In Surfer, you can load a BLN file as a base map, reshape the polygon and then use the updated polygon or BLN file to assign a NoData value to areas inside or outside the polygon.
To reshape the BLN:
- 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.
- If you'd like to enter the exact coordinates of where the vertex should be:
- Select the object in the Contents or Plot window.
- In the Properties, click the Coordinates tab.
- Click in the fields to delete and enter new coordinates. Press enter or click out of the field for the changes to take effect.
- Note: Use the Features | Edit Features | Reshape tool to see each vertex and select individually.
- If you'd like to drag the vertex to a specific location, click Features | Edit Features | Reshape.
- Click and drag a vertex to reshape the polygon. Press DELETE on your keyboard to delete a vertex and press CTRL and click to add a new vertex.
- Press ESC on the keyboard when you are done reshaping the polygon.
The edited BLN polygon can now be used as is in the base layer or saved for future use to assign NoData values to an existing grid or, you can use the polyon to Assign Nodata as you create a grid. See all options below.
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, set the Scaling source to your Map on the Scaling tab.
- On the BLN Options page, choose whether to assign no data Inside or Outside of your polygon in the NoData areas section.
- Click OK to create the file.
Assign NoData to an existing grid:
- Click Grids | Edit | Assign NoData.
- In the Assign NoData to Grid dialog, click in the Input Grid field and select the map layer containing the desired grid or click Browse to select a grid file.
- Click in the NoData Polygon Boundary field 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.
Assign NoData as you create the grid:
- Click Home | Grid Data | Grid Data.
- In the Grid Data - Select Data dialog, select the data file you will use to create your grid, select the appropriate XYZ data columns, your gridding method, and click Skip to End.
- Fill out the dialog as normal.
- In the NoData Polygon Boundary section, either click None and load the polygon from the base layer, or click Browse and navigate to the BLN file.
- Click OK to create the grid.
Updated November 1, 2021