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The standard mapsheets display both magnetic declination and topographic grids because they are created locally where we have access to the required extension.
How to calculate the current variance from the 2019 diagram used
Online resources, such as http://www.ga.gov.au/oracle/geomag/agrfform.jsp and http://www.magnetic-declination.com/ can be used to calculated current values and variation from 2019.
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