Menu: View results->Iso surfaces
Toolbar: 
Here a surface is drawn that ties a fixed value inside a volume mesh; for surface meshes a line is drawn. To create isosurfaces there are several options:
- Exact: After choosing a result or a result component of the current analysis and step, you can input several fixed values and then for each given value an isosurface is drawn.
- Automatic: Similarly, after choosing a result or a result component, you are asked for the number of isosurfaces to be created. GiD calculates the values between the Minimum and the Maximum (these are not included).
- Automatic Width: After choosing a result or result component, you are asked for a width. This width is used to create as many isosurfaces as are needed between the Minimum and Maximum defined values (these are included).
Iso lines are created for triangles and quadrilaterals when the isosurface visualization is selected. Their visualization can be enabled or disabled with Options --> Iso-surfaces --> Show isolines
Iso surfaces are supported for results defined over nodes or gauss points.
If the gauss points results can be globally smoothed, then a Smoothed sub-menu will be added with a list of the gaussian results, see View results menu
These isolines can be switched on and off with the command Results --> IsoSurfaces --> DysplayStyle --> ShowIsolines on the right buttons menu .
Go to Preferences --> Postprocess --> Iso surfaces to set and manage isosurface options, like the colours used, style which with the isosurfaces are drawn, etc. Iso surfaces