b'National differential RTP gridTechnical notefield across Australia as mapped in Figure 1 is well described by a rapid convergence in the Taylor series for which the first 5 terms are used. Figure 4 shows the 2019 differential RTP grid of Australia which can be downloaded from Geoscience Australias geophysical archive data delivery system (GADDS, https://portal.ga.gov.au/persona/gadds) or through Geoscience Australias electronic catalogue (http://pid.geoscience.gov.au/dataset/ga/131533).The quality of the differential RTP in representing the (transformed) magnetic field variation across Australia is primarily determined by the quality of the input TMI grid. The grid used is the best national compilation but includes surveys of different vintage and specifications and its reliability varies with location. A unique strength of the Australian national TMI compilation grid is constraint of long wavelength variations using the coast to coast (east-west and north-south) Australia-wide airborne geophysical or AWAGS survey (Milligan etal., 2009), which eliminates the cumulative errors that would arise in levelling using multiple grid overlaps. This validity of the long wavelength TMI components contributes significantly to validity of the differential RTP grid. As can be seen from Figure 1 there is considerable variation in the magnetic field direction across most of the States and Territories and therefore any compilation across those areas will also benefit from a differential RTP (in Figure 3.South-North profile through the two TMI anomalies plotted insome states they are already delivered by the state geological Figure 2 and a corresponding vertical field anomaly. surveys). Those state differential RTPs will perform identically Figure 4.Differential Reduction to Pole applied to the Australian national magnetic compilation, 2019.DECEMBER 2021 PREVIEW 56'