Doing a take-off from a scanned drawing or an image

This guide follows French practice: French standards (NF DTU), French regulations and French bill conventions. Most sources are in French.

Paper drawings, archives, images: what are we talking about?

Not every drawing arrives as an architect's PDF. For a refurbishment, an extension or a quick estimate, you often work from:

• a paper print handed over by the client, sometimes folded, annotated, yellowed;
• an archive drawing: an old building permit file, the original drawings of a building, prints kept for decades;
• a scanned PDF, which is only an image stored in a PDF: lines made of pixels, no text to search;
• an image (JPG, PNG, TIFF): a scan, a screenshot, a photo taken on site.

In all these cases, the file is only a grid of pixels. It knows neither the scale of the drawing nor the size of the original sheet, except through a resolution value that may be missing or wrong. Take-off is still possible; it needs more checks.

Digitising a drawing

Resolution

The resolution of a scan is expressed in dots per inch (dpi; “ppp” in French), an inch being exactly 25.4 mm. At 300 dpi, one pixel covers 25.4 ÷ 300 = 0.085 mm of sheet. Depending on the scale of the drawing, that represents in reality:

The pixel is not the limit of your accuracy: your click is, to within a few pixels. What matters most is that lines and dimensions remain legible. For text recognition, one scanner manufacturer recommends 400 dpi, and 200 to 400 dpi for a searchable PDF. Doubling the resolution quadruples the number of pixels, and roughly the file size.

File format

TIFF and PNG keep every pixel as it is; JPG compresses by discarding some information, which creates small smudges around fine lines if the quality is set low. A scanned PDF contains one of these images and is measured in the same way. A greyscale scan keeps fine lines and faint dimensions better than a pure black-and-white scan, which can wipe out anything light grey.

Flatbed, wide-format scanner or photo

• A4 or A3 flatbed scanner. It only takes a sheet of its own size: an A1 drawing (594 × 841 mm) would need several passes on an A3 (297 × 420 mm), and each join adds an error. Keep it for small sizes.
• Wide-format scanner. Reprographics shops have them; the sheet feeds past the sensor, up to A0 and beyond. One manufacturer, for example, states an accuracy of 0.1% ± 1 pixel: 1 mm over 1 m of sheet, or 1 cm over 10 m at 1:100.
• Photo. A phone is never perfectly parallel to the sheet: perspective shrinks the far side, and the scale varies from one corner to another. A photo of a drawing is for counting items or reading a dimension, not for measuring areas.

In every case, scan at 100%, with no “fit” option and no reduction.

Setting the scale: resolution is not always enough

In theory, a 100% scan of a drawing at its original size sets itself: the scale of the drawing and the resolution of the file give the ratio between pixels and metres. In practice, this calculation often fails:

• the scanned sheet was already a reduction (an A1 printed on A3 to fit in a file);
• the image was resized to send by email, but the file still states its original resolution;
• the file states no resolution, or a value unrelated to the sheet (screenshot, photo);
• the copier or scanner applied a percentage to fit the page.

The rule is therefore to calibrate on a known dimension, the longest available, clicking its extension lines as closely as possible, then to check on another dimension. With no dimension, the scale bar of the drawing will do: it has undergone the same reductions as the drawing. With neither, measure a clean length on site (a façade, a straight wall) and calibrate on it. The guide to the scale of a drawing covers these calculations in detail.

Distortion: checking both ways

A paper drawing is not dimensionally stable. Paper and tracing paper move with humidity, a folded print keeps its creases, a sheet-feed scanner can pull the sheet slightly askew. The scale is then no longer quite the same across and down the sheet, or from one end of it to the other.

Hence the cross-check: after calibrating, measure a dimension in the other direction, and if possible a third one at the far end of the sheet. Same error everywhere: the scale is wrong, recalibrate. Error in one direction only: the document is distorted. A calibration corrects only one ratio; the remaining error carries over to every measurement taken in the other direction and to every area. If it matters for the item, ask for a better document (the original file, a new scan) or survey on site; otherwise, note it in your assumptions.

Making the drawing legible for measuring

• Fade the background: a paler drawing lets your coloured lines stand out, without changing the original file.
• Switch to black and white a yellowed or bluish print: your lines become the only colours on screen.
• Turn the drawing upright by quarter turns if it was scanned sideways.
• Always aim at the same edge of the line, or its middle: on a scan, a line is several pixels thick.

Worked example

A floor plan at 1:100, printed on an A1 sheet (594 × 841 mm), scanned on a wide-format scanner at 300 dpi. An overall dimension of 12.50 m is written on the drawing.

From resolution to scale

The file says 300 dpi, the title block 1:100: with these two values, the 12.50 m dimension measures exactly 12.50 m.

The same scan, reduced before sending

To send it by email, the image was reduced to 6,000 px wide; the file still states 300 dpi.

Two remedies: set the true resolution, 181 dpi, or, more safely, calibrate directly on the 12.50 m dimension.

Distortion in one direction only

Another drawing, an old print calibrated on a horizontal 12.50 m dimension. The vertical 8.40 m dimension, checked next, measures 8.23 m: 2.0% short. A 5.00 × 4.00 m room, its long side horizontal, measures 5.00 × 3.92 = 19.60 m² (4.00 × 8.23 ÷ 8.40 = 3.92) instead of 20.00 m². Recalibrating on the vertical dimension would only move the error to the other direction: it is the document that needs improving, or the difference that needs recording.

The most common mistakes

• Trusting the resolution stated by the file without measuring a single dimension.
• Scanning a reduced copy in the belief that it is the original.
• Scanning a large drawing in several pieces on an A3 scanner, then joining them by eye.
• Measuring areas on a hand-held photo.
• Checking only one direction on an old print.
• Scanning in pure black and white, and losing fine lines or faint dimensions.
• Aiming sometimes at one edge of the line, sometimes at the other: a 0.5 mm line at 1:200 represents 10 cm.

Doing it in Kotelys

Kotelys imports drawing images (PNG, JPG, TIFF, BMP, WebP) and scanned PDFs like any other drawing, from the Home tab or by dragging and dropping from File Explorer.

• An image that does not state its resolution is flagged: calibration on a dimension then comes first, and the scale list waits for the scan resolution (200, 300, 400 dpi…). If the file does state one, Kotelys uses it: all the more reason to check.
• Check the scale on a dimension of the drawing gives the difference in % and recalibrates if needed: do it in both directions.
• Background opacity fades the drawing, importing in black and white neutralises a yellowed print, and rotation by quarter turns sets it upright.
• After that, lengths, areas, deductions and counts work as on a PDF, with the same reports: take-off calculations, summary, Excel.

Kotelys does not straighten a photo taken at an angle and does not correct distortion: a single scale applies to the whole drawing. On a scanned drawing, word search (Ctrl+F) finds nothing, as there is no text in the image.

Further reading: the measuring tools in Kotelys, and the other guides.

Sources

1. NIST, “Guide to the SI, Appendix B.8” (the inch is exactly 2.54 × 10⁻² m)
2. Epson, Epson Scan guide: « Sélection de la résolution de numérisation » (recommended resolutions by use)
3. Contex, HD Ultra X wide-format scanner datasheet (accuracy 0.1% ± 1 pixel, 1,200 dpi optical)
4. Markus Kuhn, University of Cambridge, “International standard paper sizes” (ISO 216 A size dimensions)

Thresholds and values described as “taken for the example” are not rules: those of your contract (specification, bill of quantities) always prevail.