Unit(axis) - Axis physical units

Description:

This attribute contains a textual representation of the physical units used to represent co-ordinate values on a particular axis of a Frame.

Specialised Frames (SkyFrame, SpecFrame, etc.) re-define the default Unit values to be appropriate to the particular co-ordinate system being represented.

For most classes, the Unit attribute is a purely descriptive comment intended for human readers and makes no difference to the operation of the software. However, there are some classes which have active Unit attributes. Changing the Unit attribute for such classes will result in the Mappings  within the WCS FrameSet  being modified in order to reflect the change in units. By default, only SpecFrames have an active Unit attribute.

In general, the syntax of the Unit attribute should follow the recommendations made in the FITS standard (see the paper ``Representation of world coordinates in FITS'' by Greisen & Calabretta (available at http://www.cv.nrao.edu/fits/documents/wcs/wcs.html).

Type:

String.

Notes:

• When specifying this attribute by name, it should be subscripted with the number of the Frame axis to which it applies (unless the Frame has only one axis).