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wcsfix.h File Reference
`#include "wcs.h"`
`#include "wcserr.h"`

Go to the source code of this file.

## Macros

#define | CDFIX 0
| Index of cdfix() status value in vector returned by wcsfix().
#define | DATFIX 1
| Index of datfix() status value in vector returned by wcsfix().
#define | OBSFIX 2
#define | UNITFIX 3
| Index of unitfix() status value in vector returned by wcsfix().
#define | SPCFIX 4
| Index of spcfix() status value in vector returned by wcsfix().
#define | CELFIX 5
| Index of celfix() status value in vector returned by wcsfix().
#define | CYLFIX 6
| Index of cylfix() status value in vector returned by wcsfix().
#define | NWCSFIX 7
| Number of elements in the status vector returned by wcsfix().
#define | cylfix_errmsg wcsfix_errmsg
| Deprecated.

## Enumerations

enum

wcsfix_errmsg_enum {
FIXERR_OBSGEO_FIX = -5 , FIXERR_DATE_FIX = -4 , FIXERR_SPC_UPDATE = -3 , FIXERR_UNITS_ALIAS = -2 ,
FIXERR_NO_CHANGE = -1 , FIXERR_SUCCESS = 0 , FIXERR_NULL_POINTER = 1 , FIXERR_MEMORY = 2 ,
FIXERR_SINGULAR_MTX = 3 , FIXERR_BAD_CTYPE = 4 , FIXERR_BAD_PARAM = 5 , FIXERR_BAD_COORD_TRANS = 6 ,
FIXERR_ILL_COORD_TRANS = 7 , FIXERR_BAD_CORNER_PIX = 8 , FIXERR_NO_REF_PIX_COORD = 9 , FIXERR_NO_REF_PIX_VAL = 10
}

## Functions

int | wcsfix (int ctrl, const int naxis[], struct wcsprm *wcs, int stat[])
| Translate a non-standard WCS struct.
int | wcsfixi (int ctrl, const int naxis[], struct wcsprm *wcs, int stat[], struct wcserr info[])
| Translate a non-standard WCS struct.
int | cdfix (struct wcsprm *wcs)
| Fix erroneously omitted `CDi_ja` keywords.
int | datfix (struct wcsprm *wcs)
| Translate `DATE-OBS` and derive `MJD-OBS` or vice versa.
int | obsfix (int ctrl, struct wcsprm *wcs)
| complete the `OBS`GEO-[XYZLBH] vector of observatory coordinates.
int | unitfix (int ctrl, struct wcsprm *wcs)
| Correct aberrant `CUNITia` keyvalues.
int | spcfix (struct wcsprm *wcs)
| Translate AIPS-convention spectral types.
int | celfix (struct wcsprm *wcs)
| Translate AIPS-convention celestial projection types.
int | cylfix (const int naxis[], struct wcsprm *wcs)
| Fix malformed cylindrical projections.
int | wcspcx (struct wcsprm *wcs, int dopc, int permute, double rotn[2])
| regularize PCi_j.

## Variables

const char * | wcsfix_errmsg []
| Status return messages.

## Detailed Description

Routines in this suite identify and translate various forms of construct known to occur in FITS headers that violate the FITS World Coordinate System (WCS) standard described in

"Representations of world coordinates in FITS",
Greisen, E.W., & Calabretta, M.R. 2002, A&A, 395, 1061 (WCS Paper I)
"Representations of celestial coordinates in FITS",
Calabretta, M.R., & Greisen, E.W. 2002, A&A, 395, 1077 (WCS Paper II)
"Representations of spectral coordinates in FITS",
Greisen, E.W., Calabretta, M.R., Valdes, F.G., & Allen, S.L.
2006, A&A, 446, 747 (WCS Paper III)
"Representations of time coordinates in FITS -
Time and relative dimension in space",
Rots, A.H., Bunclark, P.S., Calabretta, M.R., Allen, S.L.,
Manchester, R.N., & Thompson, W.T. 2015, A&A, 574, A36 (WCS Paper VII)

Repairs effected by these routines range from the translation of non-standard values for standard WCS keywords, to the repair of malformed coordinate representations. Some routines are also provided to check the consistency of pairs of keyvalues that define the same measure in two different ways, for example, as a date and an MJD.

A separate routine, wcspcx(), "regularizes" the linear transformation matrix component (PCi_j) of the coordinate transformation to make it more human- readable. Where a coordinate description was constructed from CDi_j, it decomposes it into PCi_j + CDELTi in a meaningful way. Optionally, it can also diagonalize the PCi_j matrix (as far as possible), i.e. undo a transposition of axes in the intermediate pixel coordinate system.

Non-standard keyvalues:
AIPS-convention celestial projection types, `NCP` and `GLS`, and spectral types, `'FREQ-LSR'`, `'FELO-HEL'`, etc., set in `CTYPEia` are translated on-the-fly by wcsset() but without modifying the relevant ctype[], pv[] or specsys members of the wcsprm struct. That is, only the information extracted from ctype[] is translated when wcsset() fills in wcsprm::cel (celprm struct) or wcsprm::spc (spcprm struct).

On the other hand, these routines do change the values of wcsprm::ctype[], wcsprm::pv[], wcsprm::specsys and other wcsprm struct members as appropriate to produce the same result as if the FITS header itself had been translated.

Auxiliary WCS header information not used directly by WCSLIB may also be translated. For example, the older `DATE-OBS` date format (wcsprm::dateobs) is recast to year-2000 standard form, and `MJD-OBS` (wcsprm::mjdobs) will be deduced from it if not already set.

Certain combinations of keyvalues that result in malformed coordinate systems, as described in Sect. 7.3.4 of Paper I, may also be repaired. These are handled by cylfix().

Non-standard keywords:
The AIPS-convention CROTAn keywords are recognized as quasi-standard and as such are accomodated by wcsprm::crota[] and translated to wcsprm::pc[][] by wcsset(). These are not dealt with here, nor are any other non-standard keywords since these routines work only on the contents of a wcsprm struct and do not deal with FITS headers per se. In particular, they do not identify or translate `CD00i00j`, `PC00i00j`, `PROJPn`, `EPOCH`, `VELREF` or `VSOURCEa` keywords; this may be done by the FITS WCS header parser supplied with WCSLIB, refer to wcshdr.h.

wcsfix() and wcsfixi() apply all of the corrections handled by the following specific functions, which may also be invoked separately:

- cdfix(): Sets the diagonal element of the `CDi_ja` matrix to 1.0 if all `CDi_ja` keywords associated with a particular axis are omitted.

- datfix(): recast an older `DATE-OBS` date format in dateobs to year-2000 standard form. Derive dateref from mjdref if not already set. Alternatively, if dateref is set and mjdref isn't, then derive mjdref from it. If both are set, then check consistency. Likewise for dateobs and mjdobs; datebeg and mjdbeg; dateavg and mjdavg; and dateend and mjdend.

- obsfix(): if only one half of obsgeo[] is set, then derive the other half from it. If both halves are set, then check consistency.

- unitfix(): translate some commonly used but non-standard unit strings in the `CUNITia` keyvalues, e.g. '`DEG`' -> '`deg`'.

- spcfix(): translate AIPS-convention spectral types, `'FREQ-LSR'`, `'FELO-HEL'`, etc., in ctype[] as set from `CTYPEia`.

- celfix(): translate AIPS-convention celestial projection types, `NCP` and `GLS`, in ctype[] as set from `CTYPEia`.

- cylfix(): fixes WCS keyvalues for malformed cylindrical projections that suffer from the problem described in Sect. 7.3.4 of Paper I.

## Macro Definition Documentation

## CDFIX

#define CDFIX 0

Index of cdfix() status value in vector returned by wcsfix().

Index of the status value returned by cdfix() in the status vector returned by wcsfix().

## DATFIX

#define DATFIX 1

Index of datfix() status value in vector returned by wcsfix().

Index of the status value returned by datfix() in the status vector returned by wcsfix().

## OBSFIX

#define OBSFIX 2

## UNITFIX

#define UNITFIX 3

Index of unitfix() status value in vector returned by wcsfix().

Index of the status value returned by unitfix() in the status vector returned by wcsfix().

## SPCFIX

#define SPCFIX 4

Index of spcfix() status value in vector returned by wcsfix().

Index of the status value returned by spcfix() in the status vector returned by wcsfix().

## CELFIX

#define CELFIX 5

Index of celfix() status value in vector returned by wcsfix().

Index of the status value returned by celfix() in the status vector returned by wcsfix().

## CYLFIX

#define CYLFIX 6

Index of cylfix() status value in vector returned by wcsfix().

Index of the status value returned by cylfix() in the status vector returned by wcsfix().

## NWCSFIX

#define NWCSFIX 7

Number of elements in the status vector returned by wcsfix().

Number of elements in the status vector returned by wcsfix().

## cylfix_errmsg

#define cylfix_errmsg wcsfix_errmsg

Deprecated.

Deprecated:
  Added for backwards compatibility, use wcsfix_errmsg directly now instead.

## Enumeration Type Documentation

## wcsfix_errmsg_enum

enum wcsfix_errmsg_enum

Enumerator
FIXERR_OBSGEO_FIX |
FIXERR_DATE_FIX |
FIXERR_SPC_UPDATE |
FIXERR_UNITS_ALIAS |
FIXERR_NO_CHANGE |
FIXERR_SUCCESS |
FIXERR_NULL_POINTER |
FIXERR_MEMORY |
FIXERR_SINGULAR_MTX |
FIXERR_BAD_CTYPE |
FIXERR_BAD_PARAM |
FIXERR_BAD_COORD_TRANS |
FIXERR_ILL_COORD_TRANS |
FIXERR_BAD_CORNER_PIX |
FIXERR_NO_REF_PIX_COORD |
FIXERR_NO_REF_PIX_VAL |

## Function Documentation

## wcsfix()

int wcsfix | ( | int | ctrl,
|  | const int | naxis[],
|  | struct wcsprm * | wcs,
|  | int | stat[]
| ) |  |

Translate a non-standard WCS struct.

wcsfix() is identical to wcsfixi(), but lacks the info argument.

## wcsfixi()

int wcsfixi | ( | int | ctrl,
|  | const int | naxis[],
|  | struct wcsprm * | wcs,
|  | int | stat[],
|  | struct wcserr | info[]
| ) |  |

Translate a non-standard WCS struct.

wcsfixi() applies all of the corrections handled separately by cdfix(), datfix(), obsfix(), unitfix(), spcfix(), celfix(), and cylfix().

Parameters

  [in] | ctrl | Do potentially unsafe translations of non-standard unit strings as described in the usage notes to wcsutrn().
  [in] | naxis | Image axis lengths. If this array pointer is set to zero then cylfix() will not be invoked.
  [in,out] | wcs | Coordinate transformation parameters.
  [out] | stat | Status returns from each of the functions. Use the preprocessor macros NWCSFIX to dimension this vector and CDFIX, DATFIX, `OBS`FIX, UNITFIX, SPCFIX, CELFIX, and CYLFIX to access its elements. A status value of -2 is set for functions that were not invoked.
  [out] | info | Status messages from each of the functions. Use the preprocessor macros NWCSFIX to dimension this vector and CDFIX, DATFIX, `OBS`FIX, UNITFIX, SPCFIX, CELFIX, and CYLFIX to access its elements. Note that the memory allocated by wcsfixi() for the message in each wcserr struct (wcserr::msg, if non-zero) must be freed by the user. See wcsdealloc().

Returns
  Status return value:
  - 0: Success.
  - 1: One or more of the translation functions returned an error.

## cdfix()

int cdfix | ( | struct wcsprm * | wcs | ) |

Fix erroneously omitted `CDi_ja` keywords.

cdfix() sets the diagonal element of the `CDi_ja` matrix to unity if all `CDi_ja` keywords associated with a given axis were omitted. According to WCS Paper I, if any `CDi_ja` keywords at all are given in a FITS header then those not given default to zero. This results in a singular matrix with an intersecting row and column of zeros.

cdfix() is expected to be invoked before wcsset(), which will fail if these errors have not been corrected.

Parameters

  [in,out] | wcs | Coordinate transformation parameters.

Returns
  Status return value:
  - -1: No change required (not an error).
  - 0: Success.
  - 1: Null wcsprm pointer passed.

## datfix()

int datfix | ( | struct wcsprm * | wcs | ) |

Translate `DATE-OBS` and derive `MJD-OBS` or vice versa.

datfix() translates the old `DATE-OBS` date format set in wcsprm::dateobs to year-2000 standard form (yyyy-mm-dd `T` hh:mm:ss). It derives wcsprm::dateref from wcsprm::mjdref if not already set. Alternatively, if dateref is set and mjdref isn't, then it derives mjdref from it. If both are set but disagree by more than 0.001 day (86.4 seconds) then an error status is returned. Likewise for wcsprm::dateobs and wcsprm::mjdobs; wcsprm::datebeg and wcsprm::mjdbeg; wcsprm::dateavg and wcsprm::mjdavg; and wcsprm::dateend and wcsprm::mjdend.

If neither dateobs nor mjdobs are set, but wcsprm::jepoch (primarily) or wcsprm::bepoch is, then both are derived from it. If jepoch and/or bepoch are set but disagree with dateobs or mjdobs by more than 0.000002 year (63.2 seconds), an informative message is produced.

The translations done by datfix() do not affect and are not affected by wcsset().

Parameters

  [in,out] | wcs | Coordinate transformation parameters. wcsprm::dateref and/or wcsprm::mjdref may be changed. wcsprm::dateobs and/or wcsprm::mjdobs may be changed. wcsprm::datebeg and/or wcsprm::mjdbeg may be changed. wcsprm::dateavg and/or wcsprm::mjdavg may be changed. wcsprm::dateend and/or wcsprm::mjdend may be changed.

Returns
  Status return value:
  - -1: No change required (not an error).
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 5: Invalid parameter value.
  For returns >= 0, a detailed message, whether informative or an error message, may be set in wcsprm::err if enabled, see wcserr_enable(), with wcsprm::err.status set to FIXERR_DATE_FIX.

Notes:

1. The MJD algorithms used by datfix() are from D.A. Hatcher, 1984, QJRAS, 25, 53-55, as modified by P.T. Wallace for use in SLALIB subroutines CLDJ and DJCL.

## obsfix()

int obsfix | ( | int | ctrl,
|  | struct wcsprm * | wcs
| ) |  |

complete the `OBS`GEO-[XYZLBH] vector of observatory coordinates.

obsfix() completes the wcsprm::obsgeo vector of observatory coordinates. That is, if only the (x,y,z) Cartesian coordinate triplet or the (l,b,h) geodetic coordinate triplet are set, then it derives the other triplet from it. If both triplets are set, then it checks for consistency at the level of 1 metre.

The operations done by obsfix() do not affect and are not affected by wcsset().

Parameters

  [in]

  ctrl

  Flag that controls behaviour if one triplet is defined and the other is only partially defined:
  - 0: Reset only the undefined elements of an incomplete coordinate triplet.
  - 1: Reset all elements of an incomplete triplet.
  - 2: Don't make any changes, check for consistency only. Returns an error if either of the two triplets is incomplete.

  [in,out] | wcs | Coordinate transformation parameters. wcsprm::obsgeo may be changed.

Returns
  Status return value:
  - -1: No change required (not an error).
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 5: Invalid parameter value.
  For returns >= 0, a detailed message, whether informative or an error message, may be set in wcsprm::err if enabled, see wcserr_enable(), with wcsprm::err.status set to FIXERR_`OBS`_FIX.

Notes:

1. While the International Terrestrial Reference System (ITRS) is based solely on Cartesian coordinates, it recommends the use of the GRS80 ellipsoid in converting to geodetic coordinates. However, while WCS Paper III recommends ITRS Cartesian coordinates, Paper VII prescribes the use of the IAU(1976) ellipsoid for geodetic coordinates, and consequently that is what is used here.

2. For reference, parameters of commonly used global reference ellipsoids:

   a (m) 1/f Standard
   --------- ------------- --------------------------------
   6378140 298.2577 IAU(1976)
   6378137 298.257222101 GRS80
   6378137 298.257223563 WGS84
   6378136 298.257 IERS(1989)
   6378136.6 298.25642 IERS(2003,2010), IAU(2009/2012)

   where f = (a - b) / a is the flattening, and a and b are the semi-major and semi-minor radii in metres.

3. The transformation from geodetic (lng,lat,hgt) to Cartesian (x,y,z) is

   x = (n + hgt)*coslng*coslat,
   y = (n + hgt)*sinlng*coslat,
   z = (n*(1.0 - e^2) + hgt)*sinlat,

   where the "prime vertical radius", n, is a function of latitude

   n = a / sqrt(1 - (e*sinlat)^2),

   and a, the equatorial radius, and e^2 = (2 - f)*f, the (first) eccentricity of the ellipsoid, are constants. obsfix() inverts these iteratively by writing

   x = rho*coslng*coslat,
   y = rho*sinlng*coslat,
   zeta = rho*sinlat,

   where

   rho = n + hgt,
   = sqrt(x^2 + y^2 + zeta^2),
   zeta = z / (1 - n*e^2/rho),

   and iterating over the value of zeta. Since e is small, a good first approximation is given by zeta = z.

## unitfix()

int unitfix | ( | int | ctrl,
|  | struct wcsprm * | wcs
| ) |  |

Correct aberrant `CUNITia` keyvalues.

unitfix() applies wcsutrn() to translate non-standard `CUNITia` keyvalues, e.g. '`DEG`' -> '`deg`', also stripping off unnecessary whitespace.

unitfix() is expected to be invoked before wcsset(), which will fail if non-standard `CUNITia` keyvalues have not been translated.

Parameters

  [in] | ctrl | Do potentially unsafe translations described in the usage notes to wcsutrn().
  [in,out] | wcs | Coordinate transformation parameters.

Returns
  Status return value:
  - -1: No change required (not an error).
  - 0: Success (an alias was applied).
  - 1: Null wcsprm pointer passed.
  When units are translated (i.e. 0 is returned), an informative message is set in wcsprm::err if enabled, see wcserr_enable(), with wcsprm::err.status set to FIXERR_UNITS_ALIAS.

## spcfix()

int spcfix | ( | struct wcsprm * | wcs | ) |

Translate AIPS-convention spectral types.

spcfix() translates AIPS-convention spectral coordinate types, '{`FREQ`,`FELO`,`VELO`}-{`LSR`,`HEL`,`OBS`}' (e.g. 'FREQ-OBS', `'FELO-HEL'`, 'VELO-LSR') set in wcsprm::ctype[], subject to `VELREF` set in wcsprm::velref.

Note that if wcs::specsys is already set then it will not be overridden.

AIPS-convention spectral types set in `CTYPEia` are translated on-the-fly by wcsset() but without modifying wcsprm::ctype[] or wcsprm::specsys. That is, only the information extracted from wcsprm::ctype[] is translated when wcsset() fills in wcsprm::spc (spcprm struct). spcfix() modifies wcsprm::ctype[] so that if the header is subsequently written out, e.g. by wcshdo(), then it will contain translated `CTYPEia` keyvalues.

The operations done by spcfix() do not affect and are not affected by wcsset().

Parameters

  [in,out] | wcs | Coordinate transformation parameters. wcsprm::ctype[] and/or wcsprm::specsys may be changed.

Returns
  Status return value:
  - -1: No change required (not an error).
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 2: Memory allocation failed.
  - 3: Linear transformation matrix is singular.
  - 4: Inconsistent or unrecognized coordinate axis types.
  - 5: Invalid parameter value.
  - 6: Invalid coordinate transformation parameters.
  - 7: Ill-conditioned coordinate transformation parameters.
  For returns >= 0, a detailed message, whether informative or an error message, may be set in wcsprm::err if enabled, see wcserr_enable(), with wcsprm::err.status set to FIXERR_SPC_UPDTE.

## celfix()

int celfix | ( | struct wcsprm * | wcs | ) |

Translate AIPS-convention celestial projection types.

celfix() translates AIPS-convention celestial projection types, `NCP` and `GLS`, set in the ctype[] member of the wcsprm struct.

Two additional pv[] keyvalues are created when translating `NCP`, and three are created when translating `GLS` with non-zero reference point. If the pv[] array was initially allocated by wcsini() then the array will be expanded if necessary. Otherwise, error 2 will be returned if sufficient empty slots are not already available for use.

AIPS-convention celestial projection types set in `CTYPEia` are translated on-the-fly by wcsset() but without modifying wcsprm::ctype[], wcsprm::pv[], or wcsprm::npv. That is, only the information extracted from wcsprm::ctype[] is translated when wcsset() fills in wcsprm::cel (celprm struct). celfix() modifies wcsprm::ctype[], wcsprm::pv[], and wcsprm::npv so that if the header is subsequently written out, e.g. by wcshdo(), then it will contain translated `CTYPEia` keyvalues and the relevant `PVi_ma`.

The operations done by celfix() do not affect and are not affected by wcsset(). However, it uses information in the wcsprm struct provided by wcsset(), and will invoke it if necessary.

Parameters

  [in,out] | wcs | Coordinate transformation parameters. wcsprm::ctype[] and/or wcsprm::pv[] may be changed.

Returns
  Status return value:
  - -1: No change required (not an error).
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 2: Memory allocation failed.
  - 3: Linear transformation matrix is singular.
  - 4: Inconsistent or unrecognized coordinate axis types.
  - 5: Invalid parameter value.
  - 6: Invalid coordinate transformation parameters.
  - 7: Ill-conditioned coordinate transformation parameters.
  For returns > 1, a detailed error message is set in wcsprm::err if enabled, see wcserr_enable().

## cylfix()

int cylfix | ( | const int | naxis[],
|  | struct wcsprm * | wcs
| ) |  |

Fix malformed cylindrical projections.

cylfix() fixes WCS keyvalues for malformed cylindrical projections that suffer from the problem described in Sect. 7.3.4 of Paper I.

cylfix() requires the wcsprm struct to have been set up by wcsset(), and will invoke it if necessary. After modification, the struct is reset on return with an explicit call to wcsset().

Parameters

  [in] | naxis | Image axis lengths.
  [in,out] | wcs | Coordinate transformation parameters.

Returns
  Status return value:
  - -1: No change required (not an error).
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 2: Memory allocation failed.
  - 3: Linear transformation matrix is singular.
  - 4: Inconsistent or unrecognized coordinate axis types.
  - 5: Invalid parameter value.
  - 6: Invalid coordinate transformation parameters.
  - 7: Ill-conditioned coordinate transformation parameters.
  - 8: All of the corner pixel coordinates are invalid.
  - 9: Could not determine reference pixel coordinate.
  - 10: Could not determine reference pixel value.
  For returns > 1, a detailed error message is set in wcsprm::err if enabled, see wcserr_enable().

## wcspcx()

int wcspcx | ( | struct wcsprm * | wcs,
|  | int | dopc,
|  | int | permute,
|  | double | rotn[2]
| ) |  |

regularize PCi_j.

wcspcx() "regularizes" the linear transformation matrix component of the coordinate transformation (`PCi_ja`) to make it more human-readable.

Normally, upon encountering a FITS header containing a `CDi_ja` matrix, wcsset() simply treats it as `PCi_ja` and sets `CDELTia` to unity. However, wcspcx() decomposes `CDi_ja` into `PCi_ja` and `CDELTia` in such a way that `CDELTia` form meaningful scaling parameters. In practice, the residual `PCi_ja` matrix will often then be orthogonal, i.e. unity, or describing a pure rotation, axis permutation, or reflection, or a combination thereof.

The decomposition is based on normalizing the length in the transformed system (i.e. intermediate pixel coordinates) of the orthonormal basis vectors of the pixel coordinate system. This deviates slightly from the prescription given by Eq. (4) of WCS Paper I, namely Sum(j=1,N)(`PCi_ja`)² = 1, in replacing the sum over j with the sum over i. Consequently, the columns of `PCi_ja` will consist of unit vectors. In practice, especially in cubes and higher dimensional images, at least some pairs of these unit vectors, if not all, will often be orthogonal or close to orthogonal.

The sign of `CDELTia` is chosen to make the `PCi_ja` matrix as close to the, possibly permuted, unit matrix as possible, except that where the coordinate description contains a pair of celestial axes, the sign of `CDELTia` is set negative for the longitude axis and positive for the latitude axis.

Optionally, rows of the `PCi_ja` matrix may also be permuted to diagonalize it as far as possible, thus undoing any transposition of axes in the intermediate pixel coordinate system.

If the coordinate description contains a celestial plane, then the angle of rotation of each of the basis vectors associated with the celestial axes is returned. For a pure rotation the two angles should be identical. Any difference between them is a measure of axis skewness.

The decomposition is not performed for axes involving a sequent distortion function that is defined in terms of `CDi_ja`, such as TPV, TNX, or ZPX, which always are. The independent variables of the polynomial are therefore intermediate world coordinates rather than intermediate pixel coordinates. Because sequent distortions are always applied before `CDELTia`, if `CDi_ja` was translated to `PCi_ja` plus `CDELTia`, then the distortion would be altered unless the polynomial coefficients were also adjusted to account for the change of scale.

wcspcx() requires the wcsprm struct to have been set up by wcsset(), and will invoke it if necessary. The wcsprm struct is reset on return with an explicit call to wcsset().

Parameters

  [in,out] | wcs | Coordinate transformation parameters.
  [in] | dopc | If 1, then `PCi_ja` and `CDELTia`, as given, will be recomposed according to the above prescription. If 0, the operation is restricted to decomposing `CDi_ja`.

  [in]

  permute

  If 1, then after decomposition (or recomposition), permute rows of `PCi_ja` to make the axes of the intermediate pixel coordinate system match as closely as possible those of the pixel coordinates. That is, make it as close to a diagonal matrix as possible. However, celestial axes are special in always being paired, with the longitude axis preceding the latitude axis.
  All WCS entities indexed by i, such as `CTYPEia`, `CRVALia`, `CDELTia`, etc., including coordinate lookup tables, will also be permuted as necessary to account for the change to `PCi_ja`. This does not apply to `CRPIXja`, nor prior distortion functions. These operate on pixel coordinates, which are not affected by the permutation.

  [out] | rotn | with the celestial axes. For a pure rotation the two angles should be identical. Any difference between them is a measure of axis skewness. May be set to the NULL pointer if this information is not required.

Returns
  Status return value:
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 2: Memory allocation failed.
  - 5: CDi_j matrix not used.
  - 6: Sequent distortion function present.

## Variable Documentation

## wcsfix_errmsg

const char * wcsfix_errmsg[]

extern

Status return messages.

Error messages to match the status value returned from each function.