Thursday, July 21, 2011

When Is a Foot Not a Foot?

I ran into an interesting issue today when dealing with some survey drawings that were done in both metric and imperial units.  Not something you run into every day but for me, today was that day.  As Civil 3D users know, Civil 3D understands that there are U.S. and International Feet.

AutoCAD, however, does not.  So even though you may have leveraged Civil 3D's ability to distinguish between different types of feet through LandXML, Survey Database, etc., if you do an XREF or insert a block and you're depending on the AutoCAD INSUNITS system variable to kick in, you might find some discrepancies.

By the way, International feet are .3048 meters while US Survey Feet are 39.37 inches per meter or 12/39.37 or 0.30480060960121920243840487680975 meters, give or take.  When dealing with large coordinate values (like state plane coordinates), this starts to make a difference.

So, what do you do about XREFs or inserting blocks.  Well, the best approach I have found is to turn off the effect of the INSUNITS system variable by setting it to zero.  Then, when you insert or XREF a file, AutoCAD won't do you the favor of trying to scale it for you.  Then once it's in there you can use the SCALE command with a factor of 1200/3937 to go from feet to meters, or the inverse of that to go from meters to feet.

Beware of another favor that AutoCAD tries to do for you.  There's a setting in options that will just go ahead and assign a scale factor for your incoming block or XREF even if you've said you wanted it to be unitless.  It's on the User Preferences tab under Insertion Scale.  AutoCAD is like that over-helpful store worker who keeps saying "Can I scale that for you?", "Can I scale that for you?".  No AutoCAD, I got it.

Do you use a different approach to handle this issue?  If so, please share.

1 comment:

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