside.
5. Enter a name to save the new grid file as.
6. Choose what type of map layer to create from this grid, and what map to add it to.
7. Click OK.

2. Assign a NoData value to grid nodes outside the outline during gridding

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 boundary. See the first set of instructions above for steps on how to draw a polygon boundary.

To assign NoData values to the grid nodes outside of the boundary during gridding:

1. Click Home | Grid Data | Grid Data.
2. In the Grid Data - Select Data dialog,
   1. Choose your data file from the Dataset 1 list on the right, or click Browse, select the data file, and click Open.
   2. Choose your Gridding Method from the list on the left.
   3. Verify your X, Y, and Z columns are set appropriately.
   4. Click Skip to End.
3. In the Grid Data - <Gridding Method> - Output dialog,
   1. In the NoData Polygon Boundary section, select your base map layer from the list, or click Browse, select the vector file, and click Open.
   2. Toggle NoData Outside.
   3. Enter a name to save the new grid file as.
   4. Choose what type of map layer to create from this grid, and what map to add it to.
   5. Click Finish.

3. 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. This is helpful for defining a hard boundary that the gridding method cannot look through, such as a retaining wall or a building.

1. Click Home | Grid Data | Grid Data.
2. In the Grid Data - Select Data dialog,
   1. Choose your data file from the Dataset 1 list on the right, or click Browse, select the data file, and click Open.
   2. Choose a Gridding Method that supports faults from the list on the left. 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. Verify your X, Y, and Z columns are set appropriately.
   4. Click Next.
3. In the Grid Data - <Gridding Method> - Options dialog,
   1. Expand the Faults section on the right.
   2. Click the open file icon in the File containing fault traces field.
   3. Select the BLN file defining your fault line, and click Open.
   4. Click Skip to End.
4. In the Grid Data - <Gridding Method> - Output dialog,
   1. Enter a name to save the new grid file as.
   2. Choose what type of map layer to create from this grid, and what map to add it to.
   3. Click Finish.

4. Use a breakline file and also assign NoData values to the grid nodes

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 (i.e. the lake edge), including a third column values in the BLN file defining the Z values for the breakline itself (for a lake, this could be an elevation of 0 for the lake edge).

1. Click Home | Grid Data | Grid Data.
2. In the Grid Data - Select Data dialog,
   1. Choose your data file from the Dataset 1 list on the right, or click Browse, select the data file, and click Open.
   2. Choose Kriging from the Gridding Method list on the left.
   3. Verify your X, Y, and Z columns are set appropriately.
   4. Click Next.
3. In the Grid Data - <Gridding Method> - Options dialog,
   1. Expand the Breaklines section on the right.
   2. Click the open file icon in the File containing breaklines field.
   3. Select the BLN file defining your breakline, and click Open.
   4. Click Skip to End.
4. In the Grid Data - <Gridding Method> - Output dialog,
   1. Enter a name to save the new grid file as.
   2. Uncheck Add grid as layer to.
   3. Click Finish
5. Click Grids | Edit | Assign NoData.
6. In the Input Grid section, click Browse, select the grid file you just created, and click Open.
7. In the NoData Polygon Boundary section, select your base map layer from the list, or click Browse, select the vector file, and click Open. 
8. Toggle NoData Outside.
9. Enter a name to save the new grid file as.
10. Choose what type of map layer to create from this grid, and what map to add it to.
11. Click OK.

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:

• Create a BLN vector boundary file, fault file, or breakline file in Surfer
• Grid only the area inside my data limits using Surfer
• Calculate the volume or area inside an irregular polygon in Surfer
  

Updated November 25, 2019