Units: Why going unitless may not be a be a good answer for your projects

When Autodesk added US Survey Feet as an actual option in Civil 3D back in 2017, I started hearing the same sentence over and over again at Autodesk University and from others in the engineering field:

“We just set everything to unitless and things work fine.”

Ah yes. Fine. One of the most dangerous words in CAD.

That immediately raised a red flag for me. Because in my experience, when someone says “it works fine,” what they really mean is “it hasn’t exploded yet.”

So I did what any mildly unhinged CAD skeptic would do: I tested it. I wanted to see whether our company’s approach—explicitly setting units—was actually worth the effort.

Spoiler alert: it was. And “unitless” is still a terrible idea.

What Does “Unitless” Actually Mean?

In AutoCAD and Civil 3D, setting a drawing to Unitless (INSUNITS = 0) doesn’t mean “flexible” or “universal.”

It means:

“AutoCAD has absolutely no clue what one unit represents.”

One unit could be a foot. Or a meter. Or a banana. Civil 3D doesn’t know and won’t ask.

That’s a problem, because Civil 3D really cares about units for things like:

• Coordinate system transformations

• Survey data imports

• Scaling blocks and XREFs

• Annotation and label styles

• Imports/exports (LandXML, GIS, IFC, etc.)

When you choose Unitless, you’re basically telling Civil 3D: “Good luck. Figure it out.”

Real-World Problems Caused by Unitless

(aka: How This Blows Up Later)

1. Misaligned Coordinate Systems

Civil 3D uses Map 3D coordinate systems—State Plane, UTM, the whole gang. These systems expect real units: feet (US or international) or meters.

If your drawing is unitless, transformations can:

• Fail completely

• Or worse… look right but be wrong

Example: Your surface looks perfect on its own. Then you overlay a GIS basemap and suddenly it’s shifted slightly or hundreds of feet. Surprise!

2. Survey Data Imports Gone Wrong

Survey databases, field books, and GNSS data all rely on declared units to apply scale factors correctly.

Unitless drawings make Civil 3D guess. Civil 3D is bad at guessing.

Example: Survey points import at the wrong elevation or spacing because the software quietly assumed meters when you meant feet.

3. XREF and Block Scaling Nightmares

Auto-scaling during XREF insertion is driven by INSUNITS. If your base drawing is unitless, AutoCAD has no reference point.

So:

• Meter-based details come in huge

• Foot-based details come in tiny

• Nothing is consistent

• Everything is annoying

Example: A curb ramp detail comes in 3× too large, and you spend the afternoon manually scaling blocks like it’s 2005.

4. Annotation and Sheet Scale Shenanigans

Civil 3D label styles depend on drawing units for:

• Distances

• Areas

• Slopes

• Stationing

Unitless drawings can produce labels that are technically correct… in an alternate reality.

Example: Plan/profile sheets plot with incorrect stationing or distorted scales because the viewport assumes a real-world unit that doesn’t exist.

5. Import/Export Failures (The Silent File Killer)

LandXML, SHP, and IFC files carry unit data. Civil 3D has to map those units to your drawing.

If your drawing is unitless, you get:

• Manual overrides

• Silent conversions

• Subtle shifts that no one notices until construction

Example: An alignment exported to LandXML comes back shifted because the receiving software assumed International Feet instead of US Survey Feet.

No warning. No error. Just chaos.

Why US Survey Feet vs International Feet Actually Matters

Even if you do pick “feet,” here’s where people still mess it up. In Civil 3D general Feet = International Feet.

• International foot = 0.3048 m

• US survey foot = 1200/3937 m (≈ 0.3048006096 m)

The difference is tiny — about 2 parts per million — which is exactly why people ignore it.

But over distance, it adds up:

• 1 mile → ~0.01 ft difference

• 10 miles → ~0.1 ft difference

• Where is the origin point for your coordinate system? The further away your project is, the greater the shift between the two different "feet".

That’s more than enough to cause:

• Layout discrepancies

• Survey vs design arguments no one enjoys

• Closure errors

Civil 3D gives you the option for a reason. Use the one your project requires.

Best Practices (aka: How to Avoid Future Regret)

• Always set drawing units explicitly (US survey feet, international feet, or meters — pick one)

• Assign the correct coordinate system

• Standardize templates so INSUNITS is never 0

• Verify units during data exchange (LandXML, SHP, DWG)

• Document grid-to-ground scale factors and apply them consistently

This isn’t overkill. It’s basic damage control.

Bottom Line

Setting your drawing to “Unitless” might feel like a shortcut, but it’s really just kicking problems down the road and hoping everything goes fine...or that Civil 3D decides to make the correct assumptions.

Explicit units—and the correct foot type—keep:

• Your data accurate

• Your workflows predictable

• Your deliverables defensible

And most importantly, they keep Civil 3D from quietly sabotaging you when you’re not looking.

Thanks for stopping by the Den! Civil 3D: It’s not a bug, it’s a feature. Allegedly.