(Re-)Enabling ArcMap datum checking

ArcMap's default behavior is to warn you if a layer you're trying to add has a different datum than the map's current coordinate system:


However, if at some point you (mistakenly, of course...) checked the "Don't warn me again ever" box, then you will, in fact, never see this warning, nor be able to access the datum transformations from its "Transformations..." button:

GCS Transformations

To restore access to the warning and its associated datum transformations:

  1. Quit any running ArcMap(s).
  2. In Windows Explorer, browse to C:\Program Files (x86)\ArcGIS\Desktop10.6\Utilities
  3. Double-click AdvancedArcMapSettings.exe (It may take almost a minute to start up; be patient...)
  4. Select the TOC / Data tab:

    ArcMap Advanced Settings Utility

  5. In Working with data:
    1. un-check Skip datum check
    2. un-check Skip 'Unknown Spatial Reference' warning
    3. check Warn when editing datasets with unmatched coordinate systems
  6. Click Apply, then Close

Subsequent ArcMap sessions should now warn you if you try to add a layer whose datum differs from that of the map's coordinate system.

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