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Tracing a photo to scale on iPad

A photo of a part, a room or an old drawing can become a sketch you measure from, as long as you know one real length in it. Here’s how to take the photo, set the scale, and check it before you trust it.

1. Shoot it square

✗ tilted ✓ square
Tilted, it shrinks away. Square, it’s true.

Shoot square to the face you’ll trace: no scale fixes a tilt. Step back and zoom in to keep edges straight.

2. Put a known length in the picture

30 mm 300 mm
Same slip, ten times less on 300 mm.

Lay a ruler in the plane you’ll trace, or use a size you know. The longer, the better.

3. Set the scale from that length

Tap both ends
THAT LENGTH IS 150 mm Make it level Set scale
Type the length
150 mm
It levels and scales
50 mm
Dimension reads true

Tap both ends of your longest reference and type its real length. The app does the sum.

4. Trace the edges that matter

✓ outline, arc, holes ✗ shadow, scratches
Outline and features, not every mark.

Trace the outline and the features you’ll size, not shadows. Real arcs dimension cleanly.

5. Check a second length

190 mm 190 mm ✓ matches
A second known size, same face.

Check a second known length before you build. If it’s off, the photo was tilted or that feature sat nearer the camera.

Tracing to scale in PencilCAD

Import… Photo Scan paper Files
More → Import…
Underlay Strength 50% Set its scale… Locked
Under the drawing, at 50%
Trace with Line and Arc
PDF SVG DXF
Hide it, then share
  1. More → Import…: Photo, Scan paper or Files (image or PDF page), or drag or paste it in. It sits under the drawing at 50%, locked.
  2. In its panel (More, or the Underlay row in Layers): Set its scale, tap both ends, type the length.
  3. Toolset chip → CAD (or Settings → Tools). Trace with Line and Arc; Dimension reads true lengths.
  4. Check a second length, hide the underlay, and share as PDF, SVG or DXF.

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