Application of the PC matrix yields a coordinate system in the direction
of the physical axes but in pixel units. Conversion to physical units is
performed by applying the scaling specfied by the `CDELTn`.

In some past applications of this process, steps (2) and (3) have been
combined into a single step, using a matrix that is the product of the
`CDELT`** n**, which may be thought of as a diagonal matrix, and the
PC matrix. This matrix, called the coordinate description (CD) matrix,
described the tranformation from the array coordinates in pixel units to
the physical coordinate in physical units. It was represented using
keywords of the form