Handle missing keys for GDT10 Mercator

docs/ref/release_notes.rst

@@ -6,6 +6,53 @@ Release Notes
 =============
 
 
+What's new in iris-grib v0.21.0
+-------------------------------
+
+:Release: 0.21.0
+:Date: [unreleased]
+
+Features
+^^^^^^^^
+
+* `@trexfeathers <https://github.com/trexfeathers>`_ added checks for invalid
+  values in the following keys when loading grid definition template 3.10
+  (mercator grids): ``orientationOfTheGrid``, ``longitudeOfLastGridPoint``,
+  ``latitudeOfLastGridPoint``.
+  `(ISSUE#118) <https://github.com/SciTools/iris-grib/issues/118>`_,
+  `(PR#446) <https://github.com/SciTools/iris-grib/pull/446>`_
+
+
+Bugs Fixed
+^^^^^^^^^^
+
+* N/A
+
+
+Documentation
+^^^^^^^^^^^^^
+
+* N/A
+
+
+Dependencies
+^^^^^^^^^^^^
+
+* N/A
+
+
+Internal
+^^^^^^^^
+
+* N/A
+
+
+New Contributors
+^^^^^^^^^^^^^^^^
+
+* N/A
+
+
 What's new in iris-grib v0.20.0
 -------------------------------
 

src/iris_grib/_load_convert.py

@@ -776,10 +776,35 @@ def grid_definition_template_10(section, metadata):
     # intersects the Earth
     standard_parallel = section["LaD"] * _GRID_ACCURACY_IN_DEGREES
 
+    if "orientationOfTheGrid" in section and not np.isclose(
+        section["orientationOfTheGrid"], 0
+    ):
+        # Could support in future by using the ObliqueMercator class.
+        message = (
+            f"{section['orientationOfTheGrid']=} . iris-grib only supports "
+            "0.0 orientation for grid definition template 10."
+        )
+        raise TranslationError(message)
+
     cs = icoord_systems.Mercator(standard_parallel=standard_parallel, ellipsoid=geog_cs)
 
     # Create the X and Y coordinates.
     x_coord, y_coord, scan = _calculate_proj_coords_from_grid_lengths(section, cs)
+    final_x_point = x_coord.points[-1]
+    final_y_point = y_coord.points[-1]
+    if not (
+        np.isclose(section["longitudeOfLastGridPoint"], final_x_point)
+        and np.isclose(section["latitudeOfLastGridPoint"], final_y_point)
+    ):
+        message = (
+            "File grid definition inconsistent. Grid specification produces "
+            f"{final_x_point=}, {final_y_point=}. But "
+            f"{section['longitudeOfLastGridPoint']=} , "
+            f"{section['latitudeOfLastGridPoint']=} .\n\n"
+            "(Grid specification for Longitude: Di, Ni, "
+            "longitudeOfFirstGridPoint, scanningMode. Latitude uses: Dj, Nj)"
+        )
+        warnings.warn(message)
 
     # Determine the lat/lon dimensions.
     y_dim, x_dim = 0, 1

src/iris_grib/_save_rules.py

@@ -483,6 +483,9 @@ def grid_definition_template_10(cube, grib):
         0x1 << _RESOLUTION_AND_COMPONENTS_GRID_WINDS_BIT,
     )
 
+    # We don't save "orientationOfTheGrid" since we can't represent it -
+    #  would need a future handling of the Iris ObliqueMercator class.
+
 
 def grid_definition_template_12(cube, grib):
     """

src/iris_grib/tests/unit/load_convert/test_grid_definition_template_10.py

@@ -12,6 +12,8 @@
 # before importing anything else.
 import iris_grib.tests as tests
 
+import warnings
+
 import numpy as np
 
 import iris.coord_systems
@@ -37,10 +39,10 @@ def section_3(self):
             "Ni": 181,
             "Nj": 213,
             "latitudeOfFirstGridPoint": 2351555,
-            "latitudeOfLastGridPoint": 25088204,
+            "latitudeOfLastGridPoint": 2797793.1090371446,
             "LaD": 14000000,
             "longitudeOfFirstGridPoint": 114990304,
-            "longitudeOfLastGridPoint": 135009712,
+            "longitudeOfLastGridPoint": 14566918.990644248,
             "resolutionAndComponentFlags": 56,
             "scanningMode": 64,
             "Di": 12000000,
@@ -83,6 +85,34 @@ def test(self):
         expected = self.expected(y_dim=0, x_dim=1)
         self.assertEqual(metadata, expected)
 
+    def test_last_point_warning(self):
+        section = self.section_3()
+        metadata = empty_metadata()
+
+        # No warnings expected with the standard values.
+        with warnings.catch_warnings():
+            warnings.simplefilter("error")
+            grid_definition_template_10(section, metadata)
+
+        # Warning expected if specified last point does not agree with the
+        #  generated one.
+        section["longitudeOfLastGridPoint"] = 0
+        expected_message = (
+            "File grid definition inconsistent. Grid specification produces "
+            "final_x_point="
+        )
+        with self.assertWarnsRegex(UserWarning, expected_message):
+            grid_definition_template_10(section, metadata)
+
+    def test_orientation_error(self):
+        section = self.section_3()
+        section["orientationOfTheGrid"] = 1
+        metadata = empty_metadata()
+        with self.assertRaisesRegex(
+            iris.exceptions.TranslationError, "iris-grib only supports 0.0 orientation"
+        ):
+            grid_definition_template_10(section, metadata)
+
 
 if __name__ == "__main__":
     tests.main()