The Smartphone D-tape

A Lightening Talk presented by:Dr. John A. Kershaw, Jr.Professor of Forest MensurationFaculty of Forestry & EM, University of New Brunswick

1813

1913

1963

2013

There are some alternatives

Close-range photogrammetric approaches• Stereo photographs with known center-point distances• Vanishing Point Horizon with laser pointer• Dual lasers with known laser distance• An alternative

Criteria

• Fast• Accurate• Repeatable• Cheap• Automated

Smartphone

• Decent high resolution camera• App development easy and cheaply distributed• Compact, and since most everyone has one, not an additional piece of

equipment

Goal

• Take two photos, with random phone movement (distance, rotation and yawl)• From these two photos determine:• Camera shift• {X,Y,Z} coordinates• Estimate DBH

Epipolar Geometry

Two photos

The “Fundamental” Matrix

Coincident Photos

Add the DBH line, map the pixels from one photo to the other

DBH Estimate

• OLS solution to the Fundamental Equation• Camera Focal Length and Camera Aspect Ratio used to convert from

pixels to real world coordinates• DBH estimated from 3D coordinate distances• Include inclination angle for upper stem diameters (Grosenbaugh

1963)