• Vertical datums are a foundational piece of the positioning puzzle that allows us make sense of height measurements - they make it possible to align height data by defining where all heights are zero. But when the vertical datum is unreliable, we lose perspective on which direction is down and this can cause strange things to happen. Water can appear to flow in the wrong direction or pool in unexpected places. • The Australian Height Datum (AHD) is the current, official, vertical datum in use in Australia. At 50 years old this year, it has stood the test of time well. But, it has a number of bumps and wrinkles (errors and distortions), relies on degrading physical infrastructure and was never intended to be used with modern positioning technology like GPS. The Australian Vertical Working Surface is a shiny new alternative vertical datum that doesn’t depend on any physical infrastructure, is free from the errors in the AHD and is designed to be directly compatible with GPS technology in the first instance.