The 304.8 gap: a unitless survey xref
You have joined a firm whose project template is set to millimeters (INSUNITS = 4). A civil consultant delivers SITE-BASE.dwg, drawn correctly in US survey feet but saved with INSUNITS = 0 (unitless). A drafter attaches it as an xref into A-101; nothing looks obviously wrong on screen, so work continues. Later you inquire the front property line - a 150.00 ft line - and AutoCAD reports 150.0000 in the host file. The building footprint still lines up with the survey, because the host geometry was traced from the same xref. The sheet is due tomorrow and the consultant is unavailable.
1. Explain what happened at attach time and give the two viable corrections, including which one you would choose under a one-day deadline and how you would prove the fix worked.
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What happened. AutoCAD applies a unit-conversion factor only when both drawings declare a unit. Host INSUNITS = 4 (mm) and source INSUNITS = 0 (unitless) means no factor at all: the xref is attached at scale 1, so 150 ft of geometry is read as 150 mm. Nothing in the host is internally inconsistent - everything is uniformly 304.8× too small - which is why the footprint still aligns with the survey. That uniformity is exactly why no one noticed.
Two fixes.
- Open SITE-BASE.dwg, set INSUNITS = 2 (feet) in Drawing Settings, save, then reattach. Setting the source unit does not retroactively rescale an xref that is already attached; the factor is recorded at attach time. Reattach at 0,0 so coordinates are preserved.
- Override the xref's scale factor to 304.8 in the Properties palette (Xref Scale).
Choice. Under a one-day deadline, take option 2. You do not own the consultant's file, and any edit you make will be overwritten by the next issue. Log the override on the sheet and request a corrected file with INSUNITS set.
Verify, do not assume. Run DIST across the front property line; it must read 45,720 mm (150 × 304.8). Do not verify against a dimension you placed yourself, and do not trust the building alignment - it was never independent of the error.
2. The next morning a colleague opens A-101 and the xref panel shows Not Found, yet the drawing is otherwise intact. What do you check, and what would you change to stop this recurring on the 14 sheet files that also reference the survey?
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Diagnosis. Xrefs store a path, and the record is saved only in the host. If A-101 was saved pointing at a network share your colleague's machine maps under a different letter, or the folder was moved, the link breaks. The xref definition, insertion point and scale factor survive in the host even with the file missing - which is the trap: the 304.8 override is still there, but a detach-and-reattach to fix the path loses it silently. Check the Xref palette for Status / Found At / Saved Path, then correct the path in the Properties palette (Saved Path field, which accepts find-and-replace) or use Reference Manager across the whole sheet set at once.
Make it portable. Set the reference type to Relative path and keep xrefs in a subfolder beside the host, so the drawing survives being copied. For handoff, use eTransmit rather than zipping files by hand. For the office, add the project folder to Options > Files > Project Files Search Path and assign the project name to the sheets, so a moved drive is fixed in one place.
Re-apply after reattaching. If you did reattach, set the 304.8 factor again and re-run the DIST check - the second failure mode after a path fix is the scale override quietly reverting to 1.