diff --git a/feature/roaming/src/main/java/com/rtbishop/look4sat/feature/roaming/RoamingScreen.kt b/feature/roaming/src/main/java/com/rtbishop/look4sat/feature/roaming/RoamingScreen.kt
index b3afbb2b..1380f32f 100644
--- a/feature/roaming/src/main/java/com/rtbishop/look4sat/feature/roaming/RoamingScreen.kt
+++ b/feature/roaming/src/main/java/com/rtbishop/look4sat/feature/roaming/RoamingScreen.kt
@@ -447,7 +447,14 @@ private val latMargin = mapOf(
 
 // Ported lines 311-611: longitude -> (20 deg zone letter, 2 deg digit, 2' letter, 30" digit)
 private fun encodeLon(inputLon: Double): List<String> {
-    var longitude = inputLon
+    // The decompiled range table uses closed bounds on both adjacent cells
+    // (e.g. -20..0 followed by 0..20), and `when` picks the first match. Exact
+    // field/square/subsquare boundaries therefore fell into the previous cell:
+    // lon=0 produced I... instead of J..., lon=108 lost one square/subsquare.
+    // A tiny positive nudge implements the standard half-open [low, high)
+    // convention without rewriting the faithful lookup table; keep +180 inside
+    // the final R cell.
+    var longitude = if (inputLon < 180.0) inputLon + 1e-10 else 180.0 - 1e-10
     var d = 0.0
     val str = when {
         (longitude >= -180.0) && (longitude <= -160.0) -> { longitude += 180.0; "A" }
@@ -528,7 +535,9 @@ private fun encodeLon(inputLon: Double): List<String> {
 
 // Ported lines 612-914: latitude -> (10 deg zone letter, 1 deg digit, 1' letter, 15" digit)
 private fun encodeLat(inputLat: Double): List<String> {
-    var latitude = inputLat
+    // Same closed-bound issue as encodeLon: nudge into the half-open cell so an
+    // exact boundary latitude does not fall back into the previous field.
+    var latitude = if (inputLat < 90.0) inputLat + 1e-10 else 90.0 - 1e-10
     var d2 = 0.0
     var d3 = 0.0
     val str5 = when {
diff --git a/feature/roaming/src/test/java/com/rtbishop/look4sat/feature/roaming/RoamingStateTest.kt b/feature/roaming/src/test/java/com/rtbishop/look4sat/feature/roaming/RoamingStateTest.kt
index 99444302..46e4439f 100644
--- a/feature/roaming/src/test/java/com/rtbishop/look4sat/feature/roaming/RoamingStateTest.kt
+++ b/feature/roaming/src/test/java/com/rtbishop/look4sat/feature/roaming/RoamingStateTest.kt
@@ -111,6 +111,36 @@ class RoamingStateTest {
         assertEquals("RR99xx99", state.loc)
     }
 
+    @Test
+    fun `exact cell boundaries belong to the upper cell`() {
+        // The decompiled range table closes both adjacent cells (-20..0 then
+        // 0..20) and `when` takes the first match, so an exact boundary used to
+        // fall back into the previous cell: (0,0) encoded as II99xx99.
+        assertEquals("JJ00aa00", stateOf(0.0, 0.0).loc)
+        assertEquals("JJ01ma00", stateOf(1.0, 1.0).loc)
+        assertEquals("OL42aa00", stateOf(22.0, 108.0).loc)
+        assertEquals("OL42gm00", stateOf(22.5, 108.5).loc)
+        assertEquals("OL42dg00", stateOf(22.25, 108.25).loc)
+    }
+
+    @Test
+    fun `matches the independent Maidenhead converter across the grid`() {
+        // Two independent implementations of the same standard must agree;
+        // divergence previously affected every exact-boundary coordinate.
+        var lat = -90.0
+        while (lat <= 90.0) {
+            var lon = -180.0
+            while (lon <= 180.0) {
+                val ported = stateOf(lat, lon).loc
+                val reference =
+                    com.rtbishop.look4sat.core.domain.utility.positionToQth(lat, lon, 8)
+                assertEquals("mismatch at ($lat, $lon)", reference?.lowercase(), ported.lowercase())
+                lon += 5.0
+            }
+            lat += 5.0
+        }
+    }
+
     @Test
     fun `gps state after fix matches showLocation`() {
         val state = stateOf(22.314066, 108.706575)