AutoCAD Civil 3D INSUNITS Scaling problems are becoming evident as pre-2017 drawings are referenced with newer versions. AutoCAD automatically scales incoming blocks and external references (xrefs) to match the receiving drawing, according to their INSUNITS settings. If one is set to Feet and another Inches, the system will scale accordingly. This is generally understood, but can be an occupational hazard. We always verify key distances after an insert or xref is added. Well, almost always.
What Went Wrong with AutoCAD Civil 3D INSUNITS Scaling
Recently I was involved with a 2016 drawing that went out with INSUNITS = 2 (feet), and after being worked with by others, returned as INSUNITS = 21 (U.S. Survey Feet). When the latter was reintroduced to the former, they did not match. The site is thousands of feet long, and as many of the old references were not shown, I did not identify the problem immediately. Due diligence by all parties caught the difference and we corrected it rapidly.
What to do to be Safe
Please check and verify that either of the following is true BEFORE using a reference drawing (including blocks and xrefs):
- Both drawings use the same INSUNITS setting
- Either your working drawing or the reference drawing’s INSUNITS setting is set to 0
Figure 1: Note the “Locate using Geographic Data” option in the above xref insert dialog. This works well, regardless of INSUNITS settings, when both AutoCAD Civil 3D or AutoCAD Map drawings are established on a good Geographic model reference.
AutoCAD 2017 and above recognizes U.S. Survey feet as a block scaling factor, and any difference between a receiving drawing and an inserted drawing will cause AutoCAD to scale the incoming drawing WITHOUT warning.
More Information and Background (if desired)
I am not sure precisely for how long, AutoCAD automatically scales incoming blocks from, for example, Architectural units to Feet. It does this by comparing the INSUNITS setting between the working drawing and the incoming block drawing. Historically, AutoCAD could not internally recognize the difference between one “Feet” unit and another. In Civil work, we all just use Survey Feet, and as far as we were concerned WITHIN OUR INDUSTRY, a foot is a foot.
Figure 2: AutoCAD Civil 3D 2017 INSUNITS Warning
When the settings for Civil 3D State Plane were married to the drawing, UNITLESS was often set into the AUTOCAD settings (INSUNITS = 0), because AutoCAD had no block setting for U.S. Survey Feet., and the INSUNITS list only went to 20.
Figure 3: Partial list of INSUNITS from the Autodesk AutoCAD Civil 3D 2015 help page
Figure 4: Partial list of INSUNITS from the Autodesk AutoCAD Civil 3D 2017 help page
As of AutoCAD version 2017, the INSUNITS setting of 21 (U.S. Survey Feet) was added to the available list. In addition, when the Civil 3D State plane settings are applied, AutoCAD now changes the INSUNITS setting to 21.
Figure 5: AutoCAD Civil 3D 2018 drawing settings. NOTE: there is nothing on here that indicates the INSUNITS that are present. The DWGUNITS setting is displayed. Note the “Set AutoCAD variables to match” checkbox.
This means that our older drawings and newer drawings will likely have differing INSUNITS settings. AutoCAD WILL scale the drawings according to the internally understood difference between unit types, regardless of where they were sourced from. For example, in State Plane, Florida North Zone, the difference in INSUNITS of 2 (FEET) and 21 (U.S Survey Feet) translates to about 3 feet on the ground.
Conclusions and Using Unitless Settings
While AutoCAD Civil 3D INSUNITS scaling features can be good, it can cost you a lot of money if it is not well managed. The people I have spoken with are in general agreement that it would be great if Autodesk would add a warning whenever a reference file is modified in the background. Add a toggle if desired, to turn the warning on and off. I guarantee most folks would keep it on.
Setting your drawings to INSUNITS = 0 will cause AutoCAD to be unable to make a scale factor change, because it has no reference of comparison. No change will be made. I just went through this company’s templates, saved the changes, and scrapped all the old template versions.
Figure 6: Entering the INSUNITS command change
Users can always change their units back in a pinch to use the scaling feature, when desired.
Due diligence, as always, is paramount. While tedious and somewhat expensive, making damn sure things match as they should is a very cheap form of insurance against a multi-million dollar disaster.