feat(geobase): convert geographic coordinates to another datum using …

…helmert 7-parameter transformation #256

dart/geobase/lib/src/geodesy/ellipsoidal/datum.dart

@@ -42,8 +42,11 @@ import 'package:meta/meta.dart';
 import '/src/common/functions/position_functions.dart';
 import '/src/common/reference/ellipsoid.dart';
 import '/src/coordinates/base/position.dart';
+import '/src/coordinates/geographic/geographic.dart';
 import '/src/utils/object_utils.dart';
 
+import 'ellipsoidal.dart';
+
 /// A geodetic datum with a reference ellipsoid and datum transformation
 /// parameters.
 ///
@@ -166,13 +169,34 @@ class Datum {
     transform: [0, 0, -4.5, -0.22, 0, 0, 0.554],
   );
 
-  /// Converts the cartesian [geocentric] position (X, Y, Z) to another datum a
-  /// specified by [to] using the Helmert 7-parameter transformation.
+  /// Converts the [geographic] position in this datum to another datum specified
+  /// by [to].
+  ///
+  /// The geographic position is first converted to geocentric cartesian, then
+  /// the Helmert 7-parameter transformation is applied to convert the position,
+  /// and finally the result is converted back to a geographic position.
+  ///
+  /// The returned position is a geographic position in the [to] datum.
+  Geographic convertGeographic(Geographic geographic, {required Datum to}) {
+    final geocentric = geographic.toGeocentricCartesian(ellipsoid: ellipsoid);
+    final converted = convertGeocentric(geocentric, to: to);
+    return EllipsoidalExtension.fromGeocentricCartesian(
+      converted,
+      ellipsoid: to.ellipsoid,
+    );
+  }
+
+  /// Converts the cartesian [geocentric] position (X, Y, Z) in this datum to
+  /// another datum a specified by [to] using the Helmert 7-parameter
+  /// transformation.
   ///
-  /// The returned position is also a cartesian geocentric position (X, Y, Z) in
-  /// the [to] datum.
+  /// The returned position is a cartesian geocentric position (X, Y, Z) in the
+  /// [to] datum.
   Position convertGeocentric(Position geocentric, {required Datum to}) {
-    if (this == WGS84) {
+    if (this == to) {
+      // no datum change
+      return geocentric;
+    } else if (this == WGS84) {
       // converting from WGS 84
       return _applyTransform(geocentric, to.transform);
     } else if (to == WGS84) {
@@ -183,7 +207,7 @@ class Datum {
       );
     }
 
-    // neither this.datum nor toDatum are WGS84: convert this to WGS84 first
+    // neither this.datum nor toDatum are WGS84: convert origin to WGS84 first
     return _applyTransform(
       convertGeocentric(geocentric, to: WGS84),
       to.transform,
@@ -204,7 +228,7 @@ class Datum {
     // z-shift in metres
     final tz = t[2];
     // scale: normalise parts-per-million to (s+1)
-    final s = t[3] / 1e6 + 1.0;
+    final s = t[3] / 1.0e6 + 1.0;
     // x-rotation: normalise arcseconds to radians
     final rx = (t[4] / 3600.0).toRadians();
     // y-rotation: normalise arcseconds to radians

dart/geobase/test/geodesy/ellipsoidal_test.dart

@@ -123,17 +123,19 @@ void main() {
   });
 
   group('Ellipsoidal calculations on datums', () {
-    test('Datum conversions using Datum class', () {
+    test('Datum conversions using Datum class and geocentric coordinates', () {
       // test values
       // https://github.com/chrisveness/geodesy/blob/master/test/latlon-ellipsoidal-datum-tests.js
 
+      const dms6 = Dms(decimals: 6);
+
       const geo1 = Geographic(lat: 51.47788, lon: -0.00147);
       final gc1 = geo1.toGeocentricCartesian();
       final gc1conv = Datum.WGS84.convertGeocentric(gc1, to: Datum.OSGB36);
       final geo1conv = EllipsoidalExtension.fromGeocentricCartesian(gc1conv,
           ellipsoid: Datum.OSGB36.ellipsoid);
-      expect(geo1conv.latDms(), '51.4774°N');
-      expect(geo1conv.lonDms(), '0.0001°E');
+      expect(geo1conv.latDms(dms6), '51.477364°N');
+      expect(geo1conv.lonDms(dms6), '0.000150°E');
 
       final gc2 = Position.create(
         x: 4027893.924,
@@ -146,5 +148,48 @@ void main() {
       expect(geo2conv.latDms(), '50.7971°N');
       expect(geo2conv.lonDms(), '4.3612°E');
     });
+
+    test('Datum conversions using Datum class and geographic coordinates', () {
+      // test values
+      // https://github.com/chrisveness/geodesy/blob/master/test/latlon-ellipsoidal-datum-tests.js
+
+      const dms5 = Dms(decimals: 5);
+      const dms6 = Dms(decimals: 6);
+      const degMinSec = Dms(type: DmsType.degMinSec, decimals: 3);
+
+      const geo1 = Geographic(lat: 51.47788, lon: -0.00147);
+      final geo1conv = Datum.WGS84.convertGeographic(geo1, to: Datum.OSGB36);
+      expect(geo1conv.latDms(dms6), '51.477364°N');
+      expect(geo1conv.lonDms(dms6), '0.000150°E');
+      final geo1b = Datum.OSGB36.convertGeographic(geo1conv, to: Datum.WGS84);
+      expect(geo1b.latDms(dms5), '51.47788°N');
+      expect(geo1b.lonDms(dms5), '0.00147°W');
+
+      final gc2 = Position.create(
+        x: 4027893.924,
+        y: 307041.993,
+        z: 4919474.294,
+      );
+      final geo2 = EllipsoidalExtension.fromGeocentricCartesian(gc2);
+      final geo2conv = Datum.WGS84.convertGeographic(geo2, to: Datum.OSGB36);
+      expect(geo2conv.latDms(), '50.7971°N');
+      expect(geo2conv.lonDms(), '4.3612°E');
+      final geo2b = Datum.OSGB36.convertGeographic(geo2conv, to: Datum.WGS84);
+      final gc2b = geo2b.toGeocentricCartesian();
+      expect(gc2b.toText(decimals: 1), gc2.toText(decimals: 1));
+
+      const geo3 = Geographic(lat: 53.0, lon: 1.0, elev: 50.0);
+      final geo3osgb = Datum.WGS84.convertGeographic(geo3, to: Datum.OSGB36);
+      expect(geo3osgb.latLonDms(format: degMinSec),
+          '52°59′58.719″N, 1°00′06.490″E, 3.99m');
+
+      final geo3ed = Datum.WGS84.convertGeographic(geo3, to: Datum.ED50);
+      expect(geo3ed.latLonDms(format: degMinSec),
+          '53°00′02.887″N, 1°00′05.101″E, 2.72m');
+
+      final geo3ed2 = Datum.OSGB36.convertGeographic(geo3osgb, to: Datum.ED50);
+      final geo3wgs2 = Datum.ED50.convertGeographic(geo3ed2, to: Datum.WGS84);
+      expect(geo3wgs2.latLonDms(elevDecimals: 1), '53.0000°N, 1.0000°E, 50.0m');
+    });
   });
 }