Example Usage

Loading WCS information from a FITS file

This example loads a FITS file (supplied on the commandline) and uses the WCS cards in its primary header to transform.

# Load the WCS information from a fits header, and use it
# to convert pixel coordinates to world coordinates.

from __future__ import division # confidence high

import numpy
from astropy import wcs
from astropy.io import fits
import sys

# Load the FITS hdulist using astropy.io.fits
hdulist = fits.open(sys.argv[-1])

# Parse the WCS keywords in the primary HDU
w = wcs.WCS(hdulist[0].header)

# Print out the "name" of the WCS, as defined in the FITS header
print w.wcs.name

# Print out all of the settings that were parsed from the header
w.wcs.print_contents()

# Some pixel coordinates of interest.
pixcrd = numpy.array([[0,0],[24,38],[45,98]], numpy.float_)

# Convert pixel coordinates to world coordinates
# The second argument is "origin" -- in this case we're declaring we
# have 1-based (Fortran-like) coordinates.
world = w.wcs_pix2world(pixcrd, 1)
print world

# Convert the same coordinates back to pixel coordinates.
pixcrd2 = w.wcs_world2pix(world, 1)
print pixcrd2

# These should be the same as the original pixel coordinates, modulo
# some floating-point error.
assert numpy.max(numpy.abs(pixcrd - pixcrd2)) < 1e-6

Building a WCS structure programmatically

This example, rather than starting from a FITS header, sets WCS values programmatically, uses those settings to transform some points, and then saves those settings to a new FITS header.

# Set the WCS information manually by setting properties of the WCS
# object.

from __future__ import division # confidence high

import numpy
from astropy import wcs
from astropy.io import fits
import sys

# Create a new WCS object.  The number of axes must be set
# from the start
w = wcs.WCS(naxis=2)

# Set up an "Airy's zenithal" projection
# Vector properties may be set with Python lists, or Numpy arrays
w.wcs.crpix = [-234.75, 8.3393]
w.wcs.cdelt = numpy.array([-0.066667, 0.066667])
w.wcs.crval = [0, -90]
w.wcs.ctype = ["RA---AIR", "DEC--AIR"]
w.wcs.set_pv([(2, 1, 45.0)])

# Print out all of the contents of the WCS object
w.wcs.print_contents()

# Some pixel coordinates of interest.
pixcrd = numpy.array([[0,0],[24,38],[45,98]], numpy.float_)

# Convert pixel coordinates to world coordinates
world = w.wcs_pix2world(pixcrd, 1)
print world

# Convert the same coordinates back to pixel coordinates.
pixcrd2 = w.wcs_world2pix(world, 1)
print pixcrd2

# These should be the same as the original pixel coordinates, modulo
# some floating-point error.
assert numpy.max(numpy.abs(pixcrd - pixcrd2)) < 1e-6

# Now, write out the WCS object as a FITS header
header = w.to_header()

# header is an astropy.io.fits.Header object.  We can use it to create a new
# PrimaryHDU and write it to a file.
hdu = fits.PrimaryHDU(header=header)
hdu.writeto('test.fits')