Evaluating low-cost topographic surveys for computations of conveyance
Authors: Samboko Hubert T. , Schurer Sten , Savenije Hubert H. G. , Makurira Hodson , Banda Kawawa , Winsemius Hessel 
Published: January 2022

Abstract. Rapid modern technological advancements have led to
significant improvements in river monitoring using unmanned aerial vehicles
(UAVs), photogrammetric reconstruction software, and low-cost real-time
kinematic Global Navigation Satellite System (RTK GNSS) equipment. UAVs
allow for the collection of dry bathymetric data in environments that are
difficult to access. Low-cost RTK GNSS equipment facilitates accurate
measurement of wet bathymetry when combined with subaqueous measuring tools
such as acoustic Doppler current profilers (ADCPs). Hydraulic models may be
constructed from these data, which in turn can be used for various
applications such as water management, forecasting, early warning and
disaster preparedness by responsible water authorities, and construction of
river rating curves. We hypothesise that the reconstruction of dry terrain
with UAV-based photogrammetry combined with RTK GNSS equipment leads
to accurate geometries particularly fit for hydraulic understanding and
simulation models. This study sought to (1) compare open-source and
commercial photogrammetry packages to verify if water authorities with low
resource availability have the option to utilise open-source packages
without significant compromise on accuracy; (2) assess the impact of
variations in the number of ground control points (GCPs) and the
distribution of the GCP markers on the quality of digital elevation models
(DEMs), with a particular emphasis on characteristics that impact
hydraulics; and (3) investigate the impact of using reconstructions based
on different GCP numbers on conveyance and hydraulic slope. A novel method
which makes use of a simple RTK tie line along the water edge measured using
a low-cost but highly accurate GNSS is presented so as to correct the
unwanted effect of lens distortion (“doming effect”) and enable the
concatenation of geometric data from different sources. Furthermore, we
describe how merging of the dry and wet bathymetry can be achieved through
gridding based on linear interpolation. We tested our approach over a
section of the Luangwa River in Zambia. Results indicate that the
open-source software photogrammetry package is capable of producing results
that are comparable to commercially available options. We determined that
GCPs are essential for vertical accuracy, but also that an increase in the
number of GCPs above a limited number of five only moderately increases the
accuracy of results, provided the GCPs are well spaced in both the horizontal
and vertical dimension. Furthermore, insignificant differences in hydraulic
geometries among the various cross sections are observed, corroborating the
fact that a limited well-spaced set of GCPs is enough to establish a
hydraulically sound reconstruction. However, it appeared necessary to make
an additional observation of the hydraulic slope. A slope derived merely
from the UAV survey was shown to be prone to considerable errors caused by
lens distortion. Combination of the photogrammetry results with the RTK GNSS
tie line was shown to be essential to correct the slope and made the
reconstruction suitable for hydraulic model setup.