Task: regrid Purpose: regrid an image dataset Categories: map analysis REGRID any combination of the first three axes of an image using cubic interpolation, with exclusion of blanked input pixels. The output coordinate system may be specified via a template image or axis descriptors. REGRID handles conversion - between different map projections, - between different map centres (reference points), - between B1950 and J2000 equatorial coordinates, - between equatorial and galactic coordinates, - between radio and optical velocity definitions, - between LSR and barycentric velocity frames. Nearest neighbour interpolation is used for axes smaller than five pixels in extent. REGRID supports the FITS Celestial World Coordinate System (WCS) standard as defined in "WCS Paper II", Calabretta & Greisen (2002), A&A 395, 1077. Key: in The input image name. In the first instance, coordinate descriptors for the output image are copied from the input. They may then be overridden by other parameters as described below. No default. Key: out The output image name. No default. Key: axes Specify the axes to which 'tin' and 'desc' refer. For example, axes=1,2 resets descriptors for axes 1 and 2. Likewise, axes=2,3 resets descriptors for axes 2 and 3. In each case, it is still possible that another axis will be regridded due to the effect of other parameters such as 'rotate', etc. Note that the output always contains the same number of axes as the input. The default is all axes. Key: tin Template image. If present, coordinate descriptors for the axes to be regridded, as selected by keyword 'axes', are taken from the template image rather than the input image. These axes must exist in the template image. Key: desc This optionally specifies the reference value (CRVAL), reference pixel (CRPIX), coordinate increment (CDELT), and number of pixels (respectively) for each and every axis of the output image selected by keyword 'axes'. Thus, if there are any, then there must be 4 x naxes values, separated by commas, where naxes is the number of axes specified by 'axes'. These values are not changed by any options other than 'offset'. Note that for celestial axes (RA/DEC, GLON/GLAT, etc.), the reference values (CRVAL) and increments (CDELT) are in degrees. The axis types themselves (CTYPE), including the equatorial coordinate system (B1950 or J2000), are taken from the template image, if given, else from the input image, subject to modification by 'project' and 'options' (see below). Key: rotate Set the rotation between the sky and the image to be this angle, in degrees. A positive value of the angle gives an eastward rotation of the sky grid relative to the pixel grid. If the celestial axis descriptors came from the template image then the default rotation comes from that, else the input image. Key: lonpole The native longitude (deg), and... Key: latpole ...the native latitude (deg) of the celestial pole, being the same as the celestial latitude of the native pole. Normally set by the LONPOLEa and LATPOLEa keywords in FITS, or else by PVi_3a and PVi_4a which have precedence, where i is the longitude axis number. Normally only lonpole is needed. Together with the two angles specified by CRVAL, these provide the third Euler angle for a spherical coordinate transformation between celestial spherical coordinates and the projection's "native" spherical coordinates as defined by WCS Paper II. You should rarely need to set these. Defaults are taken from the template image if the projection (see below) defaulted from CTYPE in the template image. Else from the input image if the projection defaulted from that. Else default values depend on the projection. Key: phi0 The native longitude (deg), and... Key: theta0 ...the native latitude (deg) of the fiducial point of the projection (i.e. the point whose celestial coordinates are given by CRVAL), as defined by WCS Paper II. Fiddling with these is "courageous" in the sense of Sir Humphrey Appleby (http://en.wikiquote.org/wiki/Yes,_Minister). Set by the PVi_1a and PVi_2a keywords in FITS, where i is the longitude axis. Defaults are taken from the template image if the projection (see below) defaulted from CTYPE in the template image. Else from the input image if the projection defaulted from that. Else default values depend on the projection. Key: xyzero A logical value. If true, apply an offset so that the origin of Cartesian coordinates in the plane of projection corresponds to the fiducial point - i.e. (x,y) = (0,0) at (phi0,theta0). This is always the case if (phi0,theta0) assume their default values, it only has effect if they are reset (see above). In FITS this logical is set by the PVi_0a keyword, where i is the longitude axis. The default is taken from the template image if the projection (see below) defaulted from CTYPE in the template image. Else from the input image if the projection defaulted from that. Else false. Key: project Three-letter code for the output map projection. Projection codes follow the FITS WCS standard where they are encoded in the CTYPE keyword. Zenithals: AZP Zenithal/azimuthal perspective (2,C?,D?,d) SZP Slant zenithal perspective (3,C?,D?,d) TAN Gnomonic (0,D) STG Stereographic (0,C,D) SIN Orthographic/synthesis (2,d) NCP North celestial pole (0,D,d) - an important special case of the SIN projection, divergent at the equator ARC Zenithal/azimuthal equidistant (0,G) ZPN Zenithal/azimuthal polynomial (30*,G|d) ZEA Zenithal/azimuthal equal area (0,E,G) AIR Airy (1,D) Cylindricals: CYP Cylindrical perspective (2,G|D) CEA Cylindrical equal area (1,E,G) CAR Plate carrée (aka Cartesian) (0,G) - please note that this is NOT the same as the simple linear system used previously unless the reference coordinates (CRVAL) are (0,0) MER Mercator (0,C,D) - note that the variant defined by AIPS memo 46 is not supported. Pseudo-cylindricals: SFL Sanson-Flamsteed (0,E,G) GLS Global sinusoid (0,E,G) - old implementation of Sanson- Flamsteed. Do not use unless to match an existing map. PAR Parabolic (0,E,G) MOL Mollweide (0,E,G) Conventional: AIT Hammer-Aitoff (0,E,G) - note that the variant defined by AIPS memo 46 is not supported. Conics: COP Conic perspective (2*,D) COE Conic equal area (2*,E,G) COD Conic equidistant (2*,G) COO Conic orthomorphic (2*,C,D) Polyconics: BON Bonne (1*,E,G) PCO Polyconic (0,G) Quad-cubes: TSC Tangential spherical cube (0,G) CSC COBE spherical cube (0,G) QSC Quadrilateralized spherical cube (0,E,G) Hybrid: HPX HEALPix (2,E,G) The number of projection parameters is indicated in parentheses. These may be set in the pv array (below). An asterisk indicates that the projection has at least one non-defaulting parameter. Refer to WCS Paper II for the mathematical definition of the projection parameters and their default values. The letters in parentheses provide a terse summary of the projection's special properties: - C conformal - C? conformal for particular projection parameters - D divergent - D? divergent for particular projection parameters - d degenerate (parts of the sphere overlap in the map plane) - E equi-areal - G global (capable of mapping the whole sphere) Some projections may be divergent, degenerate, or global depending on the projection parameters. The default is taken from CTYPE in the template image if given, else from the input image. NOTE that if any value is specified for project, even if the same as in the template or input image, then defaults for lonpole, latpole, phi0, theta0, and pv will not be taken from the template or input image. Key: pv Array of up to 30 projection parameters as per the above. Set by the PVi_ma keywords in FITS, where i is the latitude axis number. Note that at least one non-zero parameter must be given for ZPN, COP, COE, COD, COO, and BON. If no parameters are given, defaults are taken from the template image if the projection (see above) defaulted from CTYPE in the template image. Else from the input image if the projection defaulted from that. Else default values, where applicable, depend on the projection as per WCS Paper II. Key: options Extra processing options that alter the axis description defined by the template image, axis descriptors, or input image. Several can be given, separated by commas, with minimum-match. altprj Interpret a CAR (plate carée) projection in the input ot template image as a simple linear coordinate system with an additional 1/cos(lat0) scaling factor applied when computing the longitude, e.g. RA = (p1 - CRPIX1)*CDELT1/cos(CRVAL2). This interpretation differs significantly from the FITS standard when lat0 (i.e. CRVAL2) is non-zero. noscale Produce a cube where the RA/DEC cell size does not scale with frequency/velocity. offset The coordinate system described by the template or descriptors is modified (shift and expansion or contraction) by an integral number of pixels so that it completely encloses the input. equisw Switch the output coordinate system between J2000 and B1950 equatorial. The output map will be reoriented so that north is upwards, though the coordinate graticule may be oblique nonetheless. galeqsw Switch the output coordinate system between galactic and equatorial. Galactic switches implicitly to equatorial J2000. The output map will be reoriented so that the north pole (equatorial or galactic) is upwards, though the coordinate graticule may be oblique nonetheless. nearest Use nearest neighbour interpolation rather than the default cubic interpolation. If the equatorial coordinate system is not specified in the header (via the 'epoch' item), then J2000 is assumed. Key: tol Interpolation tolerance. Tolerate an error of the specified amount in converting pixel locations in the input to the output. Must be less that 0.5. The default is 0.05. Revision: 1.19, 2021/06/02 04:45:09 UTC

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