From Perspective to Plan View
An aerial photograph is a perspective projection — objects near the camera appear larger, and elevated terrain features lean radially away from the image center. Orthorectification transforms this perspective view into a planimetric (map-like) projection where every pixel corresponds to a consistent ground distance. The corrected image, called an orthophoto, can be overlaid directly on maps and used for accurate area and distance measurements.
Relief Displacement
The most significant distortion in aerial imagery is relief displacement: elevated objects appear shifted outward from the image center by an amount proportional to their height and radial distance. A building 50 m tall photographed from 500 m altitude at the image edge is displaced by approximately 5 m from its true planimetric position. Correcting this requires a digital elevation model that specifies the terrain height at every point.
The DEM Connection
Orthorectification accuracy depends critically on DEM quality. A DEM error of 1 m at 500 m flight altitude introduces approximately 0.2% positional error — acceptable for many applications but significant for precision mapping. LiDAR-derived DEMs with decimeter accuracy enable orthophotos that meet the strictest cartographic standards. Photogrammetric DEMs generated from the same image set create a bootstrapping challenge that iterative refinement addresses.
Orthomosaics at Scale
Modern drone mapping stitches hundreds of orthorectified images into seamless orthomosaics covering entire construction sites, farms, or mining operations. Color balancing, seamline optimization, and ghosting removal at image boundaries ensure visually consistent results. The combination of centimeter GSD with rigorous geometric correction makes drone orthomosaics a standard tool in surveying, agriculture, and infrastructure inspection.