8.2. WORLD COORDINATE SYSTEM REPRESENTATIONS

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CDi_j -- [floating-point; defaults: 0.0, but see below]

Linear transformation matrix (with scale) between pixel axes j and intermediate

coordinate axes i. This nomenclature is equivalent to PCi_j when CDELTi is unity.

The CDi_j matrix must not be singular. Note that the CDi_j formalism is an

exclusive alternative to PCi_j, and the CDi_j and PCi_j keywords must not appear

together within an HDU.

In addition to the restrictions noted above, if any CDi_j keywords are present in the

HDU, all other unspecified CDi_j keywords shall default to zero. If no CDi_j keywords

are present then the header shall be interpreted as being in PCi_j form whether or not

any PCi_j keywords are actually present in the HDU.

Some non-linear algorithms that describe the transformation between pixel and in-

termediate coordinate axes require parameter values. A few non-linear algorithms also

require character-valued parameters, e.g., table lookups require the names of the ta-

ble extension and the columns to be used. Where necessary parameter values must be

specified via the following keywords:

PVi_m -- [floating-point]

Numeric parameter values for intermediate world coordinate axis i, where m is

the parameter number. Leading zeros must not be used, and m may have only

values in the range 0 through 99, and that are defined for the particular non-linear

algorithm.

PSi_m -- [character]

Character-valued parameters for intermediate world coordinate axis i, where m is

the parameter number. Leading zeros must not be used, and m may have only

values in the range 0 through 99, and that are defined for the particular non-linear

algorithm.

The following keywords, while not essential for a complete specification of an im-

age WCS, can be extremely useful for readers to interpret the accuracy of the WCS

representation of the image.

CRDERi -- [floating-point; default: 0.0]

Random error in coordinate i, which must be non-negative.

CSYERi -- [floating-point; default: 0.0]

Systematic error in coordinate i, which must be non-negative.

These values should give a representative average value of the error over the range of the

coordinate in the HDU. The total error in the coordinates would be given by summing

the individual errors in quadrature.

FITS Standard