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After most of the work had been done on a project, we learned that the permitting officials require that it must be dimensioned in metric units. This was a few months ago, and at the time they said that it didn't matter if the metric units were primary or the alternative dimensions. So, I edited all my dimensions to add the alternative dimensions in metric, and spent several hours going through and adjusting dimension scales and positions to make sure the longer dimension text didn't overlap things. It went surprisingly fast for my 33 page set of drawings, since most of them are in one drawing file. Though other vendors who did some of their own drawings had to do the rework themselves.
True to form for any bureaucrat, we are now informed that the metric dimension must be primary.
I am thinking that, for my drawings, that I will just edit the dimension style. The actual units for the drawing is inches. On the dimension primary units, i will give them a scale factor [ x .0254 ] to convert to meters, and then the alt units will x 39.37. (Note that the alt linear scale factor is times whatever value is in the primary unit space). I can see how the double-conversion process may introduce some rounding errors, but since this is a large building, I don't think the errors would be large enough to matter.
Of course, I will maintain my original set of drawings, and we will consider them to be our main working document. Any changes done will have to be done on both the working set, and on the metric set separately. The public works department in the country where the project will be done will only get PDFs, and they are the only ones who require that the metric unit be primary.
I considered just using the "Metric as Primary" drawing set as the only set. But, with the dimensions having such a weird set of conversion factors, I though it would cause a lot of confusion to have to work with that drawing. Working with it right away would work fine, but a year from now I would have forgotten all the special treatment that this drawing received.
Perhaps others have had to go through this, and can suggest another approach, if they think it would work better, or any pitfalls I should look out for.