| .. _initialization: |
|
|
| **************************************** |
| Initializing axes with world coordinates |
| **************************************** |
|
|
| Basic initialization |
| ******************** |
|
|
| To make a plot using `~astropy.visualization.wcsaxes.WCSAxes`, we first read in |
| the data using `astropy.io.fits |
| <http://docs.astropy.org/en/stable/io/fits/index.html>`_ and parse the WCS |
| information. In this example, we will use an example FITS file from the |
| http://data.astropy.org server (the |
| :func:`~astropy.utils.data.get_pkg_data_filename` function downloads the file |
| and returns a filename): |
|
|
| .. plot:: |
| :context: reset |
| :nofigs: |
| :include-source: |
| :align: center |
|
|
| from astropy.wcs import WCS |
| from astropy.io import fits |
| from astropy.utils.data import get_pkg_data_filename |
|
|
| filename = get_pkg_data_filename('galactic_center/gc_msx_e.fits') |
|
|
| hdu = fits.open(filename)[0] |
| wcs = WCS(hdu.header) |
|
|
| We then create a figure using Matplotlib and create the axes using the |
| :class:`~astropy.wcs.WCS` object created above. The following example shows how |
| to do this with the Matplotlib 'pyplot' interface, keeping a reference to the |
| axes object: |
|
|
| .. plot:: |
| :context: |
| :include-source: |
| :align: center |
|
|
| import matplotlib.pyplot as plt |
| ax = plt.subplot(projection=wcs) |
|
|
| The ``ax`` object created is an instance of the |
| :class:`~astropy.visualization.wcsaxes.WCSAxes` class. Note that if no WCS |
| transformation is specified, the transformation will default to identity, |
| meaning that the world coordinates will match the pixel coordinates. |
|
|
| The field of view shown is, as for standard matplotlib axes, 0 to 1 in both |
| directions, in pixel coordinates. As soon as you show an image (see |
| :doc:`images_contours`), the limits will be adjusted, but if you want you can |
| also adjust the limits manually. Adjusting the limits is done using the |
| same functions/methods as for a normal Matplotlib plot: |
|
|
| .. plot:: |
| :context: |
| :include-source: |
| :align: center |
|
|
| ax.set_xlim(-0.5, hdu.data.shape[1] - 0.5) |
| ax.set_ylim(-0.5, hdu.data.shape[0] - 0.5) |
|
|
| .. note:: If you use the pyplot interface, you can also replace ``ax.set_xlim`` and |
| ``ax.set_ylim`` by ``plt.xlim`` and ``plt.ylim``. |
|
|
| Alternative methods |
| ******************* |
|
|
| As in Matplotlib, there are in fact several ways you can initialize the |
| :class:`~astropy.visualization.wcsaxes.WCSAxes`. |
|
|
| As shown above, the simplest way is to make use of the :class:`~astropy.wcs.WCS` |
| class and pass this to ``plt.subplot``. If you normally use the (partially) |
| object-oriented interface of Matplotlib, you can also do:: |
|
|
| fig = plt.figure() |
| ax = fig.add_subplot(1, 1, 1, projection=wcs) |
|
|
| Note that this also works with :meth:`~matplotlib.figure.Figure.add_axes` and |
| :func:`~matplotlib.pyplot.axes`, e.g.:: |
|
|
| ax = fig.add_axes([0.1, 0.1, 0.8, 0.8], projection=wcs) |
|
|
| or:: |
|
|
| plt.axes([0.1, 0.1, 0.8, 0.8], projection=wcs) |
|
|
| Any additional arguments passed to |
| :meth:`~matplotlib.figure.Figure.add_subplot`, |
| :meth:`~matplotlib.figure.Figure.add_axes`, |
| :func:`~matplotlib.pyplot.subplot`, or :func:`~matplotlib.pyplot.axes`, such |
| as ``slices`` or ``frame_class``, will be passed on to the |
| :class:`~astropy.visualization.wcsaxes.WCSAxes` class. |
|
|
| .. _initialize_alternative: |
|
|
| Directly initializing WCSAxes |
| ***************************** |
|
|
| As an alternative to the above methods of initializing |
| :class:`~astropy.visualization.wcsaxes.WCSAxes`, you can also instantiate |
| :class:`~astropy.visualization.wcsaxes.WCSAxes` directly and add it to the |
| figure:: |
|
|
| from astropy.wcs import WCS |
| from astropy.visualization.wcsaxes import WCSAxes |
| import matplotlib.pyplot as plt |
|
|
| wcs = WCS(...) |
|
|
| fig = plt.figure() |
| ax = WCSAxes(fig, [0.1, 0.1, 0.8, 0.8], wcs=wcs) |
| fig.add_axes(ax) |
|
|
