Digital Orthophotography

Digital orthophotography is a photogrammetric service that applies the mathematics of photogrammetry to remove the distortions that are inherent in aerial photography. The orthorectification process removes the distortion in the imagery caused by the tip and tilt of the camera, the relief of the terrain and the central projection perspective.

Aerial Photography Distortions
Because a photograph is a two-dimensional representation of a three-dimensional object, when you look at the center of an aerial photograph, your view is the same as if you were looking straight down from the aircraft. But as you look toward the edges of the photograph, you are viewing the ground from an oblique angle, no longer straight down. Regardless of where you look at the photo, your view passes through a common point. This is a central perspective projection.

The very center of the aerial photograph is scale correct. Points on the photo that represent a ground elevation higher than the elevation at the center of the photograph are displaced away from the center. Lower elevations are displaced toward the center. In order to create a scale correct photograph that can be accurately measured, your view needs to be straight down over every point in the photograph.

Orthorectification Process
Orthorectification begins with a digital image created from a calibrated photogrammetric scanner. The scanner produces images that are made up of a matrix of picture elements or pixels in rows and columns. The orthorectification process takes into account many variables including (1) camera calibration data, (2) scanner calibration data, (3) the position of the camera at the time of the exposure (4) the rotational parameters of the camera at the time of the exposure, and (5) the terrain relief. Applying photogrammetric mathematics, each pixel in the image is repositioned to its correct geographical location and covers a specific distance on the ground.

Orthophotography Resolution and Accuracy
The resolution of an orthophoto is expressed by the ground distance covered by a pixel, sometimes abbreviated to the initials GSD, which stands for ground sample distance.

The accuracy of an orthophoto is based upon fit of the features within the imagery to their "real world" position. The accuracy is stated as a horizontal scale, such as 1"=100' or 1:12000. Orthophotography meeting ASPRS Class 1 specification for orthophotography positions features within 1/40th of one inch at final map scale.