vertical ruler; vanished plateaus?

Years ago, Jack created a handy 9 ft vertical ruler for columns.  Again, SO helpful.  I had done a mod to make a 10 ft one back then.

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Placed them around at measured points to verify my plateau-defined elevations.  Somehow or another, 2/3 of my plateaus had disappeared?.?.?  Can’t imagine how since I’d made Terrain elements unselectable right after creating them.  Only thing I can think of is using Settings to turn contour visibility off.  At least I’m assuming it only controls visibility.  But if that was cause, I would have expected all to be gone.  Don’t know.  Went to the backups to verify that plateaus were there at one time.  Redrew the missing ones and all seems well EXCEPT when I’d try to select a ruler column, kept selecting plateau definition lines even though the Terrain selection filters were OFF for both 2D view & 3D view windows.  Strange.  After save & exit, the reload seems ok.  Weird.  Sure hope this doesn’t happen again…