In general, the logical state attribute of each axis component is independent and may be manipulated freely. However, there is one notable and important exception to this:
If any axis component is to be in a defined state, then the centre arrays for all the NDF's axes must also be defined
Thus, all an NDF's axis centre arrays behave as a single unit, and it is a pre-requisite that all of these arrays should be in a defined state before any other axis component may be assigned a value.
Of course, it would be very inconvenient if values had to be explicitly generated and assigned to all the axis centre arrays before any other axis values could be defined, so the the NDF_ system has an implicit mechanism for assigning default values to the axis centre arrays whenever they are required (i.e. whenever any axis component is assigned a value but the axis centre arrays are still undefined). A routine is also provided to perform this task explicitly if required, and is described in the next section.
If an NDF's axis centre arrays are in a defined state, then the axis coordinate system of the NDF as a whole is regarded as being defined. Otherwise (i.e. if no axis components are defined at all), then the axis coordinate system is undefined. The routine NDF_STATE can be used to test whether or not an axis coordinate system is defined by using a component name of `Axis', thus:
CALL NDF_STATE( INDF, 'Axis', STATE, STATUS )
A .TRUE. value will be returned via the logical STATE argument if the
axis coordinate system is defined.