Digital 2-Dimensional Photogrammetry Simplified by Using a Marker of a Known Size

Lee, Jeong Woo; Kim, Dong Kyu; Chu, Seung Gyun; Cho, Byung Chae; Lee, Kyung Young

Abstract

BACKGROUND
A quantitative, rather than visual, assessment of the outcomes of facial surgery has recently become increasingly important, and this is best accomplished using anthropometry. However, most anthropometric methods have limited applicability in office settings.
METHODS
We conducted preliminary studies on images of a 30-cm straight edge ruler with various camera-to-object distances (OD) and aperture sizes, and identified a convenient range of ODs and the most reliable aperture size of our camera for photogrammetry. Subsequently, we measured various lengths circumjacent to the center of the ruler in the images of different ODs with a graphics viewer program and calibrated the raw data using the central 20-mm length. We obtained a reliable circle in our camera's viewfinder from a chart of percentage differences between the calibrated data and coupled real ruler lengths. Following this, we replaced the concept of the central 20-mm length with a geometric sphere 20-mm in diameter, and applied it to clinical photogrammetry.
RESULTS
The percentage differences were almost directly proportional to the real lengths. This value was below 0.48% in the central 160-mm of all images within an OD range of 50 to 125 cm. In clinical applications, the greatest difference compared to direct measurements was 0.97 mm.
CONCLUSIONS
Using a graphics viewer program, we performed anthropometric measurements of images that included a sphere of known size, without printing the images; the difference between these and direct measurements was <1 mm. This method is simple enough for use in an office setting.