Cost effective accurate orthoimaging of inaccessible areas

Although high resolution orthoimages have been the most popular photogrammetric derivative, the cost of orthoimaging production remains high for areas where access is difficult. This paper presents a procedure for a cost effective orthoimage production, from data (digital images, GCPs and DTMs) derived solely from Airborne Laser Scanner systems. ALS systems are designed to produce a highly accurate 3D point cloud, which can be easily rasterized in an accurate, high resolution and dense DEM. Applying a hillshade effect on that dense DEM, one can visualize the objects with great detail and thus measure 3D coordinates for points to be used as GCPs. Modern ALS systems incorporate medium format digital cameras with high resolution imaging abilities. As no ground surveying is needed, the production of high end orthoimages may be performed even in regions that are inaccessible or difficult to access, such as remote islets and mountain tops. A workflow is proposed for the production of orthoimages from low density (2.5 points/m2) laser data using an ALS II-50 system and its medium format RCD105 digital camera. The area of the case study is the northern section of the Greek island Milos. Utilizing automated processes, a series of quality control tasks and a preprocessing of the data is performed. During the data processing, an interpolation of the LiDAR data’s first returns is performed, in order to rasterize a high resolution (1m) DEM. The orthorectification process is been performed with the same DEM, leading to an orthoimage with a planimetric accuracy of 1m and minimum geometric distortions. The cost analysis of the applied procedure has proved that the method is less cost demanding compared to the usual orhoimage production methods.