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28.1.15

Dispatch from Support: "Vanilla" AutoCAD vs Civil 3D Scaling

Users Question (slightly modified to protect the innocent):

I do not fully understand is the relationship between AutoCAD and Civil 3D as it relates to scales.The Issue: When a drawing file is created in AutoCAD with mm scale and is opened by a user running Civil 3D the scales get messed up; blocks, text, linestyles all related elements seem to be affected. 

My sage answer: 

Your question is not uncommon. Civil 3D can only create objects in feet or meters. The below screengrab is from the Civil 3D drawing settings. 

However, Autocad can work in any units. The following dialog box can be found by typing UNITS in the command line. These are the base AutoCAD units.


When you open a drawing in Civil 3D where the base AutoCAD units are not meters, it still creates roads, topography, and all other Civil 3D objects as if you are in meters. As a result, you will have scaling issues.  A road that should be 6 meters wide will only draw in as 6mm. 

The reason blocks, linetypes and text get goofed up is because of  Annotative scaling. If you open a base AutoCAD file in Civil 3D, it will not have correct scales set up to display properly relative to the large size of a civil jobsite.  A typical drawing scale for a metric civil project might be 1m=2000m plotted. When working in mm, typically you are plotting closer to 1mm=1mm. 

The best way to work with Autocad and Civil 3D is to XREF the Autocad drawing into Civil 3D (or vice versa).  The XREF command recognises that the files are created in different scales and will adjust the file accordingly. Similar scaling will occur if you insert the AutoCAD file into the Civil 3D file as a block. I recommend XREF over block insertion because the files remain separate and can be edited by multiple users.  



Additional problems will occur in Civil 3D if the file originated in base AutoCAD and is opened directly.  A base AutoCAD drawing does not have the Civil 3D styles that control the layers, colors and display of Civil 3D objects. 

If you absolutely MUST edit a file that originated in base AutoCAD in Civil 3D, you can do so but you must be careful to change some settings and change them back.  

First, make sure the Civil 3D styles are present.  If you start with a template (DWT) intended for Civil 3D, that will retain Civil styles. If it is too late, and you've already made progress on the base-Autocad file, you can use the IMPORTSTYLESANDSETTINGS command or, better yet, start a new file from a Civil 3D template and insert the AutoCAD file as a block. Explode the block upon insertion.  I like the latter option for getting at the Civil 3D styles better because expressions don't come across in the IMPORTSTYLESANDSETTINGS command. 

Keep in mind that base AutoCAD drawing tools are unitless. Changing the unit type in the UNITS dialog does not scale the objects.  If I draw a polyline and tell the software that it is to be 6 units, the size of the object in that drawing are relative to other objects. AutoCAD doesn't care if I mean 6 mm, 6 feet, 6 meters or 6 light years! The only time AutoCAD pays attention to the units specified in the UNITS dialog box are when you are using the XREF or INSERT command.

I hope this helps.