Guides

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

Hold the camera straight over the subject, or straight at it, so the face you want is parallel to the lens. Anything tilted comes out foreshortened, and no scale fixes that. Step back and zoom in a little rather than going close with the wide lens, which bends straight edges near the frame.

For paper drawings, a document scanner does this for you: it finds the page, squares it up and flattens it.

2. Put a known length in the picture

Lay a ruler or tape measure on the subject, in the same plane as the edges you’ll trace, or pick a size you already know, such as a door width or a printed scale bar. The longer the reference, the better: an error of one pixel matters much less across 300 mm than across 30.

3. Set the scale from that length

The scale is just the real length divided by its length in the picture. Most apps that trace do the sum for you: you mark the two ends of the reference and type its real length. Do it on the longest reference you have.

4. Trace the edges that matter

Trace the outline and the features you need sizes for, not every shadow and scratch. Use straight lines and real arcs rather than freehand, so the traced drawing has clean corners you can dimension and edit later.

5. Check a second length

Before you build from it, measure something else you know in the same photo. If it agrees, the scale is good. If it’s off, the photo was probably tilted, or that feature isn’t in the same plane as your reference. Parts nearer the camera always come out bigger, so keep every measurement on the one face you scaled.