Mapbase converts crln_obs to hgln_obs if it is found in the header

changelog/2946.bugfix.rst

@@ -0,0 +1 @@
+Mapbase now converts Carrington longitude(crln_obs) to Stonyhurst longitude(hgln_obs), if Carrington longitude(crln_obs) is found in the header.

sunpy/map/mapbase.py

@@ -611,14 +611,16 @@ def heliographic_longitude(self):
         heliographic_longitude = self.meta.get('hgln_obs', None)
 
         if heliographic_longitude is None:
-            if self._default_heliographic_longitude is None:
-                warnings.warn_explicit(
-                    "Missing metadata for heliographic longitude: "
-                    "assuming longitude of 0 degrees",
-                    Warning, __file__,
-                    inspect.currentframe().f_back.f_lineno)
-                self._default_heliographic_longitude = 0
-            heliographic_longitude = self._default_heliographic_longitude
+            if self.meta.get('crln_obs', None) is not None:
+                heliographic_longitude = self.meta['crln_obs'] * u.deg - get_sun_L0(self.date)
+            else:
+                if self._default_heliographic_longitude is None:
+                    warnings.warn_explicit("Missing metadata for heliographic longitude: "
+                                           "assuming longitude of 0 degrees",
+                                           Warning, __file__,
+                                           inspect.currentframe().f_back.f_lineno)
+                    self._default_heliographic_longitude = 0
+                heliographic_longitude = self._default_heliographic_longitude
 
         if isinstance(heliographic_longitude, str):
             heliographic_longitude = float(heliographic_longitude)

sunpy/map/tests/test_mapbase.py

@@ -3,6 +3,7 @@
 Test Generic Map
 """
 import os
+import glob
 import tempfile
 import warnings
 
@@ -27,6 +28,9 @@
 
 testpath = sunpy.data.test.rootdir
 
+@pytest.fixture
+def HMI_file():
+    return sunpy.map.Map(glob.glob(os.path.join(testpath, "resampled_hmi.fits")))
 
 @pytest.fixture
 def aia171_test_map():
@@ -194,7 +198,10 @@ def test_coordinate_frame(aia171_test_map):
     assert frame.observer.lon == aia171_test_map.observer_coordinate.frame.lon
     assert frame.observer.radius == aia171_test_map.observer_coordinate.frame.radius
     assert frame.obstime == aia171_test_map.date
-
+
+def test_heliographic_longitude_crln(HMI_file):
+    if HMI_file.heliographic_longitude == 0:
+        assert HMI_file.heliographic_longitude == HMI_file.carrington_longitude - sunpy.coordinates.get_sun_L0(HMI_file.date)
 
 # ==============================================================================
 # Test Rotation WCS conversion