By default, Geographic Calculator allows conversions with no datum transformation. When the user selects from the list of possible datum transformations, <none> will be one of the options. If <none> is selected, there will be no transformation applied, and the result may not be accurate. To prevent this, you can select "Do not allow null datum transformations" in the Administrative Settings.
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Select Options>Administrative Settings and go to the Geodetic tab.
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Clicking the checkbox for Do not allow null coordinate transformations will prevent users from selecting <none> as the transformation between two systems. Sources and Targets with no valid coordinate transformations will not be processed.