Abstract
Abstract. Accurate image-based measurement based on UAV data is attracting attention in various applications. While the external accuracy of the UAV image blocks could be mainly affected by object-space information like number and distribution of ground control points and RTK-GNSS accuracy, its internal accuracy highly depends on camera specifications, flight height, data capturing setup and accuracy of scale estimation. For many small-scale projects accurate local measurements are highly demanded. This necessitates high internal accuracy of images block which could be transferred from model space to object space by accurate estimation of the scale parameter. This research aims at improving the internal accuracy of UAV image blocks using low-altitude flight(s) over small parts of the project area without using any ground control points. Possible further improvement by using calibrated scale-bars which serve as scale-constraints is also investigated. To this end, different scenarios of the flight configuration and distance measurements in the two photogrammetric blocks are also considered and the results are analyzed. Our investigations show 50% accuracy improvement achieved by performing local flights over small parts of the scene, given that RTK information is available. Moreover, adding accurate scale-bars increased the accuracy improvement to 67%. Furthermore, when RTK information is not available, adding local low-altitude flights and scale-bars decrease the error of local distance measurement form 1–3 meters to less than 4 centimeters.
Highlights
Nowadays drones are getting more and more popular for many applications like high precision mapping and 3D reconstruction to name a few (Gerke and Przybilla, 2016; Kraft et al, 2016; Malihi et al, 2018; Martínez-Carricondo et al, 2018; Sahebdivani et al, 2020)
In order to avoid this problem and georeferencing the image block, images should be captured using a drone which is equipped with RTK GNSS or adequate and well distributed full ground control points (GCPs) in object space should be measured
This motivated this research to investigate the feasibility of using low altitude local flights and accurate scale-bars for improving the internal accuracy of the photogrammetric block without using any GCPs
Summary
Nowadays drones are getting more and more popular for many applications like high precision mapping and 3D reconstruction to name a few (Gerke and Przybilla, 2016; Kraft et al, 2016; Malihi et al, 2018; Martínez-Carricondo et al, 2018; Sahebdivani et al, 2020). Though, providing many accurate and welldistributed GCPs needs a lot of time, effort and is costly This motivated this research to investigate the feasibility of using low altitude local flights and accurate scale-bars for improving the internal accuracy of the photogrammetric block without using any GCPs. Utilizing low altitude local flights and using scalebars instead of GCPs may highly improve the time and cost competence of the UAV-based projects for local measurement which do not need geo-referenced information. Utilizing low altitude local flights and using scalebars instead of GCPs may highly improve the time and cost competence of the UAV-based projects for local measurement which do not need geo-referenced information In this regard, different scenarios of the flight configuration and distance measurements in the photogrammetric block are considered and the results are analyzed thoroughly.
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