How to Vectorize Blueprints and Technical Drawings

A drawing scanned one degree off vertical traces one degree off vertical, and afterwards there is no single rotation that fixes it — the error is distributed across hundreds of independent coordinates and every horizontal line in the file is now slightly diagonal.

Rotate the raster until a known-horizontal feature — a title block edge, a datum line — is exactly horizontal. Most scanning software has an auto-deskew that gets within a fraction of a degree; check it against a long line rather than trusting it, because auto-deskew keys off the page edge, and the page edge is not always square to the drawing.

Blueprints and line drawings are binary by intent: there is line, and there is background. Any grey in the scan is an artefact — paper tone, foxing, the blue of an actual blueprint, the shadow of a fold — and every bit of it is something the tracer has to decide about.

Threshold the image to pure black and white before converting. Choose the threshold by looking at the faintest line you actually need: if a dimension line disappears, the threshold is too aggressive; if the paper starts speckling, it is too generous. Getting this right is worth more than any setting in the converter.

The engine traces the boundary of dark regions, so a drawn line comes back as a long thin closed shape following both of its sides, not as a single stroke down its middle. For viewing, printing, archiving and marking up, that is fine and often preferable — the line has real width, exactly as drawn.

For re-entering the geometry into CAD, it usually is not what you want: a CAD line is a centreline with a lineweight property, not a filled sliver. The route there is centreline tracing, which Inkscape does from a cleaned raster, followed by DXF export. It is more work, and it is the right work if the drawing is going back into a model.

Dimension figures, part numbers and notes trace as shapes that look like letters. They print correctly and they are dimensionally accurate, but they are not searchable, not editable as text, and not selectable.

If the drawing needs to be searchable, run OCR on the raster separately and keep the recognised text as its own layer over the vector line work. Running OCR on the drawing before converting also has a side benefit: it tells you which annotations are legible enough to survive, which is useful to know before anyone relies on them.

Section hatching — the diagonal lines filling a cut face — is dozens of parallel strokes, each of which traces as its own long thin shape. A drawing with several hatched sections can produce a file an order of magnitude larger than one without, and the hatch carries almost no information: it means 'this is a cut face', which one region and one fill pattern would say just as well.

If the file is going to be edited rather than archived, consider erasing the hatching from the raster before converting and replacing it with a proper fill pattern afterwards.