Imshow and maps coordinates

How to use imshow with a map and overplot points specified by pixel coordinates and map coordinates.

import numpy as np
import matplotlib.pyplot as plt

import astropy.units as u

import sunpy.map
from astropy.coordinates import SkyCoord
from sunpy.data.sample import AIA_171_IMAGE

We start with the sample data and create a submap of a smaller region.

aia = sunpy.map.Map(AIA_171_IMAGE)
top_right = SkyCoord(0 * u.arcsec, 1000 * u.arcsec, frame=aia.coordinate_frame)
bottom_left = SkyCoord(-1000 * u.arcsec, 0 * u.arcsec, frame=aia.coordinate_frame)
smap = aia.submap(bottom_left, top_right)

By setting the projection of the axis, a WCSAxes is created and this enables the plot to be created with the map coordinates, in this case helioprojective coordinates. The standard plot command expects pixel coordinates. It is also possible to plot a point using the map coordinates if we pass the transformation but this transform expects the values to be in degrees and not arcseconds.

fig = plt.figure()
ax = plt.subplot(projection=smap)
print(ax)
ax.imshow(smap.data)
# plot a point in the middle of the image
pixel_coord = [smap.data.shape[0]/2., smap.data.shape[1]/2.] * u.pix
ax.plot(pixel_coord[0], pixel_coord[1], 'x', color='white',
        label=f'Pixel coordinate [{pixel_coord[0]}, {pixel_coord[1]}]')
ax.coords.grid(color='yellow', linestyle='solid', alpha=0.5)

map_coord = ([-300, 200] * u.arcsec)
# Using the transform command expects coordinates in degrees and not arcseconds
ax.plot(map_coord[0].to('deg'), map_coord[1].to('deg'), 'o', color='white', transform=ax.get_transform('world'),
        label=f'Map coordinate [{map_coord[0]}, {map_coord[1]}]')
plt.legend()
plt.show()
../../../_images/sphx_glr_wcs_axis_map_example_001.png

Out:

WCSAxesSubplot(0.125,0.11;0.775x0.77)

Total running time of the script: ( 0 minutes 0.573 seconds)

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