Create an irregular-shaped map in Surfer

By default, all grid based maps in Surfer are rectilinear.  To create a map in Surfer that conforms to the shape of your data, you must assign NoData to the grid.  You can use a vector file to assign NoData values to both 2D and 3D grid files during or after gridding, to act as a fault during gridding, or to act as a breakline during gridding. The polygon in this vector file will define the outline of the building, lake, mine, etc. that you wish to limit your map to.

Assign NoData outside a polygon 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.

Assign NoData to the grid nodes outside of a boundary using the steps below:

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 2D 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.

*If you do not have a file or drawn polygon, see the steps below to create one:

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.
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, or double click.

Assign NoData outside an alpha shape after gridding

Alpha Shapes allow you to create a tight polygon boundary around your data points, much like the existing Convex hull option. Alpha shapes can create a concave hull around data points and even give you control on how tight the polygon clings around the data limits. See Assign NoData outside of Alpha Shape for more information. 

Create the alpha shape:

1. Click Home | New Map | Base | Base from Data, select your data file, and click Open.
2. In the Base from XY Data dialog, make any necessary adjustments and click OK.
3. Click the Base(vector) layer to use all points in the layer or select 3 or more points inside the Base(vector) layer.
4. Click Features | New Features | Alpha Shape.
5. In the Alpha Shape dialog, enter an Alpha value. The larger the value the more convex the polygon will be, and the smaller the number the more concave the polygon will be.
6. If you'd like to see what the resulting polygon will look like, click Preview.
   Click OK.

Assign NoData outside the alpha shape polygon using the same steps as earlier.

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 March 2024