b'Data trends Data trendsTim Keeping Associate Editor for geophysicaldata management and analysis technical-standards@aseg.org.auFigure 1.ICSM diagram of the prominent spatial systems in use in Australia. Source: https://www.ga.gov.Australian elevation cheat sheet au/scientific-topics/positioning-navigation/australian-geospatial-reference-systemRecently the ASEGs Technical Standards Committee discussed calculating gravitybut an ellipsoid height would notGravimetric QuasiGeoid (AGQG) which with GPS heights (ellipsoid) instead ofneed to be derived before convertingclaims to increase accuracy again by AHD (ellipsoidgeoid), which led to theto other geoids. Geoids change inup 1 m.question - what are the various heightresponse to factors such as earthquake datums we would need to convert?displacements and seasonal shifts inElevation products can be confusing due Ideally all surveys would come withwater loading. to similar names.original GPS heights, but when they doDSM (Digital Surface Model) measured not, do we have the lineage to back- Luckily for datum calculations, the calculate? GRS80 ellipsoid is commonly usedfrom above and can include top of (Table 1). Geoids have evolved quicklycanopyGPS values from ground gravity,with computing power and satellites,DEM (Digital Elevation Model) are grids airborne radar and laser altimeter,and now integrate points from thereferenced to a datum and are meant NASA shuttle radar (SRTM) and LiDARInternational Terrestrial Referenceto be ground instead of canopycan pass through transformationsFrame (ITRF/ATRF). The ITRF, whichDTM (Digital Terrain Model) is usually before we see them. Measurements canis maintained by the Internationala DEM but can also mean a vector be tied to increasingly obscure surveyEarth Rotation and Reference Systemsdataset with extra points between the markers, a mix of six global navigationService (https://www.iers.org) isDEM pointssystems, and base stations that mayitself frequently updated. The latestGPS is WGS84 with EGM96 but devices have been further constrained bygeoid in Australia is the Australiancan output relative to local geoidsAUSPOS. Since the Herculean effort to produce AHD 1971, the height datumsTable 1.Short list of geodetic datums, their ellipsoids with names of geoids have been updated more frequently,and height datums relevant to Australian surveying. List is in chronological order with the new Australian Verticalaccording to year in the suffix.Working Surface (AVWS) incorporating refinements from airborne gravity. TheDatum Ellipsoid Geoid Vertical datumICSM website link below shows how diverse the Australian coordinatesAGD66 Local spheroidlandscape has become https://icsm. AHD71gov.au/australian-datums-and-how- GRS80move-between-them (Figure 1).AGD84 Local spheroidLets start with ellipsoids and geoidsWGS84 GRS80 EGM84and their symbiotic relationship.GDA94 GRS80Ellipsoids are the smooth, squashed spheroidal shapes that allow us toEGM96calculate an XY position. Geoids are aAUSGeoid98 AHDsimilar volume and shape, but with theAUSGeoid09 AHDsurface moved up or down to representATRF2014where gravity would be equivalent to AGQG2019that at mean sea level. Heights reported in survey data can be relative to either,GDA2020 GRS80 AUSGeoid2020 AUSGeoid2020 or AVWSOCTOBER 2023 PREVIEW 34'