The impact of number and spatial distribution of GCPs on the positional accuracy of geospatial products derived from low-cost UASs

Currently, orthophotos and Digital Terrain Models (DTMs) are commonly being generated from low-cost consumer-grade Unmanned Aerial Systems (UASs) using well-known workflows based on photogrammetric and structure-from-motion methods. To achieve high accuracy positioning standards Ground Control Points (GCPs) have to be used in the workflow. In this paper, we assess the impact of number and spatial distribution of GCPs on the positional accuracy of these two geospatial products. By using 3 well-known mapping accuracy standards and 170 checkpoints, the planimetric and altimetric accuracy is assessed for 13 cases of GCPs configuration. The results show that in general, the UAS workflow presents a better accuracy in planimetry than in altimetry. In addition, the application of the mapping standards shows that the two products are suitable for GIS and mapping applications requiring spatial details equivalent to scales less than or equal 1:3000. We also conclude that the insertion of GCPs in the central part of the block does not contribute significantly to an increase in the planimetric accuracy of the geospatial products. Finally, for the proposed minimum photogrammetric overlaps (70% and 35%), the ideal GCP distribution consists of a uniform distribution of GCPs. First, on the central part the block with a horizontal separation of 3 to 4 ground base units for the altimetric component. Second, on the periphery of the block with a separation of 7 to 8 ground base units for the planimetric component.