Enhance CoordinateConverter with custom central meridian support and refine UTM distance calculations

- Added `toUtmGridAtMeridian` for flexible UTM projections with an arbitrary central meridian.
- Updated `DistanceCalculator` to use midpoint-based central meridian for accurate planar distance computations.
- Introduced longitude normalization utilities to improve handling of antimeridian crossings.

src/main/java/dev/coph/flightscore/backend/coordinate/distance/DistanceCalculator.java

@@ -194,23 +194,39 @@ public final class DistanceCalculator {
     }
 
     private static double utmEuclidean(Coordinate from, Coordinate to, MapDatum datum) {
-        // Project the source naturally; this fixes the zone (and hemisphere) of
-        // the planar frame in which the distance is measured.
-        GridCoordinate fromUtm = CoordinateConverter.convert(from, datum, MapDatum.WGS84, MapGrid.UTM);
-
-        // Project the target into the same zone / hemisphere as the source, so
-        // both points share one central meridian and the wedge-shaped overlap
-        // between zones causes no jump in the planar coordinates.
+        // Bring both points onto WGS84 first so they share one ellipsoid.
+        Coordinate fromWgs84 = CoordinateConverter.transformDatum(from, datum, MapDatum.WGS84);
         Coordinate toWgs84 = CoordinateConverter.transformDatum(to, datum, MapDatum.WGS84);
-        GridCoordinate toUtm = fromUtm.utmZone() != null
-                ? CoordinateConverter.toUtmGrid(toWgs84, MapDatum.WGS84,
-                        fromUtm.utmZone(), fromUtm.southernHemisphere())
-                : CoordinateConverter.toGrid(toWgs84, MapDatum.WGS84, MapGrid.UTM);
+
+        // Pick the central meridian half-way between the two longitudes. This
+        // distributes the Transverse Mercator scale distortion symmetrically
+        // and roughly halves the maximum planar error compared to using either
+        // point's natural UTM zone — important when the two points are far
+        // apart in longitude or straddle a zone boundary ("Zwickel"). The
+        // shortest longitude arc is used so an antimeridian crossing is fine.
+        double dLon = normalizeLongitudeDelta(toWgs84.longitude() - fromWgs84.longitude());
+        double midLon = normalizeLongitude(fromWgs84.longitude() + dLon / 2.0);
+        boolean southern = (fromWgs84.latitude() + toWgs84.latitude()) / 2.0 < 0.0;
+
+        GridCoordinate fromUtm = CoordinateConverter.toUtmGridAtMeridian(fromWgs84, MapDatum.WGS84, midLon, southern);
+        GridCoordinate toUtm = CoordinateConverter.toUtmGridAtMeridian(toWgs84, MapDatum.WGS84, midLon, southern);
 
         return Math.hypot(toUtm.easting() - fromUtm.easting(),
                 toUtm.northing() - fromUtm.northing());
     }
 
+    private static double normalizeLongitudeDelta(double dLon) {
+        while (dLon > 180.0) dLon -= 360.0;
+        while (dLon < -180.0) dLon += 360.0;
+        return dLon;
+    }
+
+    private static double normalizeLongitude(double lon) {
+        while (lon > 180.0) lon -= 360.0;
+        while (lon <= -180.0) lon += 360.0;
+        return lon;
+    }
+
     // ------------------------------------------------------------------
     //  Result assembly
     // ------------------------------------------------------------------

src/main/java/dev/coph/flightscore/backend/map/CoordinateConverter.java

@@ -82,7 +82,7 @@ public final class CoordinateConverter {
                 return new GridCoordinate(coordinate.longitude(), coordinate.latitude(),
                         coordinate.barometricAltitude(), coordinate.gpsAltitude(), null, null, false);
             case TRANSVERSE_MERCATOR:
-                return transverseMercatorForward(coordinate, datum, grid, null, false);
+                return transverseMercatorForward(coordinate, datum, grid, null, null, false);
             case SWISS_OBLIQUE_MERCATOR:
                 return swissObliqueMercatorForward(coordinate, datum, grid);
             default:
@@ -106,7 +106,28 @@ public final class CoordinateConverter {
         if (zone < 1 || zone > 60) {
             throw new IllegalArgumentException("UTM zone must be in [1, 60], was " + zone);
         }
-        return transverseMercatorForward(coordinate, datum, MapGrid.UTM, zone, southern);
+        return transverseMercatorForward(coordinate, datum, MapGrid.UTM, zone, null, southern);
+    }
+
+    /**
+     * Projects a {@link Coordinate} (referenced to {@code datum}) onto a
+     * UTM-style Transverse Mercator grid with an arbitrary central meridian
+     * (in decimal degrees). UTM's scale factor (0.9996), false easting
+     * (500&nbsp;000&nbsp;m) and hemisphere-dependent false northing are reused;
+     * only the central meridian is overridden.
+     * <p>
+     * This is the primary tool for an accurate planar distance between two
+     * points that are far apart in longitude: picking the mid-longitude as the
+     * central meridian distributes the Transverse Mercator scale distortion
+     * symmetrically and roughly halves the maximum error.
+     * <p>
+     * The returned {@link GridCoordinate} carries no UTM zone number (since
+     * the projection is not a standard UTM zone) but reports the hemisphere
+     * as requested.
+     */
+    public static GridCoordinate toUtmGridAtMeridian(Coordinate coordinate, MapDatum datum,
+                                                     double centralMeridianDeg, boolean southern) {
+        return transverseMercatorForward(coordinate, datum, MapGrid.UTM, null, centralMeridianDeg, southern);
     }
 
     /**
@@ -288,7 +309,8 @@ public final class CoordinateConverter {
     // ------------------------------------------------------------------
 
     private static GridCoordinate transverseMercatorForward(Coordinate coordinate, MapDatum datum, MapGrid grid,
-                                                            Integer forcedZone, boolean forcedSouthern) {
+                                                            Integer forcedZone, Double forcedCentralMeridianDeg,
+                                                            boolean forcedSouthern) {
         double a = datum.semiMajorAxis();
         double f = datum.flattening();
         double e = datum.firstEccentricity();
@@ -300,7 +322,11 @@ public final class CoordinateConverter {
         boolean southern = false;
         double centralMeridian;
         double falseNorthing = grid.falseNorthing();
-        if (grid.isZoneDependent()) {
+        if (forcedCentralMeridianDeg != null) {
+            southern = forcedSouthern;
+            centralMeridian = Math.toRadians(forcedCentralMeridianDeg);
+            falseNorthing = southern ? 10_000_000.0 : 0.0;
+        } else if (grid.isZoneDependent()) {
             if (forcedZone != null) {
                 zone = forcedZone;
                 southern = forcedSouthern;