The base of a 3D surface map in Surfer is drawn from the surface down to the minimum Z value of the map by default. To adjust the depth to which the base sides are filled, you can simply edit the 3D surface layer properties.

To do so:

1. Select the 3D Surface layer in the Contents window.
2. In the Properties window, on the General page under the Base section, check the Show base check box.
3. Once the Show base check box is checked, the Base method property appears. The options here are Map minimum, Grid minimum and Custom.
   1. Map minimum shows the base down to the minimum Z value of all grids in the map.
   2. Grid minimum shows the base down to the minimum Z value of the specific grid used to create the 3D surface layer.
   3. Custom allows you to specify the Z value to fill down to. If Custom is selected, the Custom base value control is revealed to allow you to enter the custom base value.
4. Make your selections and the sides are filled from the surface down to the selected base value.

In Surfer 22 and previous versions, the base of a 3D surface layer was drawn from the surface down to the minimum Z value of the grid file used to create the layer. There were no options to change this in the layer properties, but you could perform the workaround of editing the grid display slightly. To do so:

1. Right click over the 3D Surface layer in the Contents window and click Edit Grid.
2. In the Grid Editor, the lower left node is selected. That’s fine. In the toolbar, in the Z: edit box, enter the z value you wish the base to be drawn to  (e.g. -200).
3. Click Grid Editor | Options | Update Layer.
4. Click File | Close to close the Grid Editor. 
5. Click No in the Surfer dialog asking if you want to save the changes to your grid file (you don’t want to actually change the grid file).
6. Back in the plot window, as long as the Z axis is showing automatic scaling, the base is now drawn to the new minimum Z value. 

See Also:  How can I set the Z limits for my 3D surface map in Surfer?.

Updated November, 2021