An orthomosaic is a map made from drone photographs: hundreds of overlapping aerial images stitched together and corrected for perspective and terrain so that every pixel sits in its true geographic position and scale can be measured directly from the image. Commercial software such as Pix4D and Metashape does this well, but OpenDroneMap, or ODM, offers a capable open-source alternative that runs on a local computer or a cloud server and produces orthomosaics, digital surface models, point clouds and 3D meshes from the same photo set.

Good processing starts before takeoff, in the overlap and ground control you capture. Plan for at least 75 percent forward overlap and 65 percent side overlap, fly a consistent altitude, and use a double grid over buildings or forests where a single grid leaves occluded walls and canopy gaps. Place at least five ground control points, or GCPs, spread across the site and on its edges, survey them with a GNSS receiver or a known benchmark, and photograph each marker clearly; without GCPs, a consumer drone GPS may place the final mosaic 2 to 5 metres from truth, while well-distributed GCPs routinely bring horizontal error below 10 centimetres on a 10-hectare site.

Flight logs and photo metadata are part of the survey, not paperwork to skip. Record take-off time, wind speed, cloud cover, battery swaps and any aborted lines, because a gap in coverage discovered during processing is far easier to explain — and to re-fly — when the log shows exactly which line failed. Keep the original photos with EXIF GPS intact, even after processing, because a future software version may extract better accuracy from the same images. Clients rarely ask for raw photos until a dispute arises, and on that day the archive is worth more than any polished PDF.