In Surfer, you can load a BLN file as a base map, reshape the polygon and then use the updated BLN file to assign a NoData value to areas inside or outside the polygon. The exact steps are:
- Click Home | New Map | Base | Base, select the BLN file and click Open.The BLN file is imported as a base map.
- In the Contents window, expand the Base layer group and select a polygon in the BLN file.
- Click Features | Edit Features | Reshape.
- Reshape the polygon.
- Press ESC on the keyboard when you are done reshaping the polygon.
- The edited BLN polygon can now be used to assign NoData values to the grid by following these steps:
- Click Grids | Edit | Assign NoData.
- In the Input Grid field of the Assign NoData to Grid dialog, select the map layer containing the desired grid or click Browse to select a grid file.
- In the NoData Polygon Boundary field, click None and select the Base layer containing the BLN polygon.
- Toggle NoData Outside or the desired no data option.
- Uncheck the box next to Add grid as layer to, if desired, and then click OK.
- To save the edited BLN polygon for later use:
- Uncheck the boxes next to each of the four axes in the Contents window.
- Click File | Export.
- In the Export dialog, set the Save as type to BLN Golden Software Blanking (*.bln), give your file a name, and verify Show options dialog is checked. Click Save.
- In the Export Options dialog, if your map is georeferenced, make sure to set the Scaling source dropdown list on the Scaling page to your Map, and choose an external georeferencing file type on the Spatial References page.
- On the BLN Options page, in the NoData areas section choose whether to assign no data Inside or Outside of your polygon.
- Click OK to create the file.
Updated September 18, 2017