This menu is for the generation of all the different possible geometrical entities. Usually, new entities are created inside the current layer (see Layers and groups (only Preprocessing)). Geometry Menu>Create>Point
Menu: Geometry->Create->Point
Individual points are created by entering each point in the usual way (see Point definition). The points can then be joined together to form lines.
Caution: It is impossible to create new points joining old ones.
The Number option lets you choose the label that will be assigned to the next point created. If a point with this number already exists, the old line changes its number. This option is used for example to automatize thinks in macros, knowing the identifier of the entitiy because it is forced.
Geometry Menu>Create>Straight line
Menu: Geometry->Create->Straight line
Toolbar: 
To create a straight line, start by entering just two points (see Point definition ), and then continue entering points in order to create more lines from the first one. Every part of the total line created is an independent line.
It is important to note that when creating lines, new points are also being created (if existing ones are not used).
The Close option joins the first point and the last point created with a straight line and finishes.
The Undo option undoes the creation of the last point (if new) and the last line. It is possible to continue undoing all the way back to the first point.
The Number option lets you choose the label that will be assigned to the next line created. If a line with this number already exists, the old line changes its number.
If Join is chosen, it is maintained for all points until No join is selected.
Geometry Menu>Create>NURBS line
Menu: Geometry->Create->NURBS line
Toolbar: 
Interpolation / Fitting /ByControlPts
NURBS are non-uniform rational B-splines.
NUBS is a kind of polynomial piecewise curve, defined by a degree, a control polygon and a knot vector.
The degree is the polynomial degree.
The control points are a set of points that the curve approximates smoothly. Usually the curve pass through the first and last control point.
The knot vector is a vector of increasing real numbers (from 0 to 1 in GiD) that divides the parametric space in the intervals.
A NURBS (with 'R') could also have rational weights, an extra real value by control point. The curve approximate more closely the control points with bigger relative weight values. Rational NURBS are a quotient between two polynomials, this allow to mathematically describing conical curves like circumferences or ellipses in an exact way.
There are three ways of creating a NURBS line from series of points using this command:
Interpolation (default option): the NURBS will be a cubic polynomial calculating the control polygon and knots in such a way that the curve pass through all the user points.
Fitting: Similar to the interpolation case, but the curve will not exactly interpolates the user points, only approximate them by a 'minimum squared' criteria. The degree of the curve must be set before the curve calculation.
ByControlPts: Directly defines NURBS setting its degree and control polygon. The shape of the curve is similar to the control polygon, but it does not interpolate the polygon points.
During the session the selected method become the current one until it is changed again.
All the NURBS creation options are available from the contextual menu during the curve creation or from the Right buttons toolbar
The messages showed in the command line will orientate the user about the required parameters on each case.
When defining interpolating curves, you can choose to explicitly define the tangents to one or both ends (using the Tangents option). These tangents can be customized, in that they can either be defined by picking their direction on the screen or by considering an existing line as a tangent to the NURBS if it follows a previous curve (the option ByLine). The Next option allows only one tangent to be defined.
In this way, it is possible to create a closed NURBS by selecting the initial point as the end one and choosing one of the options 'Tangent', 'Next', or 'ByLine'.
The Undo option undoes the creation of the last point; this can be done all the way back to the first point.
When a NURBS has been created, all the interior points (except the first and last) are temporary points, not really entity points unless they previously existed.
To enter rational weights on the curve, the Edit NURBS line/surface command (see Edit NURBS) can be used.
Surface constant: This option allow the creation of a curve on a parametric surface setting the parameter u or v to a constant value specified by the user.
It is possible to create also NURBS curves from the Copy window, be extrusion of points with a transformation of translation or rotation, this allow the creation of straight lines and arcs respectively.
Geometry Menu>Create>Parametric line
Menu: Geometry->Create->Parametric line
Tool to create a parametric approximated curve
Parametric line window
The data that must be input are the mathematical formulae of the coordinates X(t), Y(t) and Z(t), where 't' is the parameter of the curve, and its value belongs to the interval [ t0-t1 ]. The curve is created by approximation and is a NURBS (Non-Uniform Rational B-Spline) which is created with N points. In GiD these kinds of curves are cubic (order 3).
The valid mathematical funcions are all Tcl functions:
+ - * / %
abs cosh log sqrt acos double log10 srand asin exp pow tan atan floor rand tanh atan2 fmod round ceil hypot sin cos int sinh
EXAMPLE
We fill the formula bars with the expression of a conic helix.
That helix starts with radius R0=4 and finishes with radius R1=1, performing N=3 turns from t=0.0 to t=1.0, the height also changes from 0 to H=5.
Example of a conic helix with a unique curve or with points only
Geometry Menu>Create>Polyline
Menu: Geometry->Create->Polyline
A polyline is a set of at least two other lines of any type (including polylines themselves). Every line must share one or two of its endpoints with the endpoints of other lines.
There are two possible ways to create a polyline, either by selecting one line and searching the rest until a corner or end is reached, or by selecting several lines (see Entity selection). In the case of the latter, the order of selection is not important but all of them must join each other by sharing common points.
Polylines are drawn in green to show the difference between the other lines, which are drawn in blue.
Polylines are widely used when creating 4-sided surfaces (see 4-sided surface creation) and automatic 4-sided surfaces (see Automatic 4-sided surface creation).
When deleting a polyline, all its lines are deleted. When exploding it (see Polyline), the polyline will disappear and its individual lines will appear.
It is not possible to create third level polylines: one former polyline can be included inside another, but this is the limit and these two cannot be included within a further polyline.
The Number option lets you choose the label that will be assigned to the next created line. If a line with this number already exists, its number is changed.
Geometry Menu>Create>Arc
Menu: Geometry->Create->Arc
Toolbar: 
To create an arc you can either enter three points (By 3 points, see Point definition) or enter a radius and the two tangent lines at the arc's ends (Fillet curves).
It is important to note that when creating an arc, new points are also being created (if existing ones are not being used).
An arc that begins and ends at the same point (i.e. where the first and third points are the same) will be created as a circle. An arc will always include the second point that is entered, though this one is only used as a reference and, if it is not an existing point, is automatically erased when the arc is created.
The Undo option undoes the creation of the last point (if it is a new one). It is possible to continue undoing all the way back to the first point.
The Fillet curves option lets you input a radius and select two lines that share one common point. An arc will then be created and the two lines will be modified to be tangent and continuous with this new arc.
To convert one arc to another one with the same center and in the same plane but with a complementary angle, the Swap arc command can be used (see Swap arc).
Geometry Menu>Create>NURBS surface
Menu: Geometry->Create->NURBS surface
Toolbar: 
NURBS are non-uniform rational B-splines. They are a type of surface that is defined by its control polygon (one set of points that the surface approximates smoothly), one set of knots for the two directions u and v (a non-decreasing list of real numbers between 0 and 1) and, optionally, one set of rational weights.
To draw the isoparametric lines in u,v=0.5, check the surface drawing type option in the preferences window.
- By contour: this creates a NURBS according to its contour lines. GiD automatically calculates the interior information of the surface so as to interpolate the boundaries smoothly. To create a NURBS surface, some lines must be selected (see Entity selection). The order of selection is not important but all of them must join each other by sharing common points and must form a closed contour. The number of lines must be equal to or greater than one and their shape must be topologically similar to a triangle or a quadrilateral in the space if the algorithm is to work correctly. This last argument is not necessary if all the lines lie in one plane. In this case, the surface is created as a trimmed one and any problems with the shape are avoided. It is possible to select the boundary lines and the boundary lines of interior holes at the same time, if all the lines belong to a plane.
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- Automatic: this automatically creates all possible surfaces with the number of sides given by the user. Every new surface will be created in the current layer.
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- Trimmed: this option lets you select one existing NURBS surface and a set of closed lines that are inside the surface. Some of these lines may already belong to the contour of the existing surface. Some other lines may be created with an intersection with another surface. Another new surface will be created without changing the old one. It is possible to select the boundary lines and the boundary lines of interior holes at the same time, if all the lines belong to the surface:
Creation of a new trimmed surface with a hole
- Untrimmed: this constructs one new surface with the selected surface as its base and with the natural contours of the NURBS surface as its contours. The resulting surface is not trimmed.
- Parallel lines: this lets you create one surface given a set of parallel lines in the space. The new surface will interpolate all the selected lines.
- By points/By line points: these two options are available in the Contextual mouse menu after the NURBS surface creation tool is selected. By points creates a NURBS surface from a cloud of points, and By line points creates a NURBS surface from level curves. These two functions are very useful for creating relief and terrains. In the image below there is a NURBS surface created from level curves:
Note: This surface is an approximation to the selected points/lines, but there is no interpolation.
- Search: this lets you select one line and then creates one surface that contains that line.
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Example creating lines U and V of surface parametrization
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- Rectangle
- Polygon
- Circle
- Sphere
- Cylinder
- Cone
- Prism
- Torus
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