Click the Features | Change Type | Change Type | Polyline to Polygon command or right-click on a polyline and click Polyline to Polygon to convert one or more selected polylines or spline polylines into polygons. Each selected polyline is converted to a new separate polygon. To close the polygon, the first point and last point in the polyline are connected with a new line.
The new polygon uses the default fill properties and retains the line properties from the polyline. If the object name has not been changed from the default Polyline, the object is renamed Polygon. If the object name had been altered, the custom object name is retained.
Spline Polyline to Polygon
One way to get a smoothed polygon in Surfer is to create a spline polyline with the Features | Insert | Spline Polyline command. Click on the screen to create the polyline. Make sure that the last point is near the first point. Press ESC on the keyboard to end drawing mode. Click on the spline polyline to select it. Click the Features | Change Type | Change Type | Polyline to Polygon command. The spline polyline is converted to a smooth polygon with enough nodes in the polygon to mimic the spline polyline appearance.