WCSLIB
7.7

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Macros  
#define  CDFIX 0 
Index of cdfix() status value in vector returned by wcsfix(). More...  
#define  DATFIX 1 
Index of datfix() status value in vector returned by wcsfix(). More...  
#define  OBSFIX 2 
#define  UNITFIX 3 
Index of unitfix() status value in vector returned by wcsfix(). More...  
#define  SPCFIX 4 
Index of spcfix() status value in vector returned by wcsfix(). More...  
#define  CELFIX 5 
Index of celfix() status value in vector returned by wcsfix(). More...  
#define  CYLFIX 6 
Index of cylfix() status value in vector returned by wcsfix(). More...  
#define  NWCSFIX 7 
Number of elements in the status vector returned by wcsfix(). More...  
#define  cylfix_errmsg wcsfix_errmsg 
Deprecated. More...  
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 nonstandard WCS struct. More...  
int  wcsfixi (int ctrl, const int naxis[], struct wcsprm *wcs, int stat[], struct wcserr info[]) 
Translate a nonstandard WCS struct. More...  
int  cdfix (struct wcsprm *wcs) 
Fix erroneously omitted CDi_ja keywords. More...  
int  datfix (struct wcsprm *wcs) 
Translate DATEOBS and derive MJDOBS or vice versa. More...  
int  obsfix (int ctrl, struct wcsprm *wcs) 
complete the OBS GEO[XYZLBH] vector of observatory coordinates. More...  
int  unitfix (int ctrl, struct wcsprm *wcs) 
Correct aberrant CUNITia keyvalues. More...  
int  spcfix (struct wcsprm *wcs) 
Translate AIPSconvention spectral types. More...  
int  celfix (struct wcsprm *wcs) 
Translate AIPSconvention celestial projection types. More...  
int  cylfix (const int naxis[], struct wcsprm *wcs) 
Fix malformed cylindrical projections. More...  
int  wcspcx (struct wcsprm *wcs, int dopc, int permute, double rotn[2]) 
regularize PCi_j. More...  
Variables  
const char *  wcsfix_errmsg [] 
Status return messages. More...  
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
Repairs effected by these routines range from the translation of nonstandard 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.
Nonstandard keyvalues:
AIPSconvention celestial projection types, NCP
and GLS
, and spectral types, 'FREQLSR'
, 'FELOHEL'
, etc., set in CTYPEia
are translated onthefly 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 DATEOBS
date format (wcsprm::dateobs) is recast to year2000 standard form, and MJDOBS
(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().
Nonstandard keywords:
The AIPSconvention CROTAn keywords are recognized as quasistandard 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 nonstandard 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 DATEOBS
date format in dateobs to year2000 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 nonstandard unit strings in the CUNITia
keyvalues, e.g. 'DEG
' > 'deg
'.
spcfix(): translate AIPSconvention spectral types, 'FREQLSR'
, 'FELOHEL'
, etc., in ctype[] as set from CTYPEia
.
celfix(): translate AIPSconvention celestial projection types, NCP
and GLS
, in ctype[] as set from CTYPEia
.
#define CDFIX 0 
#define DATFIX 1 
#define OBSFIX 2 
#define UNITFIX 3 
#define SPCFIX 4 
#define CELFIX 5 
#define CYLFIX 6 
#define NWCSFIX 7 
Number of elements in the status vector returned by wcsfix().
#define cylfix_errmsg wcsfix_errmsg 
enum wcsfix_errmsg_enum 
int wcsfix  (  int  ctrl, 
const int  naxis[],  
struct wcsprm *  wcs,  
int  stat[]  
) 
wcsfix() is identical to wcsfixi(), but lacks the info argument.
wcsfixi() applies all of the corrections handled separately by cdfix(), datfix(), obsfix(), unitfix(), spcfix(), celfix(), and cylfix().
[in]  ctrl  Do potentially unsafe translations of nonstandard 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 nonzero) must be freed by the user. See wcsdealloc(). 
int cdfix  (  struct wcsprm *  wcs  ) 
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.
[in,out]  wcs  Coordinate transformation parameters. 
int datfix  (  struct wcsprm *  wcs  ) 
datfix() translates the old DATEOBS
date format set in wcsprm::dateobs to year2000 standard form (yyyymmddT
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().
[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. 
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:
int obsfix  (  int  ctrl, 
struct wcsprm *  wcs  
) 
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().
[in]  ctrl  Flag that controls behaviour if one triplet is defined and the other is only partially defined:

[in,out]  wcs  Coordinate transformation parameters. wcsprm::obsgeo may be changed. 
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:
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.
For reference, parameters of commonly used global reference ellipsoids:
where f = (a  b) / a is the flattening, and a and b are the semimajor and semiminor radii in metres.
The transformation from geodetic (lng,lat,hgt) to Cartesian (x,y,z) is
where the "prime vertical radius", n, is a function of latitude
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
where
and iterating over the value of zeta. Since e is small, a good first approximation is given by zeta = z.
int unitfix  (  int  ctrl, 
struct wcsprm *  wcs  
) 
unitfix() applies wcsutrn() to translate nonstandard CUNITia
keyvalues, e.g. 'DEG
' > 'deg
', also stripping off unnecessary whitespace.
unitfix() is expected to be invoked before wcsset(), which will fail if nonstandard CUNITia
keyvalues have not been translated.
[in]  ctrl  Do potentially unsafe translations described in the usage notes to wcsutrn(). 
[in,out]  wcs  Coordinate transformation parameters. 
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.
int spcfix  (  struct wcsprm *  wcs  ) 
spcfix() translates AIPSconvention spectral coordinate types, '{FREQ
,FELO
,VELO
}{LSR
,HEL
,OBS
}' (e.g. 'FREQOBS', 'FELOHEL'
, 'VELOLSR') 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.
AIPSconvention spectral types set in CTYPEia
are translated onthefly 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().
[in,out]  wcs  Coordinate transformation parameters. wcsprm::ctype[] and/or wcsprm::specsys may be changed. 
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.
int celfix  (  struct wcsprm *  wcs  ) 
celfix() translates AIPSconvention 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 nonzero 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.
AIPSconvention celestial projection types set in CTYPEia
are translated onthefly 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.
[in,out]  wcs  Coordinate transformation parameters. wcsprm::ctype[] and/or wcsprm::pv[] may be changed. 
For returns > 1, a detailed error message is set in wcsprm::err if enabled, see wcserr_enable().
int cylfix  (  const int  naxis[], 
struct wcsprm *  wcs  
) 
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().
[in]  naxis  Image axis lengths. 
[in,out]  wcs  Coordinate transformation parameters. 
For returns > 1, a detailed error message is set in wcsprm::err if enabled, see wcserr_enable().
int wcspcx  (  struct wcsprm *  wcs, 
int  dopc,  
int  permute,  
double  rotn[2]  
) 
wcspcx() "regularizes" the linear transformation matrix component of the coordinate transformation (PCi_ja
) to make it more humanreadable.
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().
[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. 

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