To compare a scanned drawing with a new CAD PDF, both sheets have to be rendered at the same resolution, aligned for the shift, rotation, scale and stretch that scanning adds, and compared with a sensitivity that ignores scanner speckle but keeps real edits. Drawing Revision Delta does this on Windows: it accepts scans as PDF, TIFF, PNG, JPEG or BMP, reads the sheet number from each title block with the text recognition built into Windows, aligns each pair with local stretch correction, and clouds the differences. Edited words are read from PDF text, so on the scanned side changes show as regions rather than as text.
This comes up on renovation work, record drawings, and any project where the last issued set lives in a flat file drawer or a scanned archive.
Compare a scanned old revision against the new CAD set, sheet by sheet.
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Why is comparing a scan to a CAD PDF harder?
A CAD PDF is clean geometry. A scan is a photograph of paper. Between them sit several differences that have nothing to do with design changes:
| Difference | Cause | Effect on a naive overlay |
|---|---|---|
| Shift | Paper fed off center | Everything doubles |
| Rotation | Paper fed at a slight angle | Lines diverge across the sheet |
| Scale | Scanned or plotted at a slightly different size | Edges drift apart toward the borders |
| Local stretch | Paper stretch, roller slip in large-format scanners | Parts of the sheet line up and others do not |
| Speckle and fold marks | Dust, creases, stamps, handwritten notes | Hundreds of tiny false changes |
| No text layer | The scan is an image | Text edits cannot be read as words |
A simple overlay in a PDF viewer will light up nearly every line. The comparison has to remove the geometric differences first.
How do I prepare scans for comparison?
Good inputs make a big difference:
- Scan at a consistent resolution. Use the same settings for every sheet. Black and white or grayscale is usually enough for line drawings.
- Scan straight and full. Feed carefully and include the whole sheet edge to edge, title block included.
- One sheet per page or file. Multipage PDFs and folders of single images both work, but every sheet should be its own page.
- Keep the title block legible. The sheet number is how sheets pair. A smudged number means pairing by hand.
- Do not crop or deskew heavily in other software unless you need to. Alignment in the comparison handles shift, rotation and scale.
How does Drawing Revision Delta compare a scan with a CAD PDF?
Drawing Revision Delta compares two revisions of a drawing set:
- Add the earlier set. Choose the scans: a scanned PDF, or a folder of TIFF, PNG, JPEG or BMP images.
- Add the later set. Choose the CAD PDF.
- Check the pairs. Sheets are matched by the sheet number in each title block, read from the PDF text or, for scans and CAD plots drawn as lines, with the text recognition built into Windows. Sheets with no readable number pair by title or file name, and Pair selected sets any pair by hand.
- Mask the title block. Choose Mask title block, or Draw mask over the title block and stamps area. One mask applies to every pair, and content inside it is shown in light gray and not counted.
- Set resolution and sensitivity. Choose 150 or 200 DPI. Start at Standard sensitivity; switch to Ignore small differences if speckle and line weight differences crowd the list.
- Compare all. Each pair is aligned for shift, rotation and scale, with local correction for plotter and scanner stretch. The Pairs table ranks sheets by share of change.
- Review. Open a pair in Overlay, Blink or Side by side. Red was only in the scan, blue is new in the CAD PDF. Fade unchanged lightens everything else.
- Export. The change report, the clouded set of the CAD sheets with an editable cloud around every change, two CSV files, and PNG snapshots.
Try it on one scanned sheet and its CAD revision during the 15-day free trial.
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What should I expect from the results?
Be realistic about scans:
- Changes appear as regions. Edited notes and dimensions are read from PDF text, so they are listed in words only where both sides have text. Against a scan, a changed note shows as a red and blue clouded region you read yourself.
- Hand marks show up. Stamps, signatures, red-pen notes and fold lines on the scan count as content. Mask the areas you know are stamps, or expect a few extra regions.
- Very poor scans stay poor. If the scan is faint, skewed by many degrees, or missing part of the sheet, no alignment will make it a clean source. Rescan if you can.
- Drawn-as-lines text. Some CAD plots draw text as vector lines rather than font text. Title block numbers are still read by text recognition, but edited words on those sheets show as regions.
Which settings help most with scans?
| Symptom | Try |
|---|---|
| Hundreds of tiny clouds | Ignore small differences sensitivity |
| Every sheet flagged | Mask the title block and any stamp area |
| Sheets not paired | Pair selected by hand, or check the title block is legible |
| Fine linework missing from the list | Fine sensitivity and 200 DPI |
| Very large sets slow | 150 DPI; a PC with 8 GB of RAM handles full-size 36 by 24 inch sets comfortably |
When is a different approach better?
- You have the old CAD files. Comparing CAD to CAD, as PDFs exported the same way, gives cleaner results and edited words on both sides. Use the scan only when nothing better exists.
- You need a CAD-level diff. Some CAD platforms compare drawing files natively, at object level. Drawing Revision Delta compares rendered sheets.
- A one-off check of a single detail. A careful visual check side by side may be quicker than setting up a comparison.
Frequently asked questions
Can Drawing Revision Delta compare TIFF scans?
Yes. It accepts PDF, TIFF, PNG, JPEG and BMP on either side of the comparison.
Will it read text from the scan?
It reads the title block sheet number with the Windows text recognition engine for pairing. Edited notes and dimensions are compared from PDF text, so changes on a scan are shown as clouded regions.
What if the scan is slightly crooked?
Alignment corrects shift, rotation and scale for each pair, with local correction for plotter and scanner stretch.
Does it change my scans?
No. Compared drawings are opened for reading only. The clouded set is a new PDF.
Which Windows does it need?
Windows 11 (build 22000 or later) on x64 or ARM64. A PC with 8 GB of RAM handles full-size 36 by 24 inch sets comfortably.
Try Drawing Revision Delta
Drawing Revision Delta is a one-time purchase in the Microsoft Store, with a 15-day free trial. Scan one old sheet, compare it with its new CAD revision, and review the clouds. For more, see how to compare two drawing revisions, revision clouds and delta tags explained, or the Drawing Revision Delta product page.
Find what changed since the set that only exists on paper.
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Microsoft and Windows are trademarks of the Microsoft group of companies. Other names are trademarks of their owners. Drawing Revision Delta is not affiliated with or endorsed by any of them.
Last updated 17 October 2026. Published by Automata Labs, a software company in Canada.