In evaluating geometry, the system considers the degree of freedom that
it has. In two dimensions, points and lines have two degrees of freedom,
circles have three and ellipses have five degrees of freedom. Fixed
geometry will never be moved by the system, and has no degree of freedom.
If all of the degrees of freedom of a geometry have been taken up by a
consistent combination of dimensions and fixed geometry, that geometry is
said to be isoconstrained (also known as welldefined). Geometry that
still has some degrees of freedom is said to be underconstrained (also
known as underdefined).
Status codes are given through a graphical way (colors) during the
Sketch edition. The update error dialog box when returning in 3D explicitly
gives them (check visualization of diagnosis in Tools > Options >
Sketcher > Colors).
Note that:
 The system will mark all entities that are relevant to a problem
rather than just the first item encountered. So, for instance, in the
case of an inconsistent triangle with sides 10, 10 and 50, all three
dimensions would be marked as INCONSISTENT.
 The order in which the codes are listed below is significant. The
system will test to see whether a geometry should have the status
OVERCONSTRAINED before considering whether it should be INCONSISTENT.
This chapter describes the overconstrained and inconsistent status
codes calculated by the system and explain methods for solving any
underlying problems with a Sketch. 
You will find the following information:

Overconstrained
In many sketches, the user will specify more than the minimum required
number of dimensions or constraints. In certain cases the system will
ignore redundant constraints and solve the Sketch. In other cases it will
mark parts of the Sketch as overconstrained.
The descriptions below refer to consistent constraints and dimensions.
Dimensions are said to be consistent if their values are satisfied by the
position of the geometries.
Geometry will be marked as overconstrained when it cannot be solved
because there are too many dimensions acting on it for the degrees of
freedom available.
A dimension will be marked as overdimensioned if it conflicts with one
or more other dimensions and it is not possible to vary the value of the
dimension and still find a consistent solution. For example, the geometry
and dimensions in the figure below will be overconstrained because the
dimension values cannot be varied independently, even though they can all
be satisfied by appropriate geometry positions. 

However, the system is able to cope with certain overconstrained
situations involving logical constraints. This is important because logical
constraints such as parallelism are likely to be overspecified when a
design is being built up interactively. For example, if two lines are
defined to be parallel and then a distance is subsequently given between
them the parallelism is then specified twice. The following is a list of
some of the overconstrained configurations that can be solved:
 Multiple constraints between the same geometries.
For instance, two circles can have several tangent constraints between
them.
 Multiple coincident constraints between geometries of the same type.
For instance, three points can each be made coincident to the other two.
 Multiple coincident constraints between lines and points.
For instance, two lines can be made coincident, and their endpoints can
be made coincident with the other line.
Parallel and perpendicular constraints. Any combination of parallel and
perpendicular constraints will be reduced to the minimum set required, and
any excess ones will be ignored. Note that a distance dimension between two
lines is treated as a parallel constraint, except that it will never be one
of the constraints that is ignored.
Symmetric constraints . There are many configurations where symmetric
constraints will make other constraints redundant. These are recognized by
the system. For example, if two lines are made symmetric two of the
coincidence constraints between the points and the lines are redundant. 
Resolving Overconstrained Cases
Overconstrained entities occur in loops where all of the entities in a
loop conflict with each other.
Overconstrained entities can also occur when there are too many fixed
geometries.
To resolve overconstrained problems, the user will need to:
 Set as references dimensions
 Deactivate or remove constraints
 Unfix geometry
Note that the system will evaluate as much of the geometry as possible.
It determines exactly which dimensions are contributing to the situation. 
Inconsistent
This section describes when the inconsistent status codes can occur and
how you can modify the Sketch to avoid them.
In general, the inconsistent status shows that the user is attempting to
make a change to the Sketch that is too large. In this context, "large" is
relative to the size of the Sketch.
Parts of a Sketch may become inconsistent as a result of a number of
different operations.
The most common of these are as follows:
 The user changes the value of a dimension. This will normally occur
for cases where there would be large changes to one or more geometries.
 The user adds a dimension or constraint to a Sketch, in order to move
geometry.
 When dragging geometries, the user attempts to input a large
transformation.
 When the geometric type of a useedge is changed (geometry coming
from the projection or intersection of a 3D geometry)
 When there are useedge large positions or orientations changes.
 When an element of a geometry is deleted (especially in conic
curves, connecting curves, spline offsets).
The geometry has not been solved because:
 No solution exists for the current values of dimensions.
 The system cannot find a solution, even though a solution may exist
with the current values of dimensions. This occurs when trying to make
large changes to underconstrained sketches or to parametric curves (See
section Overconstrained and Inconsistent on Parametric Curves below for
further details).
 The system has not find a solution that respects the previous
chirality.
Chirality determines the way that geometry is positioned relative to the
geometries to which it is dimensioned. A dimensioning scheme can often be
satisfied by a number of different configurations. The system will always
evaluate a new configuration that has the same chirality as the original
geometry. It is important to realize that geometry in the system always has
an original configuration, which is used for deciding the chirality.


When a sketch is inconsistent, it may contain unresolved constraints
(nonverified geometrical constraints). Make sure all the
inconsistencies in the sketch are resolved before going any further. 
Resolving Inconsistent Cases
If the inconsistent status code was a result of changing a dimension
value, the problem will be resolved by changing the dimension back to its
old value. However, in some cases the user may want to modify other parts
of the Sketch to allow the change to be made. The following sections
describe different ways that can be tried.
When attempting to solve a problem, the user should focus on the
geometries and dimensions in the Sketch with the inconsistent status code.
In order to decide how to avoid the status code it is useful to check
first if the problem comes from inconsistent dimensions.
An example of this is a triangle with sides of length 50, 50 and 120. 

Not Changed

The not changed status is used in the following cases:
 When geometry becomes overconstrained or inconsistent, the system
will not be able to position any other geometries that depend on it.
These dependent geometries and their associated dimensions (and any
others that depend on them) will be marked not changed.
 Dimensions between two fixed geometry will be given the status code
not changed.
 Dimension between two free or one free and one fixed geometry in the
same set will be given the status code not changed.

Parametric Curves
This section is an overview of specific overconstrained and
inconsistent problems on parametric curves. The Sketcher can manipulate
points, lines, circles and ellipses but can also manage splines and nurbs.
These parametric curves can be created:
 Through an Intersection or Projection of a 3D geometry in the Sketch.
After isolating it, constraint can be used to change the position of the
curve. The system is unable to directly modify the shape because the
curve, which have no internal freedoms that the system can control, have
only three degrees of freedom.
 By the Spline command. The curve is defined from other geometries.
The parametric curve is said dependent. It is constructed so it passes
through a series of control points.
Constraints and dimensions can be added between a dependent parametric
curve and other geometries in the sketch.
Solving problems will occur:
 If the position of the defining geometry depends upon the position of
the parametric curve, either directly or indirectly.
 When the other geometry of the constraint or dimension is an other
parametric curve or dependent parametric curve.
Always use the Constraint command without dialog box to specify where
the constraint must be created on the curve . Through the Constraint
Defines in Dialog Box command, the selection points are not taken
into account.
On fully underconstrained sketches, the system can have difficulty
choosing between changing the shape and/or moving its defining geometry
especially when it supposes to make large changes. Moving the geometry will
help the system find a consistent solution in that case. 