Change the NoData (blanked/null) values in a grid to another value in Surfer

Method 1: Grid Math

The easiest way is to click Grids | Calculate | Math. This is the simplest solution.

1. Click Grids | Calculate | Math.
2. In the Grid Math dialog, click the Add Grids button, select your grid file and click Open. 
3. Change the NoData Handling to Remap to and then enter the value you want for the blanked nodes as the Remap Value (e.g. 0 or 9999).
4. Change the Output Grid File if you wish and click OK. The new grid is created, and all the NoData nodes have been converted from the NoData Z value to the Z value specified.
   

Method 2: Grid Filter

Another option is to click Grids | Edit | Filter.

1. Click Grids | Edit | Filter.
2. In the Grid Filter dialog, select the Input Grid from the drop down list, or click Browse to navigate to the file. 
3. Choose Linear Convolution Filters | User Defined Filters | General User-defined (mxn) to fill a blanked area with a value. 
4. Set the Edge Effects | NoData Nodes to Fill and set the Value to whatever value you want the blanked nodes to be (e.g. 0 or 9999).
5. Change the Output Grid File if you wish and click OK. The new grid is created, and all the NoData nodes have been converted from the NoData Z value to the Z value specified.
   

Updated November, 2021