Mirror inaccurate

[Jurrian_Giezen]

See video, can anyone explain to me why the mirrored pipe is a different distance away from the line as the pipe I'm mirroring? It doesn't make any sense to me.

[RSomppi]

Impossible to say without the model.

[HVAC-Novice]

Is it possible there is a (to us) hidden line or plane? Like the dimensions attach to the centerline shown, but when you pick the mirror axis, you inadvertently pick a an invisible line that is 10mm off-set? 

Probably have to share the model for someone to re-create the situation 

[pkolarik]

I don't have a solution, but I ran into something eerily similar a couple weeks ago.

I had a completely level (in elevation) straight run of pipe. I drew a new reference plane in plan view and used that to mirror said pipe. The resulting piece of pipe was NOT level (in elevation). It was off by 32nds of an inch.

After double-checking my source pipe and everything else, I retested it a couple more times by creating a brand-new run of level piping a few feet long in plan view. Drew another brand-new reference plan in plan view and mirrored the new pipe. Same thing;  the resulting pipe was off by tiny fractions of an inch from level. Every time I tested it, this exact same result happened.

[HVAC-Novice]

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@pkolarik wrote:

I don't have a solution, but I ran into something eerily similar a couple weeks ago.

I had a completely level (in elevation) straight run of pipe. I drew a new reference plane in plan view and used that to mirror said pipe. The resulting piece of pipe was NOT level (in elevation). It was off by 32nds of an inch.

After double-checking my source pipe and everything else, I retested it a couple more times by creating a brand-new run of level piping a few feet long in plan view. Drew another brand-new reference plan in plan view and mirrored the new pipe. Same thing;  the resulting pipe was off by tiny fractions of an inch from level. Every time I tested it, this exact same result happened.

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Did you do that with just a pipe, or a pipe with fittings etc.? OP had mirrored a pipe with attached fittings/pipe. Maybe mirroring more than "just a pipe" is too complex for Revit. 

Maybe this happens more often than we know. I don't recall ever putting dimensions on mirrored items. and since this is just a tiny inaccuracy, one wouldn't just "see" it is off. 

[Jurrian_Giezen]

In our work we do dimension mirrored items, since it's necessary for the engineers on the job to place the pipe. 
It's rare for us to only mirror one pipe, usually it's much more getting copied/mirrored. I've noticed before that we often get inaccuracies due to copy/mirror in Revit. Nothing as prominent as this one though. Somehow for Revit a copy isn't an exact copy, rather a imperfect redraw. 

[Jurrian_Giezen]

Sadly I'm not allowed to share the project and I couldn't reproduce the issue in a fresh project.