diff --git a/src/main/java/dev/coph/flightscore/backend/coordinate/distance/DistanceCalculator.java b/src/main/java/dev/coph/flightscore/backend/coordinate/distance/DistanceCalculator.java
index 6a27f2c..462b494 100644
--- a/src/main/java/dev/coph/flightscore/backend/coordinate/distance/DistanceCalculator.java
+++ b/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
// ------------------------------------------------------------------
diff --git a/src/main/java/dev/coph/flightscore/backend/map/CoordinateConverter.java b/src/main/java/dev/coph/flightscore/backend/map/CoordinateConverter.java
index cc4e3fe..904b2ef 100644
--- a/src/main/java/dev/coph/flightscore/backend/map/CoordinateConverter.java
+++ b/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
+ * (500 000 m) and hemisphere-dependent false northing are reused;
+ * only the central meridian is overridden.
+ *
+ * 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.
+ *
+ * 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;