Menu: Mesh->Unstructured

Note: Size is given by the average side length (edge) of the corresponding mesh element.

In this menu user can set the mesh sizes to be used by the unstructured mesher.

Assign sizes on points, lines, surfaces or volumes:

It is possible to assign different sizes to different entities of the mesh. This means that in the vicinity of these entities, the generated elements will be approximately of that size. All the entities that do not have an assigned size when meshing take the default one. Points do not take any size if none is given.

Assigning the size 0.0 to an entity is the same as setting the default size and set the mesh of the entity as unstructured. The transition between different sizes is controlled by a parameter in preferences (see Preferences ).

Sizes by chordal error:

Menu: Mesh->Unstructured->Sizes by chordal error...

The 'By chordal error' option asks for a chordal error (the maximum distance between the generated element and the real geometry) and also minimum and maximum size limits. GiD assigns the corresponding sizes to all the entities to satisfy this condition. One size is assigned to the entire entity, based on a mean curvature computed on it. It will only change the current sizes if the new one is smaller than the one defined previously. In structured surfaces, stretching is permitted. This means that if necessary, elements can have very different sizes in the two principal directions. Note: Entities are assigned a size between the minimum and maximum; however, when generating the mesh, GiD may adapt the size of the elements if necessary, sometimes exceeding the minimum and maximum limits.

In this window you can view the effect of different chordal error values on the number of elements that will be created over a line. Any line of the geometry can be picked using the button in the lower right part of the window, and a scheme based on its curvature is used in this window in order to get an idea of the number of elements that will be generated in that line.

Sizes by background mesh:

Menu: Mesh->Unstructured->Background mesh...

With this option it is possible to assign sizes using a background mesh of triangles or tetrahedra. When this option is selected GiD asks for a file; that file must contain the background mesh. The background mesh must cover the whole domain, so it will usually be a previous mesh of the same model. The format of the file containing the background mesh is the following:

First line: BackgroundMesh V 1.0

Description of the mesh. The format of that mesh is described in the Mesh read section (see GiD mesh ).

Desired sizes in the following format:

DesiredSize (Nodes or Elements)

number of size



Background mesh file example:

BackgroundMesh V 1.0

MESH dimension 3 ElemType Triangle Nnode 3


1 5.61705 4.81504 0.00000


51 -5.64191 -1.53335 0.00000

end coordinates


1 24 16 26

2 16 10 14


76 34 31 28

end elements

DesiredSize Elements

1 0.20000

2 0.20000


75 1.50000

76 1.50000

End DesiredSize

Correct sizes:

Menu: Mesh->Unstructured->Correct sizes...

When the Correct sizes option is selected, a window appears. In this window it is possible to enter a minimum and a maximum mesh size.

If the 'By geometry' option is activated, sizes are assigned to all the entities depending on the shape of the geometry. This means that smaller surfaces will have smaller sizes assigned.

If the 'Correct incompatible sizes' option is activated, some sizes are reduced to ensure that the transitions between sizes in close entities are not too fast.

It will only change the existing sizes if the new size is smaller than that previously defined.

Note: Applying the last two options with default values is the same as setting the Automatic correct sizes preference to Normal in the preferences window.

Note: When meshing a complex volume, instead of trying to adjust element sizes and geometry detail, it may be useful to use the 'By geometry' and 'Correct incompatible sizes' options, setting the Maximum meshing size as equal to the default size for meshing, and setting the Minimum meshing size to reflect the details (10 times smaller, for example). Sometimes it is also necessary to assign sizes by cordal error.

Assign entities:

This option is used to assign an unstructured mesh to geometrical entities (lines, surfaces or volumes). Using this option it is not necessary to specify an unstructured size for entities; the default size will be set for them.

CAUTION: Be careful when assigning large sizes to entities close to others where a small size has been given. It may be impossible to obtain a mesh if the 'Automatic correct sizes' is set to 'NoNe' in the Preferences window.


This option assigns the unstructured mesher to be used to the surfaces. The different meshers available are the ones available in the Preferences window. Default option assign to the entity the mesher set in the Preferences window.

UnstructuredSpacingSize of meshBy geometryBy chordal errorChordal errorCorrect sizesbackground mesh