Orthophotogrammetry consists of two principles; orthorectification and orthomosaic generation. Orthorectification is the process of removing unwanted effects caused by the image perspective and the terrain being captured. When an aerial photo is taken, the angle is rarely looking directly downwards, resulting in photos being taken on an angle or the terrain looking different than in reality. Orthorectifying the images gives the image a constant scale and allows the image to reflect the true positions of what is in the photo.
Orthomosaic generation is the process of “stitching” together all the orthorectified images of an area into one seamless image, providing accurate measurements such as distance, angles and areas.