From d8e2f995492c22f2f7bf544813fe29dce149df05 Mon Sep 17 00:00:00 2001
From: "Morten W. Hansen" <mortenwh@met.no>
Date: Sat, 9 May 2026 14:00:06 +0000
Subject: [PATCH] Fix floating-point precision artifacts in longitude
 normalization
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When normalizing longitudes to ±180°, float arithmetic produced
representation artifacts (e.g. '41.76' → '41.75999999999999').
Fix by rounding back to the original number of decimal places.

Also handles numpy.float64 inputs by converting to str before
inspecting decimal places.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 py_mmd_tools/nc_to_mmd.py | 10 +++++++---
 tests/test_nc_to_mmd.py   | 21 +++++++++++++++++++++
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/py_mmd_tools/nc_to_mmd.py b/py_mmd_tools/nc_to_mmd.py
index 0f79e90f..2269984c 100644
--- a/py_mmd_tools/nc_to_mmd.py
+++ b/py_mmd_tools/nc_to_mmd.py
@@ -1255,10 +1255,14 @@ def get_geographic_extent_rectangle(self, mmd_element, ncin):
                     self.missing_attributes["errors"].append(
                         "%s must be convertible to float type." % acdd_key
                     )
-        # Make sure longitudes are within +/-180 degrees
+        # Make sure longitudes are within +/-180 degrees, preserving original
+        # string precision to avoid floating-point representation artifacts.
         if not fail:
-            data["east"] = str((float(data["east"]) + 180.) % 360. - 180.)
-            data["west"] = str((float(data["west"]) + 180.) % 360. - 180.)
+            for key in ("east", "west"):
+                val_str = str(data[key])
+                decimal_places = len(val_str.split(".")[-1]) if "." in val_str else 0
+                normalized = (float(val_str) + 180.) % 360. - 180.
+                data[key] = str(round(normalized, decimal_places))
 
         return data
 
diff --git a/tests/test_nc_to_mmd.py b/tests/test_nc_to_mmd.py
index e7387429..2841ee9d 100644
--- a/tests/test_nc_to_mmd.py
+++ b/tests/test_nc_to_mmd.py
@@ -1192,6 +1192,27 @@ def test_geographic_extent_is_string(self):
         self.assertIsInstance(value["north"], str)
         self.assertIsInstance(value["south"], str)
 
+    def test_geographic_extent_no_float_precision_artifacts(self):
+        """Test that longitude normalization does not introduce floating-point
+        precision artifacts. E.g., input '41.76' should remain '41.76', not
+        become '41.75999999999999'.
+        """
+        mmd_yaml = yaml.load(
+            resource_string('py_mmd_tools', 'mmd_elements.yaml'), Loader=yaml.FullLoader
+        )
+        md = Nc_to_mmd(os.path.abspath('tests/data/reference_nc.nc'), check_only=True)
+        ncin = Dataset(md.netcdf_file, "w", diskless=True)
+        ncin.geospatial_lat_max = "60.158733"
+        ncin.geospatial_lat_min = "59.78492"
+        ncin.geospatial_lon_max = "41.76"
+        ncin.geospatial_lon_min = "10.5"
+        value = md.get_geographic_extent_rectangle(
+            mmd_yaml['geographic_extent']['rectangle'], ncin)
+        self.assertEqual(value["east"], "41.76")
+        self.assertEqual(value["west"], "10.5")
+        self.assertEqual(value["north"], "60.158733")
+        self.assertEqual(value["south"], "59.78492")
+
     def test_geographic_extent_rectangle_is_floatable(self):
         """ Test that the provided geospatial coordinates can be
         converted to float.