Add accessor for RFC compliant JSON in Coordinate3D

Sources/GISTools/GeoJson/Coordinate3D.swift

@@ -390,6 +390,22 @@ extension Coordinate3D: GeoJsonReadable {
         return result
     }
 
+    /// Dump the coordinate as a JSON object, as defined in the specification.
+    ///
+    /// - important: The returned array will always contain ``latitude`` and ``longitude``,
+    ///              and ``altitude`` only if it exists.
+    public var asMinimalJson: [Double] {
+        var result: [Double] = (projection == .epsg4326 || projection == .noSRID
+            ? [longitude, latitude]
+            : [longitudeProjected(to: .epsg4326), latitudeProjected(to: .epsg4326)])
+
+        if let altitude {
+            result.append(altitude)
+        }
+
+        return result
+    }
+
 }
 
 // Custom implementation, because the protocol has different prerequisites

Tests/GISToolsTests/GeoJson/CoordinateTests.swift

@@ -47,6 +47,9 @@ final class CoordinateTests: XCTestCase {
 
         XCTAssertEqual(String(data: coordinateDataM, encoding: .utf8), "[10,15,null,1234]")
         XCTAssertEqual(String(data: coordinateDataZ, encoding: .utf8), "[10,15,500]")
+
+        XCTAssertEqual(coordinateM.asMinimalJson, [10, 15])
+        XCTAssertEqual(coordinateZ.asMinimalJson, [10, 15, 500])
     }
 
     func testEncodable3857() throws {
@@ -97,6 +100,7 @@ final class CoordinateTests: XCTestCase {
         let decodedCoordinateZM = try JSONDecoder().decode(Coordinate3D.self, from: coordinateDataZM)
 
         XCTAssertEqual(decodedCoordinateM.asJson, [10.0, 15.0, nil, 1234])
+        XCTAssertEqual(decodedCoordinateM.asMinimalJson, [10.0, 15.0])
         XCTAssertEqual(decodedCoordinateZ.asJson, [10.0, 15.0, 500])
         XCTAssertEqual(decodedCoordinateZM.asJson, [10.0, 15.0, 500])
     }