diff --git a/dart/geobase/lib/src/geodesy/ellipsoidal/datum.dart b/dart/geobase/lib/src/geodesy/ellipsoidal/datum.dart
index d5c0be45..47a97659 100644
--- a/dart/geobase/lib/src/geodesy/ellipsoidal/datum.dart
+++ b/dart/geobase/lib/src/geodesy/ellipsoidal/datum.dart
@@ -178,11 +178,11 @@ class Datum {
   ///
   /// 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 geocentric = geographic.toGeocentricCartesianDatum(datum: this);
     final converted = convertGeocentric(geocentric, to: to);
-    return EllipsoidalExtension.fromGeocentricCartesian(
+    return EllipsoidalExtension.fromGeocentricCartesianDatum(
       converted,
-      ellipsoid: to.ellipsoid,
+      datum: to,
     );
   }
 
diff --git a/dart/geobase/lib/src/geodesy/ellipsoidal/ellipsoidal.dart b/dart/geobase/lib/src/geodesy/ellipsoidal/ellipsoidal.dart
index 9639fb2b..52b8d362 100644
--- a/dart/geobase/lib/src/geodesy/ellipsoidal/ellipsoidal.dart
+++ b/dart/geobase/lib/src/geodesy/ellipsoidal/ellipsoidal.dart
@@ -17,7 +17,7 @@
 
 // Adaptations on the derivative work (the Dart port):
 //
-// Copyright (c) 2020-2024 Navibyte (https://navibyte.com). All rights reserved.
+// Copyright (c) 2020-2025 Navibyte (https://navibyte.com). All rights reserved.
 // Use of this source code is governed by a “BSD-3-Clause”-style license that is
 // specified in the LICENSE file.
 //
@@ -36,6 +36,8 @@ import '/src/common/reference/ellipsoid.dart';
 import '/src/coordinates/base/position.dart';
 import '/src/coordinates/geographic/geographic.dart';
 
+import 'datum.dart';
+
 /// The base class for calculations related to the Earth surface modeled by
 /// ellipsoidal reference frames.
 ///
@@ -61,7 +63,18 @@ import '/src/coordinates/geographic/geographic.dart';
 /// accessing these transformations.
 @immutable
 class Ellipsoidal {
+  /// An optional datum used for calculations with a reference ellipsoid and
+  /// datum transformation parameters.
+  ///
+  /// See also [ellipsoid].
+  final Datum? datum;
+
   /// The reference ellipsoid used for calculations.
+  ///
+  /// When [datum] is provided, this [ellipsoid] property equals to the
+  /// ellipsoid of the datum.
+  ///
+  /// See also [datum].
   final Ellipsoid ellipsoid;
 
   /// The origin geographic position for calculations.
@@ -69,7 +82,7 @@ class Ellipsoidal {
 
   /// Create an object for ellipsoidal calculations with [origin] as the
   /// current geographic position (latitude and longitude as geodetic
-  /// coordinates).
+  /// coordinates) based on the given [ellipsoid].
   ///
   /// {@template geobase.geodesy.ellipsoidal.parameters}
   ///
@@ -77,10 +90,26 @@ class Ellipsoidal {
   /// * [ellipsoid]: A reference ellipsoid with ellipsoidal parameters.
   ///
   /// {@endtemplate}
-  const Ellipsoidal(this.origin, {this.ellipsoid = Ellipsoid.WGS84});
+  const Ellipsoidal(this.origin, {this.ellipsoid = Ellipsoid.WGS84})
+      : datum = null;
+
+  /// Create an object for ellipsoidal calculations with [origin] as the
+  /// current geographic position (latitude and longitude as geodetic
+  /// coordinates) based on the given [datum].
+  ///
+  /// {@template geobase.geodesy.ellipsoidal.datum}
+  ///
+  /// Parameters:
+  /// * [datum]: A datum with a reference ellipsoid and datum transformation
+  ///   parameters.
+  ///
+  /// {@endtemplate}
+  Ellipsoidal.datum(this.origin, {Datum this.datum = Datum.WGS84})
+      : ellipsoid = datum.ellipsoid;
 
   /// Create an object for ellipsoidal calculations with a origin position
-  /// transformed from [geocentric] cartesian coordinates (X, Y, Z).
+  /// transformed from [geocentric] cartesian coordinates (X, Y, Z) based on the
+  /// given [ellipsoid].
   ///
   /// {@macro geobase.geodesy.ellipsoidal.ecef}
   ///
@@ -99,6 +128,27 @@ class Ellipsoidal {
     );
   }
 
+  /// Create an object for ellipsoidal calculations with a origin position
+  /// transformed from [geocentric] cartesian coordinates (X, Y, Z) based on the
+  /// given [datum].
+  ///
+  /// {@macro geobase.geodesy.ellipsoidal.ecef}
+  ///
+  /// {@macro geobase.geodesy.ellipsoidal.datum}
+  factory Ellipsoidal.fromGeocentricCartesianDatum(
+    Position geocentric, {
+    Datum datum = Datum.WGS84,
+  }) {
+    // an instance with target geographic position
+    return Ellipsoidal.datum(
+      EllipsoidalExtension.fromGeocentricCartesianDatum(
+        geocentric,
+        datum: datum,
+      ),
+      datum: datum,
+    );
+  }
+
   /// Transform the geographic position at [origin] (latitude and longitude as
   /// geodetic coordinates) to geocentric cartesian coordinates (X, Y, Z).
   ///
@@ -156,7 +206,8 @@ class Ellipsoidal {
 /// these transformations.
 extension EllipsoidalExtension on Geographic {
   /// Transform this geographic position (latitude and longitude as
-  /// geodetic coordinates) to geocentric cartesian coordinates (X, Y, Z).
+  /// geodetic coordinates) to geocentric cartesian coordinates (X, Y, Z) based
+  /// on the given [ellipsoid].
   ///
   /// {@macro geobase.geodesy.ellipsoidal.ecef}
   ///
@@ -164,13 +215,37 @@ extension EllipsoidalExtension on Geographic {
   Position toGeocentricCartesian({Ellipsoid ellipsoid = Ellipsoid.WGS84}) =>
       Ellipsoidal(this, ellipsoid: ellipsoid).toGeocentricCartesian();
 
+  /// Transform this geographic position (latitude and longitude as
+  /// geodetic coordinates) to geocentric cartesian coordinates (X, Y, Z) based
+  /// on the given [datum].
+  ///
+  /// {@macro geobase.geodesy.ellipsoidal.ecef}
+  ///
+  /// {@macro geobase.geodesy.ellipsoidal.datum}
+  Position toGeocentricCartesianDatum({Datum datum = Datum.WGS84}) =>
+      Ellipsoidal.datum(this, datum: datum).toGeocentricCartesian();
+
   /// Transform the given [geocentric] cartesian coordinates (X, Y, Z) to
-  /// geographic coordinates (latitude and longitude).
+  /// geographic coordinates (latitude and longitude) based on the given
+  /// [datum].
   ///
   /// {@macro geobase.geodesy.ellipsoidal.ecef}
   ///
-  /// Parameters:
-  /// * [ellipsoid]: A reference ellipsoid with ellipsoidal parameters.
+  /// {@macro geobase.geodesy.ellipsoidal.datum}
+  static Geographic fromGeocentricCartesianDatum(
+    Position geocentric, {
+    Datum datum = Datum.WGS84,
+  }) {
+    return fromGeocentricCartesian(geocentric, ellipsoid: datum.ellipsoid);
+  }
+
+  /// Transform the given [geocentric] cartesian coordinates (X, Y, Z) to
+  /// geographic coordinates (latitude and longitude) based on the given
+  /// [ellipsoid].
+  ///
+  /// {@macro geobase.geodesy.ellipsoidal.ecef}
+  ///
+  /// {@macro geobase.geodesy.ellipsoidal.parameters}
   static Geographic fromGeocentricCartesian(
     Position geocentric, {
     Ellipsoid ellipsoid = Ellipsoid.WGS84,
diff --git a/dart/geobase/test/geodesy/ellipsoidal_test.dart b/dart/geobase/test/geodesy/ellipsoidal_test.dart
index daca1702..e40ba08c 100644
--- a/dart/geobase/test/geodesy/ellipsoidal_test.dart
+++ b/dart/geobase/test/geodesy/ellipsoidal_test.dart
@@ -132,8 +132,9 @@ void main() {
       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);
+      final geo1conv = EllipsoidalExtension.fromGeocentricCartesianDatum(
+          gc1conv,
+          datum: Datum.OSGB36);
       expect(geo1conv.latDms(dms6), '51.477364°N');
       expect(geo1conv.lonDms(dms6), '0.000150°E');
 
@@ -143,8 +144,9 @@ void main() {
         z: 4919474.294,
       );
       final gc2conv = Datum.WGS84.convertGeocentric(gc2, to: Datum.OSGB36);
-      final geo2conv = EllipsoidalExtension.fromGeocentricCartesian(gc2conv,
-          ellipsoid: Datum.OSGB36.ellipsoid);
+      final geo2conv = EllipsoidalExtension.fromGeocentricCartesianDatum(
+          gc2conv,
+          datum: Datum.OSGB36);
       expect(geo2conv.latDms(), '50.7971°N');
       expect(geo2conv.lonDms(), '4.3612°E');
     });