An orthophoto or orthoimage is an image that is free of distortion (it has been ortho-rectified) and which is characterized by a uniform scale over its entire surface. We can consider by simplifying that it is like if each element shown on the image has been photographed directly from the vertical over it.
In other words, an orthophoto is a kind of scaled photographic map, on which it is perfectly possible to perform measurements as if it were a standard map. It is part of the photogrammetry field and is generally performed by Unmanned Aerial Vehicles (UAV).
The orthoimage can be overlaid with other maps containing other urban or technical elements like a power supply network, a dam, a road, a cable television network, a construction project, etc. Photogrammetry allows to obtain useful maps containing a lot of information helping making decisions.
Orthophotos are much faster and easier to create than establishing of a new area conventional map and they can be reproduced on a regular basis thanks to the cost efficiency and quick operability of drones.
These photographs are shot by air and the ease with which our Unmanned Aircraft Systems (UAS) can perform low altitude slow flight is particularly adapted for their acquisition.
The precision of an orthophoto is directly proportional to the resolution of the image captured by the embedded digital camera on board.
With the automated navigation flight, the Remotely Piloted Aircraft System (RPAS) will fly over the area to explore in a systematized way covering it completely. A percentage of overlap between the pictures will bring out the reliefs, the same way than the human stereoscopic vision does. The flight plan used by the operator will be uploaded on the drone on a case by case basis and will be adjusted to correspond to each specific photogrammetry mission.
Depending on the navigation path, the drone will scan the area on parallel axes and with a shift of a few degrees to precisely reach this stereoscopic human vision effect through the onboard camera.
Once on the ground, the images are analyzed, processed and corrected in order to eliminate distortions including the effect of the relief displacement. It calculates the Digital Elevation Model (DEM which is a topographic representation of an area) in order to correct them and adjust any associated terrain deforming errors.
Used in several fields, such as the urban & land use planning, administrative departments management, communication, agriculture, archeology and others, photogrammetry allows the identification of objects and geometric shapes projecting their measurements on the horizontal plane (Planimetrics).
