Photogrammetry is the art, science and technology of making measurements from photographs. It is based on the principle of optical parallax, where an object will appear to be in a slightly different position relative to the viewer depending on the viewer’s location. This is the effect that gives us our ability to judge distance and depth as we see the same object from slightly different perspectives with each eye. Applied to a series of aerial photographs taken from a moving camera, parallax allows us to accurately calculate the position (elevation as well as location) of a feature on the ground.
A camera attached to a UAV is flown across the landscape in a regular grid pattern, while the camera acquires photographs at regular intervals. The images are arranged to overlap, so that any given point on the ground is visible in many photographs. The overlap is usually arranged to be 80% or more in the direction of travel (‘vertical overlap’) and 60% or more between adjacent rows of the grid pattern (horizontal overlap’).
Using the principle of ‘relief displacement’ – an extension of the parallax principle - the elevation of each point on the ground can be calculated as a deviation from the position it would occupy on a flat (planimetric) surface. The process of estimating the elevation of an entire surface by using a series of sequential images is known as ‘structure from motion’.