Method for Estimating Targets’ Dimensions Using Aerial Surveillance Cameras

Determining the size of objects (static or moving) appearing in the aerial imagery can foster targets’ reconnaissance and support rapid decision-making during the flight. This article presents a novel method, based on previous consolidated approaches, to measure the dimensions of any target appearing in an aerial image, either vertically (height), horizontally (length, width), or even areas. It works with optical cameras; no other payload sensors are needed. It requires a single optical image and a short processing time (it can work in real time during the flight). The required parameters for the calculation are the focal length, detector dimensions, Pitch and Roll (Euler) angles, and the relative elevation of the aerial camera from the points where the targets are. This latter parameter is strongly dependent on the positional accuracy of the aerial system, which is not always very high; therefore, this article also proposes an approach to retrieve the accurate elevation by considering only two points of known coordinates visible in the image. The coordinates of these points can be obtained using Digital Elevation Models (DEMs), which can subsequently allow retrieving the relative elevation of the camera at any other point of the image. The method was tested using an accurate DEM and 24 aerial images acquired by a drone. The results showed that in most of the cases considered, the metrics were estimated with an error lower than 5 cm. In the last part of the article, potential limitations and solutions to automatize the processes were discussed.