Comparison of bottom-track to global positioning system referenced discharges measured using an acoustic Doppler current profiler

SummaryA negative bias in discharge measurements made with an acoustic Doppler current profiler (ADCP) can be caused by the movement of sediment on or near the streambed. The integration of a global positioning system (GPS) to track the movement of the ADCP can be used to avoid the systematic negative bias associated with a moving streambed. More than 500 discharge transects from 63 discharge measurements with GPS data were collected at sites throughout the US, Canada, and New Zealand with no moving bed to compare GPS and bottom-track-referenced discharges. Although the data indicated some statistical bias depending on site conditions and type of GPS data used, these biases were typically about 0.5% or less. An assessment of differential correction sources was limited by a lack of data collected in a range of different correction sources and different GPS receivers at the same sites. Despite this limitation, the data indicate that the use of Wide Area Augmentation System (WAAS) corrected positional data is acceptable for discharge measurements using GGA as the boat-velocity reference. The discharge data based on GPS-referenced boat velocities from the VTG data string, which does not require differential correction, were comparable to the discharges based on GPS-referenced boat velocities from the differentially-corrected GGA data string. Spatial variability of measure discharges referenced to GGA, VTG and bottom-tracking is higher near the channel banks. The spatial variability of VTG-referenced discharges is correlated with the spatial distribution of maximum Horizontal Dilution of Precision (HDOP) values and the spatial variability of GGA-referenced discharges is correlated with proximity to channel banks.

► Discharge measured using GGA or VTG are within ±0.5% on average of bottom-tracking data.
► Measured discharges referenced to VTG are within 1% on average of GGA referenced discharge data.
► More random errors in GPS discharges for sites with mean boat speeds <0.25 m/s and/or channel widths <25 m.
► Spatial variability of VTG-referenced discharges is correlated with spatial distribution of maximum HDOP.
► Spatial variability of GGA-referenced discharges is correlated with proximity to channel banks.