Merge pull request sunpy#3116 from Cadair/fix_wcs-to-frame

More reliably determine lon/lat from WCS

changelog/3116.bugfix.rst

@@ -0,0 +1,3 @@
+Make a correction to `sunpy.coordinates.wcs_utils.solar_wcs_frame_mapping` so
+that `astropy.wcs.WCS` objects are correctly converted to
+`sunpy.coordinates.frames` objects irrespective of the ordering of the axes.

sunpy/coordinates/tests/test_wcs_utils.py

@@ -18,6 +18,15 @@ def test_hpc():
     assert isinstance(result, Helioprojective)
 
 
+def test_hpc_flipped():
+    wcs = WCS(naxis=2)
+    wcs.wcs.ctype = ['HPLT', 'HPLN']
+
+    result = solar_wcs_frame_mapping(wcs)
+
+    assert isinstance(result, Helioprojective)
+
+
 def test_hgs():
     wcs = WCS(naxis=2)
     wcs.wcs.ctype = ['HGLN', 'HGLT']

sunpy/coordinates/wcs_utils.py

@@ -14,7 +14,7 @@ def solar_wcs_frame_mapping(wcs):
     type values in the `astropy.wcs.utils.wcs_to_celestial_frame` registry.
     """
 
-    dateobs = wcs.wcs.dateobs if wcs.wcs.dateobs else None
+    dateobs = wcs.wcs.dateobs or None
 
     # SunPy Map adds 'heliographic_observer' and 'rsun' attributes to the WCS
     # object. We check for them here, and default to None.
@@ -28,27 +28,19 @@ def solar_wcs_frame_mapping(wcs):
     else:
         rsun = None
 
-    # First we try the Celestial sub, which rectifies the order.
-    # It will return anything matching ??LN*, ??LT*
-    wcss = wcs.sub([WCSSUB_CELESTIAL])
+    # Truncate the ctype to the first four letters
+    ctypes = {c[:4] for c in wcs.wcs.ctype}
 
-    # If the SUB works, use it.
-    if wcss.naxis == 2:
-        wcs = wcss
-
-    xcoord = wcs.wcs.ctype[0][0:4]
-    ycoord = wcs.wcs.ctype[1][0:4]
-
-    if xcoord == 'HPLN' and ycoord == 'HPLT':
+    if {'HPLN', 'HPLT'} <= ctypes:
         return Helioprojective(obstime=dateobs, observer=observer, rsun=rsun)
 
-    if xcoord == 'HGLN' and ycoord == 'HGLT':
+    if {'HGLN', 'HGLT'} <= ctypes:
         return HeliographicStonyhurst(obstime=dateobs)
 
-    if xcoord == 'CRLN' and ycoord == 'CRLT':
+    if {'CRLN', 'CRLT'} <= ctypes:
         return HeliographicCarrington(obstime=dateobs)
 
-    if xcoord == 'SOLX' and ycoord == 'SOLY':
+    if {'SOLX', 'SOLY'} <= ctypes:
         return Heliocentric(obstime=dateobs, observer=observer)