fix: crossing latitude for flat

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased]
 
+### Fixed
+
+- Crossing latitude for flat geometries ([#188](https://github.com/gadomski/antimeridian/pull/188))
+
 ## [0.4.3] - 2025-07-25
 
 ### Fixed

src/antimeridian/_implementation.py

@@ -573,7 +573,7 @@ def crossing_latitude_great_circle(start: XY, end: XY) -> float:
 
 def crossing_latitude_flat(start: XY, end: XY) -> float:
     latitude_delta = end[1] - start[1]
-    if end[0] > 0:
+    if end[0] < 0:
         return round(
             start[1]
             + (180.0 - start[0]) * latitude_delta / (end[0] + 360.0 - start[0]),

tests/data/input/issues-187.json

@@ -0,0 +1,11 @@
+{
+  "type": "Polygon",
+  "coordinates": [
+    [
+      [177.6545884597184, 67.05574774066811],
+      [179.0195867605756, 65.33232820668778],
+      [198.4723636216472, 66.06909015550372],
+      [198.7828129097253, 68.14247939909886]
+    ]
+  ]
+}

tests/data/output/flat/great-circle.json

@@ -1,53 +1 @@
-{
-    "type": "MultiPolygon",
-    "coordinates": [
-        [
-            [
-                [
-                    180.0,
-                    -65.7501218
-                ],
-                [
-                    179.45,
-                    -65.5
-                ],
-                [
-                    178.3,
-                    -66.0
-                ],
-                [
-                    179.75,
-                    -66.5
-                ],
-                [
-                    180.0,
-                    -66.2497739
-                ],
-                [
-                    180.0,
-                    -65.7501218
-                ]
-            ]
-        ],
-        [
-            [
-                [
-                    -180.0,
-                    -66.2497739
-                ],
-                [
-                    -179.1,
-                    -66.0
-                ],
-                [
-                    -180.0,
-                    -65.7501218
-                ],
-                [
-                    -180.0,
-                    -66.2497739
-                ]
-            ]
-        ]
-    ]
-}
+{"type": "MultiPolygon", "coordinates": [[[[180.0, -65.6896552], [179.45, -65.5], [178.3, -66.0], [179.75, -66.5], [180.0, -66.3913043], [180.0, -65.6896552]]], [[[-180.0, -66.3913043], [-179.1, -66.0], [-180.0, -65.6896552], [-180.0, -66.3913043]]]]}

tests/data/output/flat/issues-124.json

@@ -1 +1 @@
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