Measuring distances using digital cameras

This paper presents a generic method of calculating accurate horizontal and vertical object distances from digital images taken with any digital camera and lens combination, where the object plane is parallel to the image plane or tilted in the vertical plane. This method was developed for a project investigating the size, density and spatial distribution of shrubs growing as hedges.

The geometry of objects projected on to parallel and oblique image planes was used to develop an equation for calculating horizontal and vertical distances from image pixel counts. Images of a grid were taken with a variety of digital cameras at different focal lengths, image-object distances and vertical tilt angles. The equations were also tested by taking a set of photos of shrubs in hedge field experiment.

The results show a very strong correlation between calculated distances and physical measurements across a range of cameras, focal lengths, distances and vertical tilt angles. A small constant error was found in two of the three cameras tested suggesting that effective image size may vary from published sensor dimensions in some cameras. Cameras should be calibrated to check for any constant error before using the equations described in this paper.