Create dashed contour lines in the NoData areas of a contour map in Surfer

1. Click Grids | Edit | Assign NoData to blank to the grid file inside the polygon boundary.
2. Assign NoData to the original grid file again outside the polygon boundary. 
3. Click Home | New Map | Contour, select one of the blanked grid files and click Open.
4. Click Home | Add to Map | Layer | Contour, select the other blanked grid file and click Open.
5. Change the line style for the contour map that has contours in the area you want changed. 

 
Surfer contour map showing dashed contours outside the area of interest.

If there is too large a gap between the two contour maps (where the boundary polygon line is), you may need to regrid the original grid file with higher resolution (more grid nodes) or click Grid | Spline Smooth to insert more nodes in the original grid file, and then repeat the process above. Smaller grid cells (more grid nodes) will reduce the gap at the blanked edge. The Dashed in Blanked area.srf example attached was created with 300 x 300 grid lines.

Updated December 6, 2021