Students might remember from previous activities that their image size changes as they move farther away or closer to the camera. To make this technique work, students will have to be the same distance from the camera as the object they’re measuring, or their image size will be distorted. One student will have to stand right next to the objects or perhaps flat against them (if they’re measuring a building or a tree.)  Also, the photographer needs to hold the camera perpendicular to the floor and not tilt it up or down.Urge students to take care both when they take their photos and when they adjust their calipers.  The data they’ll get is only as good as their measurement techniques.

This is particularly useful for Lessons such as You are the Ruler.