Revision C lands in your inbox. The transmittal says "minor coordination updates." You know from experience that "minor" sometimes hides a moved gridline, a resized opening, or a note that changes the whole detail. Comparing the two PDFs by flipping between tabs will catch the obvious moves and miss the quiet ones — and the quiet ones are the RFIs.
Here's how to compare drawing revisions so every change is visible, not just the ones you happen to notice.
Why eyeballing revisions fails
Human visual comparison is good at spotting large, high-contrast differences and bad at everything else. Between two dense drawing sheets, the eye reliably misses:
- Small dimensional changes (a 3600 that became 3650)
- Elements that moved a few millimetres
- Added or deleted notes buried in a schedule
- Changes on a sheet you assumed was unchanged because its revision letter didn't move
Revision clouds are supposed to flag changes, but they're applied by hand — so they're only as complete as the person who drew them was careful. A cloud you trust is a cloud someone remembered to add.
How do I compare two revisions of a drawing?
A visual diff overlays the two revisions and highlights exactly what differs:
- Line the two revisions up. Compare the same sheet from revision A and revision B. The tool aligns the pages so identical content sits exactly on top of itself.
- Read the highlights. Additions, deletions, and moved elements are colour-coded, so a moved gridline or a changed dimension stands out even if it's tiny.
- Choose the view that fits. Side-by-side is best for reading both sheets at once; overlay is best for catching small positional shifts; a swipe view is good for walking a reviewer through the changes live.
- Export the marked-up result. Save the highlighted comparison as a PDF so the change record travels with the transmittal — evidence, not memory.
The free Arsoom revision diff does exactly this in the browser: upload two versions, get every change highlighted, and download an annotated PDF. It's the tool that catches the clash before it becomes a claim.
What kinds of change does a visual diff catch?
- Geometry: moved walls, resized openings, shifted gridlines, relocated equipment.
- Dimensions and text: edited values, added or removed notes, revised schedules.
- Deletions: elements that were removed — the hardest change to notice by eye, because you're looking for something that isn't there anymore.
Frequently asked questions
Do the two revisions need to be the same scale and size? Same sheet at the same size compares cleanest. If the page size changed between revisions, align on a common reference (like the title block) so the comparison isn't thrown off by a shift that isn't a real change.
Can I compare scanned drawings? Yes, but expect more noise — a scan introduces tiny pixel differences that aren't real changes. Vector-to-vector comparison (PDF exported from CAD both times) is the cleanest and most trustworthy.
Is this a substitute for revision clouds? It's a safety net for them. Clouds communicate intent; a diff verifies completeness. Use the diff to confirm every actual change was clouded — and to find the ones that weren't.
Can I keep a record of what changed? Yes — export the annotated PDF and attach it to the transmittal or file it with the revision. That turns "we think these were the changes" into a documented record.
Related: organizing the issued set and compressing it for email.
