A regular aerial photo is inherently distorted: camera tilt, terrain height and lens geometry make the scale vary from point to point. An orthophoto is an image in which these distortions have been corrected, so it works as a map.
1. Planning and acquisition
Flight lines are planned with overlap between images and captured with a large-format digital metric camera. Overlap lets every point be seen from at least two angles and 3D to be reconstructed.
2. Aerial triangulation
Using ground control points measured in the field, the position and orientation of the camera for each image are computed precisely, tying all images into one block and one coordinate system.
3. Rectification and production
Based on the surface model, each pixel is corrected to its true position and the images are mosaicked into a seamless product with colour balancing.
4. Quality control
Positional accuracy is verified against independent check points; seams and distortions are located and fixed before delivery.