Prioritize date_start when defining WCS and coordinate frame

eispac/core/eismap.py

@@ -126,13 +126,20 @@ def date_average(self):
         return self._get_date('date_avg') or super().date_average
 
     @property
-    def date(self):
-        # NOTE: we override this property to prioritize date_average 
-        # over DATE-OBS (or DATE_OBS). In GenericMap, this is reversed.
-        # We do this because we want to make sure we are constructing our 
-        # coordinate frames from DATE_AVG (the midpoint of the raster) and 
-        # not DATE-OBS which is the beginning of the raster.
-        return self.date_average or super().date
+    def reference_date(self):
+        """
+        The reference date for the coordinate system
+
+        According to Section 2.4 of
+        `EIS Software Note 9 <https://solarb.mssl.ucl.ac.uk/SolarB/eis_docs/eis_notes/09_POINTING/eis_swnote_09.pdf>`_,
+        the pointing keywords are defined at the start of the raster. As such, this property returns the
+        time at the beginning of the raster.
+
+        .. note:: This property is overridden because `sunpy.map.GenericMap` sets
+                  this to be the ``.date_average`` which in this case is the midpoint
+                  of the raster.
+        """
+        return self.date_start
 
     @classmethod
     def is_datasource_for(cls, data, header, **kwargs):

eispac/tests/test_eismap.py

@@ -1,7 +1,6 @@
 """
 Tests for `eispac.EISMap`
 """
-import copy
 from textwrap import dedent
 
 import numpy as np
@@ -104,30 +103,19 @@ def test_date_props(test_eis_map, attr, key):
     assert getattr(test_eis_map, attr).isot == test_eis_map.meta[key]
 
 
-def test_date(test_eis_map, test_header):
-    # Case 1: date-average exists
-    assert test_eis_map.date.isot == test_eis_map.meta['date_avg']
-    # Case 2: date-average is None so default to date_obs
-    header = copy.deepcopy(test_header)
-    del header['date_beg']
-    del header['date_end']
-    del header['date_avg']
-    new_map = sunpy.map.Map(test_eis_map.data, header)
-    assert new_map.date.isot == new_map.meta['date_obs']
-    # Case 3: date_avg is None so default to date_start
-    header = copy.deepcopy(test_header)
-    del header['date_avg']
-    del header['date_end']
-    del header['date_obs']
-    new_map = sunpy.map.Map(test_eis_map.data, header)
-    assert new_map.date.isot == new_map.meta['date_beg']
-    # Case 4: date_end and date_avg do not exist
-    header = copy.deepcopy(test_header)
-    del header['date_avg']
-    del header['date_beg']
-    del header['date_obs']
-    new_map = sunpy.map.Map(test_eis_map.data, header)
-    assert new_map.date.isot == new_map.meta['date_end']
+def test_date_is_dateobs(test_eis_map):
+    "Test that .date returns DATE_OBS"
+    assert test_eis_map.date.isot == test_eis_map.meta['date_obs']
+
+
+def test_date_is_date_start(test_eis_map):
+    "Test that .date returns the same date as .date_start"
+    assert test_eis_map.date == test_eis_map.date_start
+
+
+def test_reference_date_is_date_start(test_eis_map):
+    "Test that the reference date is the time at the beginning of the raster"
+    assert test_eis_map.reference_date == test_eis_map.date_start
 
 
 def test_plot_settings(test_eis_map):