public void ApproximateVelocityTest()
        {
            DateTime    dtg    = DateTime.Now;
            Observation target = new Observation {
                DateTime = dtg, X = 1, Y = 1, Z = 1
            };
            Observation pastObservation = new Observation {
                DateTime = dtg.AddSeconds(3), X = 0, Y = 1, Z = 2
            };
            Velocity expected = new Velocity {
                DateTime = dtg, X = (-1F / 3F), Y = 0, Z = (1F / 3F)
            };
            Velocity actual;

            actual = target.ApproximateVelocity(pastObservation);
            Assert.IsTrue(Math.Abs(actual.X - expected.X) < 0.0001);
            Assert.IsTrue(Math.Abs(actual.Y - expected.Y) < 0.0001);
            Assert.IsTrue(Math.Abs(actual.Z - expected.Z) < 0.0001);
        }