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Basic Information on regrid
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
Generated by miriad@atnf.csiro.au on 02 Jun 2021