b'National differential RTP gridTechnical note(171 and 176) in the Australian Remanent Anomalies DatabaseRTP is wasted, and spatial location errors of several kilometres accessed in the AUSGIN portal (https://portal.geoscience.gov.au/ might go un-noticed for some time (this problem is not specific restore/6727640b-5fb1-4e75-964a-c59c0ab5ca91) from whereto the differential RTP). In response to this concern the ASEG the models can be downloaded. Technical Committee has recently decided to create a table of formalised projections for use in Australia.Figure 7 contours also show that differential or variable RTP changes the shape of the anomaly, and also moves the anomalyAcknowledgementshigh hot-spot, in the case of the right hand fried-egg anomaly, around 700 m in a mostly Easterly direction. The national differential RTP grid was championed by Peter Milligan who played a key role in generating the national TMI Conclusions compilations, acquiring and applying the AWAGS survey data The differential RTP grid is a valuable component of the nationaland implementing the differential RTP. Peter worked on this magnetic field datasets and is fit for purpose for regionaltransform and checked its implementation within the Intrepid geological mapping and national crustal studies. The variabletechnology product. This method of differential RTP has been RTP is also useful as an input to further transforms used forused by many Australian and International geological surveys regional mapping such as vertical derivatives. For smaller scaleover the last 10 years. There are several other key contributors studies well covered by a single survey it is better to use theto this happy situation from times past that are not being standard RTP of the local survey. However, if an anomaly ofacknowledged here.interest crosses or lies close to the northern or southern edgeThe authors wish to acknowledge that the datasets presented of an individual survey it may be preferable to buffer the datahere include components of the Australian national magnetic with a merged adjacent grid rather than relying on syntheticcompilation. This includes surveys acquired over more than 50 padding. years, funded, and managed by Geoscience Australia, the State A major concern with application of any RTP transform is theand Territory Geological Surveys and industry.assumption that magnetisation direction is parallel to theReferenceslocal geomagnetic field. In the example presented here, we established that there is no significant rotation of magnetisationArkani-Hamed, J., 1988. Differential reduction to the pole of away from the geomagnetic field. Such rotations are moreregional magnetic anomalies. Geophysics, 53 (12), 15921600.common than is generally appreciated, with anomalies due toBaranov, V., 1957. A new method for interpretation of clearly rotated magnetisations just extreme examples of a muchaeromagnetic maps: pseudo-gravimetric anomalies. larger population (Foss, 2021). The differential RTP is neitherGeophysics, 22 (2), 359383.more nor less susceptible than a conventional RTP to distortionsBlakely, R.J., 1995. Potential Theory in Gravity and Magnetic arising from inappropriate source magnetisations or anyApplications. Cambridge University Press, New York, NY imperfections in the TMI data from which it is derived. Without435pp.validation of magnetisation direction, RTP transformationCooper, G.R.J. and D.R. Cowan, 2005. Differential reduction to cannot be relied upon. It is recommended that locating orthe pole. Computers & Geosciences, 31, 989-999.designing drillholes should always be based on modelling and/ Foss C., 2021. Recovery of Source Magnetization Direction from or inversion studies applied directly to TMI data (and ideally lineMagnetic Field Data. In: Gupta H.K. (eds) Encyclopedia of rather than grid data). Solid Earth Geophysics. Encyclopedia of Earth Sciences Series. End note: What else could go wrong in using continentalSpringer, Cham. https://doi.org/10.1007/978-3-030-58631-scale geophysical processing methods to systematically locate7_265anomalous geology? Milligan, P., Minty, B., Richardson, M, and R. Franklin, 2009. The Australia-wide airborne geophysical survey - accurate Enter the cartographers. The standard practise for continentalcontinental magnetic coverage, ASEG Extended scale work is to use GEODETIC coordinates. One cannotAbstracts, 2009:1, 1-9, DOI: 10.1071/ASEG2009ab075use Mercator projections as there is no continuity at ZoneNakamura, A. and P.R. Milligan, 2015. Total Magnetic Intensity Boundaries. Cartographers would like to produce flat-land(TMI) Grid of Australia with Variable Reduction to Pole maps where a scale ruler can be used. This requires a process that(VRTP) - sixth edition. Geoscience Australia dataset. https://warps the signal from the ellipsoid to flat-land. Only in the lastresearchdata.edu.au/total-magnetic-intensity-sixth-edition/ 5 years, has consensus emerged on what coordinate transforms1302712should be used so that geophysics data can be confidentlyPoudjom Djomani, Y. and B.R.S. Minty, 2019. Total Magnetic passed between 3D visualisation, GIS, creation of Data CubesIntensity (TMI) Grid of Australia with Variable Reduction to Pole and traditional map products without losing registration(VRTP) 2019 - seventh edition. Geoscience Australia dataset. and while minimising resampling. The candidate coordinatehttp://pid.geoscience.gov.au/dataset/ga/131519projections are Lambert Conic Conformal, and Albers Equal Area.Swain, C.J., 2000. Reduction to the pole of regional magnetic If there is a mismatch in the specification and understanding ofdata with variable field direction, and its stabilisation at low this data warping process all the care in producing a differentialinclinations. Exploration Geophysics 31, 7883.DECEMBER 2021 PREVIEW 58'