Analyzing the effect of distribution pattern and number of GCPs on overall accuracy of UAV photogrammetric results

Abstract

Unmanned Aerial Vehicle (UAV) based photogrammetry is becoming a valuable source of data for topographic mapping, volume calculations, terrain mapping, and generating 3D models. However, the use of UAVs for any purpose requires basic knowledge of various flight settings. The number and distribution of Ground Control Points (GCPs) are the most crucial, therefore, the number of GCPs should be used economically. This paper discusses the accuracy of UAV based photogrammetric products in Corridor mapping and area with Undulating terrain for different sets of flight settings. Influence of number of GCPs and their distribution patterns are assessed for optimal accuracy. For the accuracy assessment of GCP distribution, various configurations of GCPs were tested. The acquired accuracy was then compared for each of these configurations and the most suitable ones were determined for each terrain type. In corridor mapping, the distribution of GCPs depends upon the length of an area with GCPs alternating each side of the linear feature, separated by an offset distance along with the feature. In our study, the optimum number of GCPs was found to be four, along with the feature being mapped. Similarly, in the area with undulating terrain, the GCPs should be established in places covering all elevations with a minimum of five GCPs in shape of a die. Our results show that distribution and number of GCP used during UAV based survey play a major role in the accuracy of Digital Surface Model (DSM) and orthomosaics. The accuracy not only depends upon the number of GCPs but also on its distribution pattern. Therefore, the choice of suitable pattern and number of GCPs for a particular mission can help obtain results with desired accuracy as well as economic feasibility.