Evidence of a north-south trend between AUSGeoid98 and AHD in southwest Australia

Abstract

The AUSGeoid98 gravimetric model has been compared with 48 GPD-levelling points at a ~50km spacing across part of the southwest of Western Australia. This is arguably the best subset of GPS-derived ellipsoidal heights in Australia with na internally estimated precision of < +-9mm. The spirit-levelled heights were tied to the Australian Height Datum(AHD)using class C techniques [12mm-root-km allowable misclose]. The comparisons shoe that AUSGeoid98 gives a GPS height transformation to the AHD with a precision of ~+-13cm, which is less than reported earlier (~+-36 cm)for a nationwide dataset. A clear north-south trend of ~0.81 mm/km [ppm] is also evident in the differences; of which approximately one-third is attributable to a north-south error in the AHD induced by dominant north-south sea surface topography effects at the nearby fixed tide gauges. After removal of this north-south trend, the standard deviation of the differences reduces to ~5 cm.