feat: Add UTM coordinates #38

ref:

lib/utils/location_formats.dart

@@ -1,3 +1,169 @@
+import 'dart:math' as math;
+
+enum CoordinateDisplayFormat { decimal, dms, utm }
+
+String formatCoordinates(
+  double latitude,
+  double longitude,
+  CoordinateDisplayFormat format,
+) {
+  switch (format) {
+    case CoordinateDisplayFormat.decimal:
+      return '${latitude.toStringAsFixed(5)}, ${longitude.toStringAsFixed(5)}';
+    case CoordinateDisplayFormat.dms:
+      return '${formatDmsCoordinate(latitude, true)} ${formatDmsCoordinate(longitude, false)}';
+    case CoordinateDisplayFormat.utm:
+      return formatUtm(latitude, longitude);
+  }
+}
+
+String formatDmsCoordinate(double degrees, bool isLatitude) {
+  final direction = isLatitude
+      ? (degrees >= 0 ? 'N' : 'S')
+      : (degrees >= 0 ? 'E' : 'W');
+
+  final absolute = degrees.abs();
+  final deg = absolute.floor();
+  final minDecimal = (absolute - deg) * 60;
+  final min = minDecimal.floor();
+  final sec = (minDecimal - min) * 60;
+
+  return '$deg°${min.toString().padLeft(2, '0')}\'${sec.toStringAsFixed(2).padLeft(5, '0')}"$direction';
+}
+
+String formatUtm(double latitude, double longitude) {
+  if (latitude < -80 || latitude > 84) {
+    return formatCoordinates(
+      latitude,
+      longitude,
+      CoordinateDisplayFormat.decimal,
+    );
+  }
+
+  final zone = _utmZone(latitude, longitude);
+  final band = _utmLatitudeBand(latitude);
+  final latRad = _degreesToRadians(latitude);
+  final lonRad = _degreesToRadians(longitude);
+  final centralMeridianRad = _degreesToRadians((zone - 1) * 6 - 180 + 3);
+
+  const semiMajorAxis = 6378137.0;
+  const flattening = 1 / 298.257223563;
+  const scaleFactor = 0.9996;
+  final eccentricitySquared = flattening * (2 - flattening);
+  final secondEccentricitySquared =
+      eccentricitySquared / (1 - eccentricitySquared);
+
+  final sinLat = math.sin(latRad);
+  final cosLat = math.cos(latRad);
+  final tanLat = math.tan(latRad);
+  final n = semiMajorAxis /
+      math.sqrt(1 - eccentricitySquared * sinLat * sinLat);
+  final t = tanLat * tanLat;
+  final c = secondEccentricitySquared * cosLat * cosLat;
+  final a = cosLat * (lonRad - centralMeridianRad);
+
+  final meridianArc = semiMajorAxis *
+      ((1 -
+                  eccentricitySquared / 4 -
+                  3 * eccentricitySquared * eccentricitySquared / 64 -
+                  5 *
+                      eccentricitySquared *
+                      eccentricitySquared *
+                      eccentricitySquared /
+                      256) *
+              latRad -
+          (3 * eccentricitySquared / 8 +
+                  3 * eccentricitySquared * eccentricitySquared / 32 +
+                  45 *
+                      eccentricitySquared *
+                      eccentricitySquared *
+                      eccentricitySquared /
+                      1024) *
+              math.sin(2 * latRad) +
+          (15 * eccentricitySquared * eccentricitySquared / 256 +
+                  45 *
+                      eccentricitySquared *
+                      eccentricitySquared *
+                      eccentricitySquared /
+                      1024) *
+              math.sin(4 * latRad) -
+          (35 *
+                  eccentricitySquared *
+                  eccentricitySquared *
+                  eccentricitySquared /
+                  3072) *
+              math.sin(6 * latRad));
+
+  final easting = scaleFactor *
+          n *
+          (a +
+              (1 - t + c) * a * a * a / 6 +
+              (5 - 18 * t + t * t + 72 * c - 58 * secondEccentricitySquared) *
+                  a *
+                  a *
+                  a *
+                  a *
+                  a /
+                  120) +
+      500000;
+  var northing = scaleFactor *
+      (meridianArc +
+          n *
+              tanLat *
+              (a * a / 2 +
+                  (5 - t + 9 * c + 4 * c * c) * a * a * a * a / 24 +
+                  (61 -
+                          58 * t +
+                          t * t +
+                          600 * c -
+                          330 * secondEccentricitySquared) *
+                      a *
+                      a *
+                      a *
+                      a *
+                      a *
+                      a /
+                      720));
+  if (latitude < 0) {
+    northing += 10000000;
+  }
+
+  return '$zone$band ${easting.round()}E ${northing.round()}N';
+}
+
+int _utmZone(double latitude, double longitude) {
+  var zone = ((longitude + 180) / 6).floor() + 1;
+  if (longitude == 180) {
+    zone = 60;
+  }
+  if (latitude >= 56 &&
+      latitude < 64 &&
+      longitude >= 3 &&
+      longitude < 12) {
+    zone = 32;
+  }
+  if (latitude >= 72 && latitude < 84) {
+    if (longitude >= 0 && longitude < 9) {
+      zone = 31;
+    } else if (longitude >= 9 && longitude < 21) {
+      zone = 33;
+    } else if (longitude >= 21 && longitude < 33) {
+      zone = 35;
+    } else if (longitude >= 33 && longitude < 42) {
+      zone = 37;
+    }
+  }
+  return zone.clamp(1, 60);
+}
+
+String _utmLatitudeBand(double latitude) {
+  const bands = 'CDEFGHJKLMNPQRSTUVWX';
+  final index = ((latitude + 80) / 8).floor().clamp(0, bands.length - 1);
+  return bands[index];
+}
+
+double _degreesToRadians(double degrees) => degrees * math.pi / 180;
+
 String formatPlusCode(double lat, double lon) {
   const base = '23456789CFGHJMPQRVWX';
 

lib/widgets/map/compass/compass_header.dart

@@ -6,6 +6,7 @@ import 'package:latlong2/latlong.dart';
 import '../../../l10n/app_localizations.dart';
 import '../../../models/contact.dart';
 import '../../../models/sar_marker.dart';
+import '../../../utils/location_formats.dart';
 import 'compass_math.dart';
 
 /// Header component for the compass dialog showing compass rose,
@@ -563,21 +564,7 @@ class _LocationFormatToggle extends StatefulWidget {
 }
 
 class _LocationFormatToggleState extends State<_LocationFormatToggle> {
-  bool _showDMS = false;
+  CoordinateDisplayFormat _format = CoordinateDisplayFormat.decimal;
-
-  String _formatDMS(double degrees, bool isLatitude) {
-    final direction = isLatitude
-        ? (degrees >= 0 ? 'N' : 'S')
-        : (degrees >= 0 ? 'E' : 'W');
-
-    final absolute = degrees.abs();
-    final deg = absolute.floor();
-    final minDecimal = (absolute - deg) * 60;
-    final min = minDecimal.floor();
-    final sec = (minDecimal - min) * 60;
-
-    return '$deg°${min.toString().padLeft(2, '0')}\'${sec.toStringAsFixed(2).padLeft(5, '0')}"$direction';
-  }
 
   @override
   Widget build(BuildContext context) {
@@ -589,20 +576,24 @@ class _LocationFormatToggleState extends State<_LocationFormatToggle> {
     final l10n = AppLocalizations.of(context)!;
     final String displayText;
 
-    if (_showDMS) {
+    if (_format == CoordinateDisplayFormat.decimal) {
-      displayText =
-          '${_formatDMS(position.latitude, true)} ${_formatDMS(position.longitude, false)}';
-    } else {
       displayText = l10n.latLonFormat(
         position.latitude.toStringAsFixed(5),
         position.longitude.toStringAsFixed(5),
       );
+    } else {
+      displayText = formatCoordinates(
+        position.latitude,
+        position.longitude,
+        _format,
+      );
     }
 
     return GestureDetector(
       onTap: () {
         setState(() {
-          _showDMS = !_showDMS;
+          _format = CoordinateDisplayFormat.values[
+              (_format.index + 1) % CoordinateDisplayFormat.values.length];
         });
       },
       behavior: HitTestBehavior.opaque,

lib/widgets/map_markers.dart

@@ -4,6 +4,7 @@ import '../models/contact.dart';
 import '../models/sar_marker.dart';
 import '../models/sar_template.dart';
 import '../l10n/app_localizations.dart';
+import '../utils/location_formats.dart';
 
 class MapMarkers {
   static List<Marker> createTeamMemberMarkers(
@@ -253,7 +254,11 @@ class MapMarkers {
             if (contact.displayLocation != null) ...[
               _InfoRow(
                 'Location',
-                '${contact.displayLocation!.latitude.toStringAsFixed(6)}, ${contact.displayLocation!.longitude.toStringAsFixed(6)}',
+                formatCoordinates(
+                  contact.displayLocation!.latitude,
+                  contact.displayLocation!.longitude,
+                  CoordinateDisplayFormat.utm,
+                ),
               ),
             ],
             if (contact.telemetry?.batteryMilliVolts != null)
@@ -313,7 +318,11 @@ class MapMarkers {
           children: [
             _InfoRow(
               'Location',
-              '${marker.location.latitude.toStringAsFixed(6)}, ${marker.location.longitude.toStringAsFixed(6)}',
+              formatCoordinates(
+                marker.location.latitude,
+                marker.location.longitude,
+                CoordinateDisplayFormat.utm,
+              ),
             ),
             _InfoRow('Reported', marker.timeAgo),
             if (marker.senderName != null)

test/utils/location_formats_test.dart

@@ -0,0 +1,31 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_sar_app/utils/location_formats.dart';
+
+void main() {
+  group('formatUtm', () {
+    test('formats northern hemisphere UTM coordinates', () {
+      expect(formatUtm(37.7749, -122.4194), '10S 551131E 4180999N');
+    });
+
+    test('formats southern hemisphere UTM coordinates', () {
+      expect(formatUtm(-33.8688, 151.2093), '56H 334369E 6250948N');
+    });
+
+    test('falls back to decimal outside the UTM latitude range', () {
+      expect(formatUtm(85, 14.5), '85.00000, 14.50000');
+    });
+  });
+
+  group('formatCoordinates', () {
+    test('formats DMS coordinates', () {
+      expect(
+        formatCoordinates(
+          46.0569,
+          14.5058,
+          CoordinateDisplayFormat.dms,
+        ),
+        '46°03\'24.84"N 14°30\'20.88"E',
+      );
+    });
+  });
+}