Ground control point distribution for accurate kilometre-scale topographic mapping using an RTK-GNSS Unmanned Aerial Vehicle and SfM photogrammetry

Stott, E., Williams, R. D. and Hoey, T. B. (2020) Ground control point distribution for accurate kilometre-scale topographic mapping using an RTK-GNSS Unmanned Aerial Vehicle and SfM photogrammetry. Drones, 4(3), 55. (doi: 10.3390/drones4030055)

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Abstract

Unmanned Aerial Vehicles (UAVs) have revolutionised the availability of high resolution topographic data in many disciplines due to their relatively low-cost and ease of deployment. Consumer-grade Real Time Kinematic Global Navigation Satellite System (RTK-GNSS) equipped UAVs offer potential to reduce or eliminate ground control points (GCPs) from SfM photogrammetry surveys, removing time-consuming target deployment. Despite this, the removal of ground control can substantially reduce the georeferencing accuracy of SfM photogrammetry outputs. Here, a DJI Phantom 4 RTK UAV is deployed to survey a 2 × 0.5 km reach of the braided River Feshie, Scotland that has local channel-bar relief of c.1 m and median grain size c.60 mm. Five rectangular adjacent blocks were flown, with images collected at 20° from the nadir across a double grid, with strips flown in opposing directions to achieve locally convergent imagery geometry. Check point errors for seven scenarios with varying configurations of GCPs were tested. Results show that, contrary to some published Direct Georeferencing UAV investigations, GCPs are not essential for accurate kilometre-scale topographic modelling. Using no GCPs, 3300 independent spatially-distributed RTK-GNSS surveyed check points have mean z-axis error −0.010 m (RMSE = 0.066 m). Using 5 GCPs gave 0.016 m (RMSE = 0.072 m). Our check point results do not show vertical systematic errors, such as doming, using either 0 or 5 GCPs. However, acquiring spatially distributed independent check points to check for systematic errors is recommended. Our results imply that an RTK-GNSS UAV can produce acceptable errors with no ground control, alongside spatially distributed independent check points, demonstrating that the technique is versatile for rapid kilometre-scale topographic survey in a range of geomorphic environments.

Item Type:Articles
Additional Information:ES was funded by UK Natural Environment Research (NERC) Doctoral Training Grant NE/R007934/1, in partnership with the Scottish Environment Protection Agency (SEPA). GNSS equipment was provided by NERC Geophysical Equipment Facility (GEF) loan 1118.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Williams, Dr Richard and Hoey, Professor Trevor and Stott, Miss Eilidh
Creator Roles:
Stott, E.Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization, Funding acquisition
Williams, R. D.Conceptualization, Methodology, Investigation, Writing – review and editing, Supervision, Project administration, Funding acquisition
Hoey, T. B.Writing – review and editing, Supervision, Funding acquisition
Authors: Stott, E., Williams, R. D., and Hoey, T. B.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Drones
Publisher:MDPI
ISSN:2504-446X
ISSN (Online):2504-446X
Published Online:08 September 2020
Copyright Holders:Copyright © 2020 The Authors
First Published:First published in Drones 4(3): 55
Publisher Policy:Reproduced under a Creative Commons License

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