Understanding coordinate systems, datums and transformations in Australia

Spatial professionals are required to handle an increasingly wide range of positioning information obtained from various sources including terrestrial surveying, Global Navigation Satellite System (GNSS) observations and online GNSS processing services. These positions refer to a multitude of local, national and global datums. A clear understanding of the different coordinate systems and datums in use today and the appropriate transformations between these is therefore essential to ensure rigorous consideration of reference frame variations in order to produce high-quality positioning results. This paper provides a compendium for spatial practitioners, reviewing the concepts and definitions of coordinate systems and datums in the Australian context and outlining the practical procedures for coordinate transformations in Australia, in relation to both horizontal and vertical datums. The differences between Cartesian, curvilinear and projection coordinates are explained and practical solutions for the required coordinate conversions and transformations are presented. The computational procedure for the transformation between orthometric and ellipsoidal heights in the absence of geoid undulations referenced to a regional ellipsoid is outlined.