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# WCS User Tools

`grid_from_bounding_box` is a function which returns a grid of input points based on the bounding_box of the WCS.

    >>> from gwcs.wcstools import grid_from_bounding_box
    >>> bounding_box = ((0, 4096), (0, 2048))
    >>> x, y = grid_from_bounding_box(bounding_box)
    >>> ra, dec = w(x, y)

The `wcstools` module contains functions of general usability.

`wcs_from_fiducial` is a function which given a fiducial in some coordinate system, returns a WCS object.

    >>> from gwcs.wcstools import wcs_from_fiducial
    >>> from astropy import coordinates as coord
    >>> from astropy import units as u
    >>> from astropy.modeling import models

To create a WCS from a pointing on the sky, as a minimum pass a sky coordinate and a projection to the function.

      >>> fiducial = coord.SkyCoord(5.46 * u.deg, -72.2 * u.deg, frame='icrs')
      >>> tan = models.Pix2Sky_TAN()

Any additional transforms are prepended to the projection and sky rotation.

    >>> trans = models.Shift(-2048) & models.Shift(-1024) | models.Scale(1.38*10**-5) & models.Scale(1.38*10**-5)
    >>> w = wcs_from_fiducial(fiducial, projection=tan, transform=trans)
    >>> w(2048, 1024)
        (5.46, -72.2)

Generate a WCS object from matching (X, Y) and (RA, DEC) points. Fitting a WCS to input pixels & sky positions