# wcshdr.h File Reference

`#include "wcs.h"`

Go to the source code of this file.

## Defines
#define | WCSHDR_none 0x00000000
| Bit mask for wcspih() and wcsbth() - reject all extensions.
#define | WCSHDR_all 0x000FFFFF
| Bit mask for wcspih() and wcsbth() - accept all extensions.
#define | WCSHDR_reject 0x10000000
| Bit mask for wcspih() and wcsbth() - reject non-standard keywords.
#define | WCSHDR_CROTAia 0x00000001
| Bit mask for wcspih() and wcsbth() - accept `CROTAia`, `iCROTna`, `TCROTna`.
#define | WCSHDR_EPOCHa 0x00000002
| Bit mask for wcspih() and wcsbth() - accept `EPOCHa`.
#define | WCSHDR_VELREFa 0x00000004
| Bit mask for wcspih() and wcsbth() - accept `VELREFa`.
#define | WCSHDR_CD00i00j 0x00000008
| Bit mask for wcspih() and wcsbth() - accept `CD00i00j`.
#define | WCSHDR_PC00i00j 0x00000010
| Bit mask for wcspih() and wcsbth() - accept `PC00i00j`.
#define | WCSHDR_PROJPn 0x00000020
| Bit mask for wcspih() and wcsbth() - accept `PROJPn`.
#define | WCSHDR_RADECSYS 0x00000040
| Bit mask for wcspih() and wcsbth() - accept `RADECSYS`.
#define | WCSHDR_VSOURCE 0x00000080
| Bit mask for wcspih() and wcsbth() - accept `VSOURCEa`.
#define | WCSHDR_DOBSn 0x00000100
| Bit mask for wcspih() and wcsbth() - accept `DOBSn`.
#define | WCSHDR_LONGKEY 0x00000200
| Bit mask for wcspih() and wcsbth() - accept long forms of the alternate binary table and pixel list WCS keywords.
#define | WCSHDR_CNAMn 0x00000400
| Bit mask for wcspih() and wcsbth() - accept `iCNAMn`, `TCNAMn`, `iCRDEn`, `TCRDEn`, `iCSYEn`, `TCSYEn`.
#define | WCSHDR_AUXIMG 0x00000800
| Bit mask for wcspih() and wcsbth() - allow the image-header form of an auxiliary WCS keyword to provide a default value for all images.
#define | WCSHDR_ALLIMG 0x00001000
| Bit mask for wcspih() and wcsbth() - allow the image-header form of all image header WCS keywords to provide a default value for all images.
#define | WCSHDR_IMGHEAD 0x00010000
| Bit mask for wcsbth() - restrict to image header keywords only.
#define | WCSHDR_BIMGARR 0x00020000
| Bit mask for wcsbth() - restrict to binary table image array keywords only.
#define | WCSHDR_PIXLIST 0x00040000
| Bit mask for wcsbth() - restrict to pixel list keywords only.
#define | WCSHDO_none 0x00
| Bit mask for wcshdo() - don't write any extensions.
#define | WCSHDO_all 0xFF
| Bit mask for wcshdo() - write all extensions.
#define | WCSHDO_safe 0x0F
| Bit mask for wcshdo() - write safe extensions only.
#define | WCSHDO_DOBSn 0x01
| Bit mask for wcshdo() - write `DOBSn`.
#define | WCSHDO_TPCn_ka 0x02
| Bit mask for wcshdo() - write `TPCn_ka`.
#define | WCSHDO_PVn_ma 0x04
| Bit mask for wcshdo() - write `iPVn_ma`, `TPVn_ma`, `iPSn_ma`, `TPSn_ma`.
#define | WCSHDO_CRPXna 0x08
| Bit mask for wcshdo() - write `jCRPXna`, `TCRPXna`, `iCDLTna`, `TCDLTna`, `iCUNIna`, `TCUNIna`, `iCTYPna`, `TCTYPna`, `iCRVLna`, `TCRVLna`.
#define | WCSHDO_CNAMna 0x10
| Bit mask for wcshdo() - write `iCNAMna`, `TCNAMna`, `iCRDEna`, `TCRDEna`, `iCSYEna`, `TCSYEna`.
#define | WCSHDO_WCSNna 0x20
| Bit mask for wcshdo() - write `WCSNna` instead of `TWCSna`.
## Enumerations
enum | wcshdr_errmsg_enum { WCSHDRERR_SUCCESS = 0, WCSHDRERR_NULL_POINTER = 1, WCSHDRERR_MEMORY = 2, WCSHDRERR_BAD_COLUMN = 3, WCSHDRERR_PARSER = 4, WCSHDRERR_BAD_TABULAR_PARAMS = 5 }
## Functions
int | wcspih (char *header, int nkeyrec, int relax, int ctrl, int *nreject, int *nwcs, struct wcsprm **wcs)
| FITS WCS parser routine for image headers.
int | wcsbth (char *header, int nkeyrec, int relax, int ctrl, int keysel, int *colsel, int *nreject, int *nwcs, struct wcsprm **wcs)
| FITS WCS parser routine for binary table and image headers.
int | wcstab (struct wcsprm *wcs)
| Tabular construction routine.
int | wcsidx (int nwcs, struct wcsprm **wcs, int alts[27])
| Index alternate coordinate representations.
int | wcsbdx (int nwcs, struct wcsprm **wcs, int type, short alts[1000][28])
| Index alternate coordinate representions.
int | wcsvfree (int *nwcs, struct wcsprm **wcs)
| Free the array of wcsprm structs.
int | wcshdo (int relax, struct wcsprm *wcs, int *nkeyrec, char **header)
| Write out a wcsprm struct as a FITS header.
## Variables
const char * | wcshdr_errmsg []
| Status return messages.

## Detailed Description

Routines in this suite are aimed at extracting WCS information from a FITS file. They provide the high-level interface between the FITS file and the WCS coordinate transformation routines.

Additionally, function wcshdo() is provided to write out the contents of a wcsprm struct as a FITS header.

Briefly, the anticipated sequence of operations is as follows:

- 1: Open the FITS file and read the image or binary table header, e.g. using CFITSIO routine fits_hdr2str().

- 2: Parse the header using wcspih() or wcsbth(); they will automatically interpret `'TAB'` header keywords using wcstab().

- 3: Allocate memory for, and read `'TAB'` arrays from the binary table extension, e.g. using CFITSIO routine fits_read_wcstab() - refer to the prologue of getwcstab.h. wcsset() will automatically take control of this allocated memory, in particular causing it to be free'd by wcsfree().

- 4: Translate non-standard WCS usage using wcsfix(), see wcsfix.h.

- 5: Initialize wcsprm struct(s) using wcsset() and calculate coordinates using wcsp2s() and/or wcss2p(). Refer to the prologue of wcs.h for a description of these and other high-level WCS coordinate transformation routines.

- 6: Clean up by freeing memory with wcsvfree().

In detail:

- wcspih() is a high-level FITS WCS routine that parses an image header. It returns an array of up to 27 wcsprm structs on each of which it invokes wcstab().

- wcsbth() is the analogue of wcspih() for use with binary tables; it handles image array and pixel list keywords. As an extension of the FITS WCS standard, it also recognizes image header keywords which may be used to provide default values via an inheritance mechanism.

- wcstab() assists in filling in members of the wcsprm struct associated with coordinate lookup tables (`'TAB'`). These are based on arrays stored in a FITS binary table extension (BINTABLE) that are located by `PVi_ma` keywords in the image header.

- wcsidx() and wcsbdx() are utility routines that return the index for a specified alternate coordinate descriptor in the array of wcsprm structs returned by wcspih() or wcsbth().

- wcsvfree() deallocates memory for an array of wcsprm structs, such as returned by wcspih() or wcsbth().

- wcshdo() writes out a wcsprm struct as a FITS header.

## Define Documentation

#define WCSHDR_none 0x00000000

Bit mask for the relax argument of wcspih() and wcsbth() - reject all extensions.

Refer to wcsbth() note 5.

#define WCSHDR_all 0x000FFFFF

Bit mask for the relax argument of wcspih() and wcsbth() - accept all extensions.

Refer to wcsbth() note 5.

#define WCSHDR_reject 0x10000000

Bit mask for the relax argument of wcspih() and wcsbth() - reject non-standard keywords.

Refer to wcsbth() note 5.

#define WCSHDR_CROTAia 0x00000001

Bit mask for the relax argument of wcspih() and wcsbth() - accept `CROTAia`, `iCROTna`, `TCROTna`.

Refer to wcsbth() note 5.

#define WCSHDR_EPOCHa 0x00000002

Bit mask for the relax argument of wcspih() and wcsbth() - accept `EPOCHa`.

Refer to wcsbth() note 5.

#define WCSHDR_VELREFa 0x00000004

Bit mask for the relax argument of wcspih() and wcsbth() - accept `VELREFa`.

Refer to wcsbth() note 5.

#define WCSHDR_CD00i00j 0x00000008

Bit mask for the relax argument of wcspih() and wcsbth() - accept `CD00i00j`.

Refer to wcsbth() note 5.

#define WCSHDR_PC00i00j 0x00000010

Bit mask for the relax argument of wcspih() and wcsbth() - accept `PC00i00j`.

Refer to wcsbth() note 5.

#define WCSHDR_PROJPn 0x00000020

Bit mask for the relax argument of wcspih() and wcsbth() - accept `PROJPn`.

Refer to wcsbth() note 5.

#define WCSHDR_RADECSYS 0x00000040

Bit mask for the relax argument of wcspih() and wcsbth() - accept `RADECSYS`.

Refer to wcsbth() note 5.

#define WCSHDR_VSOURCE 0x00000080

Bit mask for the relax argument of wcspih() and wcsbth() - accept `VSOURCEa`.

Refer to wcsbth() note 5.

#define WCSHDR_DOBSn 0x00000100

Bit mask for the relax argument of wcspih() and wcsbth() - accept `DOBSn`.

Refer to wcsbth() note 5.

#define WCSHDR_LONGKEY 0x00000200

Bit mask for the relax argument of wcspih() and wcsbth() - accept long forms of the alternate binary table and pixel list WCS keywords.

Refer to wcsbth() note 5.

#define WCSHDR_CNAMn 0x00000400

Bit mask for the relax argument of wcspih() and wcsbth() - accept `iCNAMn`, `TCNAMn`, `iCRDEn`, `TCRDEn`, `iCSYEn`, `TCSYEn`.

Refer to wcsbth() note 5.

#define WCSHDR_AUXIMG 0x00000800

Bit mask for the relax argument of wcspih() and wcsbth() - allow the image-header form of an auxiliary WCS keyword with representation-wide scope to provide a default value for all images.

Refer to wcsbth() note 5.

#define WCSHDR_ALLIMG 0x00001000

Bit mask for the relax argument of wcspih() and wcsbth() - allow the image-header form of all image header WCS keywords to provide a default value for all image arrays in a binary table (n.b. not pixel list).

Refer to wcsbth() note 5.

#define WCSHDR_IMGHEAD 0x00010000

Bit mask for the keysel argument of wcsbth() - restrict keyword types considered to image header keywords only.

#define WCSHDR_BIMGARR 0x00020000

Bit mask for the keysel argument of wcsbth() - restrict keyword types considered to binary table image array keywords only.

#define WCSHDR_PIXLIST 0x00040000

Bit mask for the keysel argument of wcsbth() - restrict keyword types considered to pixel list keywords only.

#define WCSHDO_none 0x00

Bit mask for the relax argument of wcshdo() - don't write any extensions.

Refer to the notes for wcshdo().

#define WCSHDO_all 0xFF

Bit mask for the relax argument of wcshdo() - write all extensions.

Refer to the notes for wcshdo().

#define WCSHDO_safe 0x0F

Bit mask for the relax argument of wcshdo() - write only extensions that are considered safe.

Refer to the notes for wcshdo().

#define WCSHDO_DOBSn 0x01

Bit mask for the relax argument of wcshdo() - write `DOBSn`, the column-specific analogue of DATE-OBS for use in binary tables and pixel lists.

Refer to the notes for wcshdo().

#define WCSHDO_TPCn_ka 0x02

Bit mask for the relax argument of wcshdo() - write `TPCn_ka` if less than eight characters instead of `TPn_ka`.

Refer to the notes for wcshdo().

#define WCSHDO_PVn_ma 0x04

Bit mask for the relax argument of wcshdo() - write `iPVn_ma`, `TPVn_ma`, `iPSn_ma`, `TPSn_ma`, if less than eight characters instead of `iVn_ma`, `TVn_ma`, `iSn_ma`, `TSn_ma`.

Refer to the notes for wcshdo().

#define WCSHDO_CRPXna 0x08

Bit mask for the relax argument of wcshdo() - write `jCRPXna`, `TCRPXna`, `iCDLTna`, `TCDLTna`, `iCUNIna`, `TCUNIna`, `iCTYPna`, `TCTYPna`, `iCRVLna`, `TCRVLna`, if less than eight characters instead of `jCRPna`, `TCRPna`, `iCDEna`, `TCDEna`, `iCUNna`, `TCUNna`, `iCTYna`, `TCTYna`, `iCRVna`, `TCRVna`.

Refer to the notes for wcshdo().

#define WCSHDO_CNAMna 0x10

Bit mask for the relax argument of wcshdo() - write `iCNAMna`, `TCNAMna`, `iCRDEna`, `TCRDEna`, `iCSYEna`, `TCSYEna`, if less than eight characters instead of `iCNAna`, `TCNAna`, `iCRDna`, `TCRDna`, `iCSYna`, `TCSYna`.

Refer to the notes for wcshdo().

#define WCSHDO_WCSNna 0x20

Bit mask for the relax argument of wcshdo() - write `WCSNna` instead of `TWCSna`.

Refer to the notes for wcshdo().

## Enumeration Type Documentation

enum wcshdr_errmsg_enum

Enumerator:

  WCSHDRERR_SUCCESS |
  WCSHDRERR_NULL_POINTER |
  WCSHDRERR_MEMORY |
  WCSHDRERR_BAD_COLUMN |
  WCSHDRERR_PARSER |
  WCSHDRERR_BAD_TABULAR_PARAMS |

## Function Documentation

int wcspih | ( | char * | header,
|  | int | nkeyrec,
|  | int | relax,
|  | int | ctrl,
|  | int * | nreject,
|  | int * | nwcs,
|  | struct wcsprm ** | wcs |
| ) |  |  |

wcspih() is a high-level FITS WCS routine that parses an image header, either that of a primary HDU or of an image extension. All WCS keywords defined in Papers I, II, and III are recognized, and also those used by the AIPS convention and certain other keywords that existed in early drafts of the WCS papers as explained in wcsbth() note 5.

Given a character array containing a FITS image header, wcspih() identifies and reads all WCS keywords for the primary coordinate representation and up to 26 alternate representations. It returns this information as an array of wcsprm structs.

wcspih() invokes wcstab() on each of the wcsprm structs that it returns.

Use wcsbth() in preference to wcspih() for FITS headers of unknown type; wcsbth() can parse image headers as well as binary table and pixel list headers.

Parameters:

  `[in,out]` | header | Character array containing the (entire) FITS image header from which to identify and construct the coordinate representations, for example, as might be obtained conveniently via the CFITSIO routine fits_hdr2str(). Each header "keyrecord" (formerly "card image") consists of exactly 80 7-bit ASCII printing characters in the range 0x20 to 0x7e (which excludes NUL, BS, TAB, LF, FF and CR) especially noting that the keyrecords are NOT null-terminated. For negative values of ctrl (see below), header[] is modified so that WCS keyrecords processed by wcspih() are removed from it.
  `[in]` | nkeyrec | Number of keyrecords in header[].
  `[in]` | relax | Degree of permissiveness: 0: Recognize only FITS keywords defined by the published WCS standard. WCSHDR_all: Admit all recognized informal extensions of the WCS standard. Fine-grained control of the degree of permissiveness is also possible as explained in wcsbth() note 5.
  `[in]` | ctrl | Error reporting and other control options for invalid WCS and other header keyrecords: 0: Do not report any rejected header keyrecords. 1: Produce a one-line message stating the number of WCS keyrecords rejected (nreject). 2: Report each rejected keyrecord and the reason why it was rejected. 3: As above, but also report all non-WCS keyrecords that were discarded, and the number of coordinate representations (nwcs) found. The report is written to stderr. For ctrl < 0, WCS keyrecords processed by wcspih() are removed from header[]: -1: Remove only valid WCS keyrecords whose values were successfully extracted, nothing is reported. -2: Also remove WCS keyrecords that were rejected, reporting each one and the reason that it was rejected. -3: As above, and also report the number of coordinate representations (nwcs) found. -11: Same as -1 but preserving the basic keywords `'{DATE`,`MJD}-{OBS`,`AVG}'` and `'OBSGEO-{X`,`Y`,`Z}'`. If any keyrecords are removed from header[] it will be null-terminated (NUL not being a legal FITS header character), otherwise it will contain its original complement of nkeyrec keyrecords and possibly not be null-terminated.
  `[out]` | nreject | Number of WCS keywords rejected for syntax errors, illegal values, etc. Keywords not recognized as WCS keywords are simply ignored. Refer also to wcsbth() note 5.
  `[out]` | nwcs | Number of coordinate representations found.
  `[out]` | wcs | Pointer to an array of wcsprm structs containing up to 27 coordinate representations. Memory for the array is allocated by wcspih() which also invokes wcsini() for each struct to allocate memory for internal arrays and initialize their members to default values. Refer also to wcsbth() note 8. Note that wcsset() is not invoked on these structs. This allocated memory must be freed by the user, first by invoking wcsfree() for each struct, and then by freeing the array itself. A routine, wcsvfree(), is provided to do this (see below).

Returns:
  Status return value:
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 2: Memory allocation failed.
  - 4: Fatal error returned by Flex parser.

Notes:
Refer to wcsbth() notes 1, 2, 3, 5, 7, and 8.

int wcsbth | ( | char * | header,
|  | int | nkeyrec,
|  | int | relax,
|  | int | ctrl,
|  | int | keysel,
|  | int * | colsel,
|  | int * | nreject,
|  | int * | nwcs,
|  | struct wcsprm ** | wcs |
| ) |  |  |

wcsbth() is a high-level FITS WCS routine that parses a binary table header. It handles image array and pixel list WCS keywords which may be present together in one header.

As an extension of the FITS WCS standard, wcsbth() also recognizes image header keywords in a binary table header. These may be used to provide default values via an inheritance mechanism discussed in note 5 (c.f. WCSHDR_AUXIMG and WCSHDR_ALLIMG), or may instead result in wcsprm structs that are not associated with any particular column. Thus wcsbth() can handle primary image and image extension headers in addition to binary table headers (it ignores `NAXIS` and does not rely on the presence of the `TFIELDS` keyword).

All WCS keywords defined in Papers I, II, and III are recognized, and also those used by the AIPS convention and certain other keywords that existed in early drafts of the WCS papers as explained in note 5 below.

wcsbth() sets the colnum or colax[] members of the wcsprm structs that it returns with the column number of an image array or the column numbers associated with each pixel coordinate element in a pixel list. wcsprm structs that are not associated with any particular column, as may be derived from image header keywords, have colnum == 0.

Note 6 below discusses the number of wcsprm structs returned by wcsbth(), and the circumstances in which image header keywords cause a struct to be created. See also note 9 concerning the number of separate images that may be stored in a pixel list.

The API to wcsbth() is similar to that of wcspih() except for the addition of extra arguments that may be used to restrict its operation. Like wcspih(), wcsbth() invokes wcstab() on each of the wcsprm structs that it returns.

Parameters:

  `[in,out]` | header | Character array containing the (entire) FITS binary table, primary image, or image extension header from which to identify and construct the coordinate representations, for example, as might be obtained conveniently via the CFITSIO routine fits_hdr2str(). Each header "keyrecord" (formerly "card image") consists of exactly 80 7-bit ASCII printing characters in the range 0x20 to 0x7e (which excludes NUL, BS, TAB, LF, FF and CR) especially noting that the keyrecords are NOT null-terminated. For negative values of ctrl (see below), header[] is modified so that WCS keyrecords processed by wcsbth() are removed from it.
  `[in]` | nkeyrec | Number of keyrecords in header[].
  `[in]` | relax | Degree of permissiveness: 0: Recognize only FITS keywords defined by the published WCS standard. WCSHDR_all: Admit all recognized informal extensions of the WCS standard. Fine-grained control of the degree of permissiveness is also possible, as explained in note 5 below.
  `[in]` | ctrl | Error reporting and other control options for invalid WCS and other header keyrecords: 0: Do not report any rejected header keyrecords. 1: Produce a one-line message stating the number of WCS keyrecords rejected (nreject). 2: Report each rejected keyrecord and the reason why it was rejected. 3: As above, but also report all non-WCS keyrecords that were discarded, and the number of coordinate representations (nwcs) found. The report is written to stderr. For ctrl < 0, WCS keyrecords processed by wcsbth() are removed from header[]: -1: Remove only valid WCS keyrecords whose values were successfully extracted, nothing is reported. -2: Also remove WCS keyrecords that were rejected, reporting each one and the reason that it was rejected. -3: As above, and also report the number of coordinate representations (nwcs) found. -11: Same as -1 but preserving the basic keywords `'{DATE`,`MJD}-{OBS`,`AVG}'` and `'OBSGEO-{X`,`Y`,`Z}'`. If any keyrecords are removed from header[] it will be null-terminated (NUL not being a legal FITS header character), otherwise it will contain its original complement of nkeyrec keyrecords and possibly not be null-terminated.
  `[in]` | keysel | Vector of flag bits that may be used to restrict the keyword types considered: WCSHDR_IMGHEAD: Image header keywords. WCSHDR_BIMGARR: Binary table image array. WCSHDR_PIXLIST: Pixel list keywords. If zero, there is no restriction. Keywords such as `EQUIna` or `RFRQna` that are common to binary table image arrays and pixel lists (including `WCSNna` and `TWCSna`, as explained in note 4 below) are selected by both WCSHDR_BIMGARR and WCSHDR_PIXLIST. Thus if inheritance via WCSHDR_ALLIMG is enabled as discussed in note 5 and one of these shared keywords is present, then WCSHDR_IMGHEAD and WCSHDR_PIXLIST alone may be sufficient to cause the construction of coordinate descriptions for binary table image arrays.
  `[in]` | colsel | Pointer to an array of table column numbers used to restrict the keywords considered by wcsbth(). A null pointer may be specified to indicate that there is no restriction. Otherwise, the magnitude of cols[0] specifies the length of the array: cols[0] > 0: the columns are included, cols[0] < 0: the columns are excluded. For the pixel list keywords `TPn_ka` and `TCn_ka` (and `TPCn_ka` and `TCDn_ka` if WCSHDR_LONGKEY is enabled), it is an error for one column to be selected but not the other. This is unlike the situation with invalid keyrecords, which are simply rejected, because the error is not intrinsic to the header itself but arises in the way that it is processed.
  `[out]` | nreject | Number of WCS keywords rejected for syntax errors, illegal values, etc. Keywords not recognized as WCS keywords are simply ignored, refer also to note 5 below.
  `[out]` | nwcs | Number of coordinate representations found.
  `[out]` | wcs | Pointer to an array of wcsprm structs containing up to 27027 coordinate representations, refer to note 6 below. Memory for the array is allocated by wcsbth() which also invokes wcsini() for each struct to allocate memory for internal arrays and initialize their members to default values. Refer also to note 8 below. Note that wcsset() is not invoked on these structs. This allocated memory must be freed by the user, first by invoking wcsfree() for each struct, and then by freeing the array itself. A routine, wcsvfree(), is provided to do this (see below).

Returns:
  Status return value:
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 2: Memory allocation failed.
  - 3: Invalid column selection.
  - 4: Fatal error returned by Flex parser.

Notes:

1. wcspih() determines the number of coordinate axes independently for each alternate coordinate representation (denoted by the "a" value in keywords like `CTYPEia`) from the higher of

   1. `NAXIS`,
   2. `WCSAXESa`,
   3. The highest axis number in any parameterized WCS keyword. The keyvalue, as well as the keyword, must be syntactically valid otherwise it will not be considered.

   If none of these keyword types is present, i.e. if the header only contains auxiliary WCS keywords for a particular coordinate representation, then no coordinate description is constructed for it.

   wcsbth() is similar except that it ignores the `NAXIS` keyword if given an image header to process.

   The number of axes, which is returned as a member of the wcsprm struct, may differ for different coordinate representations of the same image.

2. wcspih() and wcsbth() enforce correct FITS "keyword = value" syntax with regard to "= " occurring in columns 9 and 10.

   However, they do recognize free-format character (NOST 100-2.0, Sect. 5.2.1), integer (Sect. 5.2.3), and floating-point values (Sect. 5.2.4) for all keywords.

3. Where `CROTAn`, `CDi_ja`, and `PCi_ja` occur together in one header wcspih() and wcsbth() treat them as described in the prologue to wcs.h.

4. WCS Paper I mistakenly defined the pixel list form of `WCSNAMEa` as `TWCSna` instead of `WCSNna`; the `'T'` is meant to substitute for the axis number in the binary table form of the keyword - note that keywords defined in WCS Papers II and III that are not parameterised by axis number have identical forms for binary tables and pixel lists. Consequently wcsbth() always treats `WCSNna` and `TWCSna` as equivalent.

5. wcspih() and wcsbth() interpret the relax argument as a vector of flag bits to provide fine-grained control over what non-standard WCS keywords to accept. The flag bits are subject to change in future and should be set by using the preprocessor macros (see below) for the purpose.

   - WCSHDR_none: Don't accept any extensions (not even those in the errata). Treat non-conformant keywords in the same way as non-WCS keywords in the header, i.e. simply ignore them.

   - WCSHDR_all: Accept all extensions recognized by the parser.

   - WCSHDR_reject: Reject non-standard keywords (that are not otherwise accepted). A message will optionally be printed on stderr, as determined by the ctrl argument, and nreject will be incremented.

     This flag may be used to signal the presence of non-standard keywords, otherwise they are simply passed over as though they did not exist in the header.

     Useful for testing conformance of a FITS header to the WCS standard.

   - WCSHDR_CROTAia: Accept `CROTAi`a (wcspih()), `iCROTn`a (wcsbth()), `TCROTna` (wcsbth()).
   - WCSHDR_EPOCHa: Accept ``EPOCH`a`.
   - WCSHDR_VELREFa: Accept ``VELREF`a`. wcspih() always recognizes the AIPS-convention keywords, `CROTAn`, `EPOCH`, and `VELREF` for the primary representation (a = ' ') but alternates are non-standard.

     wcsbth() accepts ``EPOCH`a` and ``VELREF`a` only if WCSHDR_AUXIMG is also enabled.

   - WCSHDR_CD00i00j: Accept `CD00i00j` (wcspih()).
   - WCSHDR_PC00i00j: Accept `PC00i00j` (wcspih()).
   - WCSHDR_PROJPn: Accept `PROJPn` (wcspih()). These appeared in early drafts of WCS Paper I+II (before they were split) and are equivalent to `CDi_ja`, `PCi_ja`, and `PVi_ma` for the primary representation (a = ' '). `PROJPn` is equivalent to `PVi_ma` with `m = n` 9, and is associated exclusively with the latitude axis.

   - WCSHDR_RADECSYS: Accept `RADECSYS`. This appeared in early drafts of WCS Paper I+II and was subsequently replaced by `RADESYSa`.

     wcsbth() accepts `RADECSYS` only if WCSHDR_AUXIMG is also enabled.

   - WCSHDR_VSOURCE: Accept `VSOURCEa` or `VSOUna` (wcsbth()). This appeared in early drafts of WCS Paper III and was subsequently dropped in favour of `ZSOURCEa` and `ZSOUna`.

     wcsbth() accepts `VSOURCEa` only if WCSHDR_AUXIMG is also enabled.

   - WCSHDR_DOBSn (wcsbth() only): Allow `DOBSn`, the column-specific analogue of `DATE-OBS`. By an oversight this was never formally defined in the standard.

   - WCSHDR_LONGKEY (wcsbth() only): Accept long forms of the alternate binary table and pixel list WCS keywords, i.e. with "a" non- blank. Specifically

     `jCRPXn`a | `TCRPXn`a | : | `jCRPXn` | `jCRPna` | `TCRPXn` | `TCRPna` | `CRPIXja`
     | `TPCn_ka` | : |  | `ijPCna` |  | `TPn_ka` | `PCi_ja`
     | `TCDn_ka` | : |  | `ijCDna` |  | `TCn_ka` | `CDi_ja`
     `iCDLTn`a | `TCDLTn`a | : | `iCDLTn` | `iCDEna` | `TCDLTn` | `TCDEna` | `CDELTia`
     `iCUNIn`a | `TCUNIn`a | : | `iCUNIn` | `iCUNna` | `TCUNIn` | `TCUNna` | `CUNITia`
     `iCTYPn`a | `TCTYPn`a | : | `iCTYPn` | `iCTYna` | `TCTYPn` | `TCTYna` | `CTYPEia`
     `iCRVLn`a | `TCRVLn`a | : | `iCRVLn` | `iCRVna` | `TCRVLn` | `TCRVna` | `CRVALia`
     `iPVn_ma` | `TPVn_ma` | : |  | `iVn_ma` |  | `TVn_ma` | `PVi_ma`
     `iPSn_ma` | `TPSn_ma` | : |  | `iSn_ma` |  | `TSn_ma` | `PSi_ma`

     where the primary and standard alternate forms together with the image-header equivalent are shown rightwards of the colon.

     The long form of these keywords could be described as quasi- standard. `TPCn_ka`, `iPVn_ma`, and `TPVn_ma` appeared by mistake in the examples in WCS Paper II and subsequently these and also `TCDn_ka`, `iPSn_ma` and `TPSn_ma` were legitimized by the errata to the WCS papers.

     Strictly speaking, the other long forms are non-standard and in fact have never appeared in any draft of the WCS papers nor in the errata. However, as natural extensions of the primary form they are unlikely to be written with any other intention. Thus it should be safe to accept them provided, of course, that the resulting keyword does not exceed the 8-character limit.

     If WCSHDR_CNAMn is enabled then also accept

     `iCNAMn`a | `TCNAMn`a | : | --- | `iCNAna` | --- | `TCNAna` | `CNAMEia`
     `iCRDEn`a | `TCRDEn`a | : | --- | `iCRDna` | --- | `TCRDna` | `CRDERia`
     `iCSYEn`a | `TCSYEn`a | : | --- | `iCSYna` | --- | `TCSYna` | `CSYERia`

     Note that `CNAMEia`, `CRDERia`, `CSYERia`, and their variants are not used by WCSLIB but are stored in the wcsprm struct as auxiliary information.

   - WCSHDR_CNAMn (wcsbth() only): Accept `iCNAMn`, `iCRDEn`, `iCSYEn`, `TCNAMn`, `TCRDEn`, and `TCSYEn`, i.e. with "a" blank. While non-standard, these are the obvious analogues of `iCTYPn`, `TCTYPn`, etc.

   - WCSHDR_AUXIMG (wcsbth() only): Allow the image-header form of an auxiliary WCS keyword with representation-wide scope to provide a default value for all images. This default may be overridden by the column-specific form of the keyword.

     For example, a keyword like `EQUINOXa` would apply to all image arrays in a binary table, or all pixel list columns with alternate representation "a" unless overridden by `EQUIna`.

     Specifically the keywords are:

     `LATPOLEa` | for `LATPna`
     `LONPOLEa` | for `LONPna`
     `RESTFREQ` | for `RFRQna`
     `RESTFRQa` | for `RFRQna`
     `RESTWAVa` | for `RWAVna`

     whose keyvalues are actually used by WCSLIB, and also keywords that provide auxiliary information that is simply stored in the wcsprm struct:

     `EPOCH` |  | ... (No column-specific form.)
     ``EPOCH`a` |  | ... Only if WCSHDR_EPOCHa is set.
     `EQUINOXa` | for `EQUIna`
     `RADESYSa` | for `RADEna`
     `RADECSYS` | for `RADEna` | ... Only if WCSHDR_RADECSYS is set.
     `SPECSYSa` | for `SPECna`
     `SSYSOBSa` | for `SOBSna`
     `SSYSSRCa` | for `SSRCna`
     `VELOSYSa` | for `VSYSna`
     `VELANGLa` | for `VANGna`
     `VELREF` |  | ... (No column-specific form.)
     ``VELREF`a` |  | ... Only if WCSHDR_VELREFa is set.
     `VSOURCEa` | for `VSOUna` | ... Only if WCSHDR_VSOURCE is set.
     `WCSNAMEa` | for `WCSNna` | ... Or `TWCSna` (see below).
     `ZSOURCEa` | for `ZSOUna`

     `DATE-AVG` | for `DAVGn`
     `DATE-OBS` | for `DOBSn`
     `MJD-AVG` | for `MJDAn`
     `MJD-OBS` | for `MJDOBn`
     `OBSGEO-X` | for `OBSGXn`
     `OBSGEO-Y` | for `OBSGYn`
     `OBSGEO-Z` | for `OBSGZn`

     where the image-header keywords on the left provide default values for the column specific keywords on the right.

     Keywords in the last group, such as `MJD-OBS`, apply to all alternate representations, so `MJD-OBS` would provide a default value for all images in the header.

     This auxiliary inheritance mechanism applies to binary table image arrays and pixel lists alike. Most of these keywords have no default value, the exceptions being `LONPOLEa` and `LATPOLEa`, and also `RADESYSa` and `EQUINOXa` which provide defaults for each other. Thus the only potential difficulty in using WCSHDR_AUXIMG is that of erroneously inheriting one of these four keywords.

     Unlike WCSHDR_ALLIMG, the existence of one (or all) of these auxiliary WCS image header keywords will not by itself cause a wcsprm struct to be created for alternate representation "a". This is because they do not provide sufficient information to create a non-trivial coordinate representation when used in conjunction with the default values of those keywords, such as `CTYPEia`, that are parameterized by axis number.

   - WCSHDR_ALLIMG (wcsbth() only): Allow the image-header form of *all* image header WCS keywords to provide a default value for all image arrays in a binary table (n.b. not pixel list). This default may be overridden by the column-specific form of the keyword.

     For example, a keyword like `CRPIXja` would apply to all image arrays in a binary table with alternate representation "a" unless overridden by `jCRPna`.

     Specifically the keywords are those listed above for WCSHDR_AUXIMG plus

     `WCSAXESa` | for `WCAXna`

     which defines the coordinate dimensionality, and the following keywords which are parameterized by axis number:

     `CRPIXja` | for `jCRPna`
     `PCi_ja` | for `ijPCna`
     `CDi_ja` | for `ijCDna`
     `CDELTia` | for `iCDEna`
     `CROTAi` | for `iCROTn`
     `CROTAi`a |  | ... Only if WCSHDR_CROTAia is set.
     `CUNITia` | for `iCUNna`
     `CTYPEia` | for `iCTYna`
     `CRVALia` | for `iCRVna`
     `PVi_ma` | for `iVn_ma`
     `PSi_ma` | for `iSn_ma`

     `CNAMEia` | for `iCNAna`
     `CRDERia` | for `iCRDna`
     `CSYERia` | for `iCSYna`

     where the image-header keywords on the left provide default values for the column specific keywords on the right.

     This full inheritance mechanism only applies to binary table image arrays, not pixel lists, because in the latter case there is no well-defined association between coordinate axis number and column number.

     Note that `CNAMEia`, `CRDERia`, `CSYERia`, and their variants are not used by WCSLIB but are stored in the wcsprm struct as auxiliary information.

     Note especially that at least one wcsprm struct will be returned for each "a" found in one of the image header keywords listed above:

     - If the image header keywords for "a" are not inherited by a binary table, then the struct will not be associated with any particular table column number and it is up to the user to provide an association.

     - If the image header keywords for "a" are inherited by a binary table image array, then those keywords are considered to be "exhausted" and do not result in a separate wcsprm struct.

   For example, to accept `CD00i00j` and `PC00i00j` and reject all other extensions, use

                relax = WCSHDR_reject | WCSHDR_CD00i00j | WCSHDR_PC00i00j;

   The parser always treats `EPOCH` as subordinate to `EQUINOXa` if both are present, and `VSOURCEa` is always subordinate to `ZSOURCEa`.

   Likewise, `VELREF` is subordinate to the formalism of WCS Paper III, see spcaips().

   Neither wcspih() nor wcsbth() currently recognize the AIPS-convention keywords `ALTRPIX` or `ALTRVAL` which effectively define an alternative representation for a spectral axis.

6. Depending on what flags have been set in its relax argument, wcsbth() could return as many as 27027 wcsprm structs:

   - Up to 27 unattached representations derived from image header keywords.

   - Up to 27 structs for each of up to 999 columns containing an image arrays.

   - Up to 27 structs for a pixel list.

   Note that it is considered legitimate for a column to contain an image array and also form part of a pixel list, and in particular that wcsbth() does not check the `TFORM` keyword for a pixel list column to check that it is scalar.

   In practice, of course, a realistic binary table header is unlikely to contain more than a handful of images.

   In order for wcsbth() to create a wcsprm struct for a particular coordinate representation, at least one WCS keyword that defines an axis number must be present, either directly or by inheritance if WCSHDR_ALLIMG is set.

   When the image header keywords for an alternate representation are inherited by a binary table image array via WCSHDR_ALLIMG, those keywords are considered to be "exhausted" and do not result in a separate wcsprm struct. Otherwise they do.

7. Neither wcspih() nor wcsbth() check for duplicated keywords, in most cases they accept the last encountered.

8. wcspih() and wcsbth() use wcsnpv() and wcsnps() (refer to the prologue of wcs.h) to match the size of the pv[] and ps[] arrays in the wcsprm structs to the number in the header. Consequently there are no unused elements in the pv[] and ps[] arrays, indeed they will often be of zero length.

9. The FITS WCS standard for pixel lists assumes that a pixel list defines one and only one image, i.e. that each row of the binary table refers to just one event, e.g. the detection of a single photon or neutrino.

   In the absence of a formal mechanism for identifying the columns containing pixel coordinates (as opposed to pixel values or ancillary data recorded at the time the photon or neutrino was detected), Paper I discusses how the WCS keywords themselves may be used to identify them.

   In practice, however, pixel lists have been used to store multiple images. Besides not specifying how to identify columns, the pixel list convention is also silent on the method to be used to associate table columns with image axes.

   wcsbth() simply collects all WCS keywords for a particular coordinate representation (i.e. the "a" value in `TCTYna`) into one wcsprm struct. However, these alternates need not be associated with the same table columns and this allows a pixel list to contain up to 27 separate images. As usual, if one of these representations happened to contain more than two celestial axes, for example, then an error would result when wcsset() is invoked on it. In this case the "colsel" argument could be used to restrict the columns used to construct the representation so that it only contained one pair of celestial axes.

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

wcstab() assists in filling in the information in the wcsprm struct relating to coordinate lookup tables.

Tabular coordinates (`'TAB'`) present certain difficulties in that the main components of the lookup table - the multidimensional coordinate array plus an index vector for each dimension - are stored in a FITS binary table extension (BINTABLE). Information required to locate these arrays is stored in `PVi_ma` and `PSi_ma` keywords in the image header.

wcstab() parses the `PVi_ma` and `PSi_ma` keywords associated with each `'TAB'` axis and allocates memory in the wcsprm struct for the required number of tabprm structs. It sets as much of the tabprm struct as can be gleaned from the image header, and also sets up an array of wtbarr structs (described in the prologue of wcs.h) to assist in extracting the required arrays from the BINTABLE extension(s).

It is then up to the user to allocate memory for, and copy arrays from the BINTABLE extension(s) into the tabprm structs. A CFITSIO routine, fits_read_wcstab(), has been provided for this purpose, see getwcstab.h. wcsset() will automatically take control of this allocated memory, in particular causing it to be free'd by wcsfree(); the user must not attempt to free it after wcsset() has been called.

Note that wcspih() and wcsbth() automatically invoke wcstab() on each of the wcsprm structs that they return.

Parameters:

  `[in,out]` | wcs | Coordinate transformation parameters (see below). wcstab() sets ntab, tab, nwtb and wtb, allocating memory for the tab and wtb arrays. This allocated memory will be free'd automatically by wcsfree().

Returns:
  Status return value:
  - 0: Success.
  - 1: Null wcsprm pointer passed.
  - 2: Memory allocation failed.
  - 3: Invalid tabular parameters.
  For returns > 1, a detailed error message is set in wcsprm::err if enabled, see wcserr_enable().

int wcsidx | ( | int | nwcs,
|  | struct wcsprm ** | wcs,
|  | int | alts[27] |
| ) |  |  |

wcsidx() returns an array of 27 indices for the alternate coordinate representations in the array of wcsprm structs returned by wcspih(). For the array returned by wcsbth() it returns indices for the unattached (colnum == 0) representations derived from image header keywords - use wcsbdx() for those derived from binary table image arrays or pixel lists keywords.

Parameters:

  `[in]` | nwcs | Number of coordinate representations in the array.
  `[in]` | wcs | Pointer to an array of wcsprm structs returned by wcspih() or wcsbth().
  `[out]` | alts | Index of each alternate coordinate representation in the array: alts[0] for the primary, alts[1] for 'A', etc., set to -1 if not present. For example, if there was no 'P' representation then
                                alts['P'-'A'+1] == -1;
  Otherwise, the address of its wcsprm struct would be
                                wcs + alts['P'-'A'+1];

Returns:
  Status return value:
  - 0: Success.
  - 1: Null wcsprm pointer passed.

int wcsbdx | ( | int | nwcs,
|  | struct wcsprm ** | wcs,
|  | int | type,
|  | short | alts[1000][28] |
| ) |  |  |

wcsbdx() returns an array of 999 x 27 indices for the alternate coordinate representions for binary table image arrays xor pixel lists in the array of wcsprm structs returned by wcsbth(). Use wcsidx() for the unattached representations derived from image header keywords.

Parameters:

  `[in]` | nwcs | Number of coordinate representations in the array.
  `[in]` | wcs | Pointer to an array of wcsprm structs returned by wcsbth().
  `[in]` | type | Select the type of coordinate representation: 0: binary table image arrays, 1: pixel lists.
  `[out]` | alts | Index of each alternate coordinate represention in the array: alts[col][0] for the primary, alts[col][1] for 'A', to alts[col][26] for 'Z', where col is the 1-relative column number, and col == 0 is used for unattached image headers. Set to -1 if not present. alts[col][27] counts the number of coordinate representations of the chosen type for each column. For example, if there was no 'P' represention for column 13 then
                                alts[13]['P'-'A'+1] == -1;
  Otherwise, the address of its wcsprm struct would be
                                wcs + alts[13]['P'-'A'+1];

Returns:
  Status return value:
  - 0: Success.
  - 1: Null wcsprm pointer passed.

int wcsvfree | ( | int * | nwcs,
|  | struct wcsprm ** | wcs |
| ) |  |  |

wcsvfree() frees the memory allocated by wcspih() or wcsbth() for the array of wcsprm structs, first invoking wcsfree() on each of the array members.

Parameters:

  `[in,out]` | nwcs | Number of coordinate representations found; set to 0 on return.
  `[in,out]` | wcs | Pointer to the array of wcsprm structs; set to 0 on return.

Returns:
  Status return value:
  - 0: Success.
  - 1: Null wcsprm pointer passed.

int wcshdo | ( | int | relax,
|  | struct wcsprm * | wcs,
|  | int * | nkeyrec,
|  | char ** | header |
| ) |  |  |

wcshdo() translates a wcsprm struct into a FITS header. If the colnum member of the struct is non-zero then a binary table image array header will be produced. Otherwise, if the colax[] member of the struct is set non-zero then a pixel list header will be produced. Otherwise, a primary image or image extension header will be produced.

If the struct was originally constructed from a header, e.g. by wcspih(), the output header will almost certainly differ in a number of respects:

- The output header only contains WCS-related keywords. In particular, it does not contain syntactically-required keywords such as `SIMPLE`, `NAXIS`, `BITPIX`, or `END`.

- Deprecated (e.g. `CROTAn`) or non-standard usage will be translated to standard (this is partially dependent on whether wcsfix() was applied).

- Quantities will be converted to the units used internally, basically SI with the addition of degrees.

- Floating-point quantities may be given to a different decimal precision.

- Elements of the `PCi_ja` matrix will be written if and only if they differ from the unit matrix. Thus, if the matrix is unity then no elements will be written.

- Additional keywords such as `WCSAXESa`, `CUNITia`, `LONPOLEa` and `LATPOLEa` may appear.

- The original keycomments will be lost, although wcshdo() tries hard to write meaningful comments.

- Keyword order may be changed.

Keywords can be translated between the image array, binary table, and pixel lists forms by manipulating the colnum or colax[] members of the wcsprm struct.

Parameters:

  `[in]` | relax | Degree of permissiveness: 0: Recognize only FITS keywords defined by the published WCS standard. -1: Admit all informal extensions of the WCS standard. Fine-grained control of the degree of permissiveness is also possible as explained in the notes below.
  `[in,out]` | wcs | Pointer to a wcsprm struct containing coordinate transformation parameters. Will be initialized if necessary.
  `[out]` | nkeyrec | Number of FITS header keyrecords returned in the "header" array.
  `[out]` | header | Pointer to an array of char holding the header. Storage for the array is allocated by wcshdo() in blocks of 2880 bytes (32 x 80-character keyrecords) and must be free'd by the user to avoid memory leaks. Each keyrecord is 80 characters long and is *NOT* null-terminated, so the first keyrecord starts at (*header)[0], the second at (*header)[80], etc.

Returns:
  Status return value (associated with wcs_errmsg[]):
  - 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().

Notes:
wcshdo() interprets the relax argument as a vector of flag bits to provide fine-grained control over what non-standard WCS keywords to write. The flag bits are subject to change in future and should be set by using the preprocessor macros (see below) for the purpose.

- WCSHDO_none: Don't use any extensions.

- WCSHDO_all: Write all recognized extensions, equivalent to setting each flag bit.

- WCSHDO_safe: Write all extensions that are considered to be safe and recommended.

- WCSHDO_DOBSn: Write `DOBSn`, the column-specific analogue of `DATE-OBS` for use in binary tables and pixel lists. WCS Paper III introduced `DATE-AVG` and `DAVGn` but by an oversight `DOBSn` (the obvious analogy) was never formally defined by the standard. The alternative to using `DOBSn` is to write `DATE-OBS` which applies to the whole table. This usage is considered to be safe and is recommended.

- WCSHDO_TPCn_ka: WCS Paper I defined

  - `TPn_ka` and `TCn_ka` for pixel lists

  but WCS Paper II uses `TPCn_ka` in one example and subsequently the errata for the WCS papers legitimized the use of

  - `TPCn_ka` and `TCDn_ka` for pixel lists

  provided that the keyword does not exceed eight characters. This usage is considered to be safe and is recommended because of the non-mnemonic terseness of the shorter forms.

- WCSHDO_PVn_ma: WCS Paper I defined

  - `iVn_ma` and `iSn_ma` for bintables and
  - `TVn_ma` and `TSn_ma` for pixel lists

  but WCS Paper II uses `iPVn_ma` and `TPVn_ma` in the examples and subsequently the errata for the WCS papers legitimized the use of

  - `iPVn_ma` and `iPSn_ma` for bintables and
  - `TPVn_ma` and `TPSn_ma` for pixel lists

  provided that the keyword does not exceed eight characters. This usage is considered to be safe and is recommended because of the non-mnemonic terseness of the shorter forms.

- WCSHDO_CRPXna: For historical reasons WCS Paper I defined

  - `jCRPXn`, `iCDLTn`, `iCUNIn`, `iCTYPn`, and `iCRVLn` for bintables and
  - `TCRPXn`, `TCDLTn`, `TCUNIn`, `TCTYPn`, and `TCRVLn` for pixel lists

  for use without an alternate version specifier. However, because of the eight-character keyword constraint, in order to accommodate column numbers greater than 99 WCS Paper I also defined

  - `jCRPna`, `iCDEna`, `iCUNna`, `iCTYna` and `iCRVna` for bintables and
  - `TCRPna`, `TCDEna`, `TCUNna`, `TCTYna` and `TCRVna` for pixel lists

  for use with an alternate version specifier (the "a"). Like the PC, CD, PV, and PS keywords there is an obvious tendency to confuse these two forms for column numbers up to 99. It is very unlikely that any parser would reject keywords in the first set with a non-blank alternate version specifier so this usage is considered to be safe and is recommended.

- WCSHDO_CNAMna: WCS Papers I and III defined

  - `iCNAna`, `iCRDna`, and `iCSYna` for bintables and
  - `TCNAna`, `TCRDna`, and `TCSYna` for pixel lists

  By analogy with the above, the long forms would be

  - `iCNAMn`a, `iCRDEn`a, and `iCSYEn`a for bintables and
  - `TCNAMn`a, `TCRDEn`a, and `TCSYEn`a for pixel lists

  Note that these keywords provide auxiliary information only, none of them are needed to compute world coordinates. This usage is potentially unsafe and is not recommended at this time.

- WCSHDO_WCSNna: In light of wcsbth() note 4, write `WCSNna` instead of `TWCSna` for pixel lists. While wcsbth() treats `WCSNna` and `TWCSna` as equivalent, other parsers may not. Consequently, this usage is potentially unsafe and is not recommended at this time.

## Variable Documentation

const char * wcshdr_errmsg[]

Error messages to match the status value returned from each function. Use wcs_errmsg[] for status returns from wcshdo().