Free PDF measurement check
Check your PDF scale before you trust the takeoff.
Compare two known dimensions with your PDF readings. See the percentage error against your project’s tolerance and download a record of the checks.
How do you check the scale of a scanned plan?
Calibrate against a labeled dimension, then measure independent horizontal and vertical spans. For each, divide the measured length minus the known length by the known length, then multiply by 100. Compare the absolute error with your project’s agreed tolerance. If directions or locations disagree, recheck units and endpoints and investigate the source. One calibration cannot fix a scan stretched differently in two directions.
Enter real-world readings from your PDF measuring software. A ruler held against the screen measures the display, not the PDF’s calibrated geometry. This checker compares your numbers; it does not open, measure or repair a PDF.
Reference check
Your scale check
Enter known dimensions and the readings from your calibrated PDF, then choose the allowed error. Use independent spans in both directions, away from the original calibration line.
Worked example: one direction needs a recheck
A labeled 20 ft span reads 20.2 ft after calibration. A separate 12 ft span reads 12 ft. For this example only, the allowed error is 0.5%.
Horizontal: (20.2 − 20) ÷ 20 × 100 = 1% error.
Vertical: (12 − 12) ÷ 12 × 100 = 0% error.
The horizontal check exceeds 0.5%; the vertical check meets it. That is a reason to investigate, not average the two errors into one scale. It does not prove the scan is distorted: confirm the endpoints, known values and units first.
What to do with the result
- If both checks agree, repeat at other locations and on each differently scaled detail.
- If both are wrong by a similar percentage, recheck the calibration distance and units, then recalibrate and measure again.
- If directions disagree, check the endpoint choices and source geometry. Obtain a better issued PDF when one proportional scale cannot satisfy the checks.
- If only a detail is wrong, use that detail’s own scale and verify it separately.
A uniform 1% length-scale error produces a 2.01% area error: 1.01 × 1.01 − 1 = 0.0201. This assumes the same error in both directions. It is not a correction to apply blindly to exported quantities.
Keep the check with the drawing
The CSV records both references, units, signed errors and the tolerance you entered. Add the sheet, revision, app version, date and reviewer to your project record. Keep source dimensions and assumptions alongside any takeoff worksheet.
In PolyPDF Free, calibration and up to 3 hand-created measurements per document let you try a small check. See the Free and Pro limits.
Try the drawing workflow in PolyPDF.
Free with no trial timer: markup, review, calibration, 3 hand-created measurements per document, and Revision Package viewing. Symbol Search, plugins, and Revision Package changes or publishing require Pro.