Surfer allows you to filter data during gridding to prevent null values from impacting the grid results. To specify a value in the data to be ignored when gridding, follow these steps:

1. Click Home | Grid Data | Grid Data.
2. In the Grid Data - Select Data dialog, click Browse in the Dataset 1 field, select the data file and click Open.
3. Assign the X, Y, and Z data columns.
4. Click the Filter Data button.
5. In the Filter dialog, enter the Z value to ignore in the Data Exclusion Filter field.  For example, enter z=-999 to ignore all rows of data that have -999 in the Z data column.
   
6. Click OK.
7. Click Skip to End or Next to apply advanced options.
8. Select any other gridding parameters you wish and click Finish.

To specify a value to ignore in the Surfer worksheet when using Data | Data | Transform, you can convert the number to a text value. Surfer can ignore any text string (non-numeric values) when performing a transform.  To convert a number to a text value in the worksheet, follow these steps:

1. Select the column of data that contains the values you want to convert to text.
2. Click Data | Find | Replace.
3. In the Find and Replace dialog, enter the value you want to ignore in the Find field, and in the Replace with field, you can enter either NA, null, or simply enter the number and prefix it with an apostrophe or quotation mark ('999). 
4. Click Replace All. Now the values are converted to text.
5. Click Data | Data | Transform.
6. Enter the Transform equation you wish, and under Text cells, select Are treated as text.
7. Click OK and the transform is applied, ignoring the cells with text in them.

Updated November, 2021