## 3 Methods and Data
### Field Data
#### 3.1.2 Positioning (GNSS)  
Creating 3D point clouds of flow data requires accurate absolute positioning of each measurement. In order to obtain the best possible absolute positioning, we mounted a Trimble R8 Global Navigation Satellite System (GNSS) operating in virtual reference station (VRS) mode onto the RiverSurveyor's GPS antenna pole instead of the ADCP's own GPS antenna, which, even when using real-time kinematic (RTK) correction, only achieves a relative positional accuracy of 2 cm, due to the fact that the base station estimates its own position using its own GPS antenna, and does so anew, every time the base station is booted. This gives the ADCP's positional data DGPS-level accuracy in absolute terms, even though the points within one measurement have RTK-level precision relative to each other. If it were possible to set up the ADCP's RTK base station over a known reference point and input those coordinates to the station along with the necessary offsets at millimeter accuracy, the use of a second GNSS would not be necessary. The location of the base station's antenna cannot be input using Sontek's RiverSurveyor Live software, making an external GNSS solution necessary. However, in the interest of completeness, it should be noted that the more recent optional HydroSurveyor software does give the option to input base station coordinates, which results in absolute RTK-GPS positioning. VRS-GNSS (also known as network RTK) is a virtualized version of RTK-GNSS, using real-time kinematic positioning, but rather than using a physical base station in the field, the VRS receiver gets its correction data through a GPRS-based (general packet radio service) Internet connection. A computing centre gathers real-time satellite data from a network of base stations around the country, based on which a virtual base station in the vicinity of the rover is computed and the virtual correction data transmitted to the receiver in real time. It is also possible to download the correction data at a later time for post-processing purposes. Given that the field site is within range of a GPRS network, the VRS system is much more convenient and faster to set up than a physical base station. Furthermore, the fact that the virtual base station can be located very closely to the measurement and can even be updatedwhen measurements spread over a large area, minimizes baseline length-dependent errors common in GNSS positioning. The Trimble R8 GNSS makes use of both the GPS and GLONASS satellite systems, which greatly improves coverage, particularly at these high latitudes. Despite having a large number of satellites available, a river channel remains a challenging environment for rapid and accurate GNSS positioning, particularly near steep banks. Fortunately, trees at the riverbank did not represent much of an obstacle during the spring flood, since they had not grown any foliage yet. The Sontek's own GPS antenna was mounted on the bow of the RCl flow or the Hydroboard to collect satellite timestamp data (_cf._ Figures 2 and 3). We use GPS timestamps later to merge the GNSS and ADCP data. The appropriate GPS offsets were included in the ADCP set-up parameters, so as not to affect the tracking ability of the ADCP. The integrated GPS receives differential corrections (DGPS) from the Satellite-based Augmentation System (SBAS) satellite network, giving it a nominal accuracy of 1 m or better. These DGPS data are used by the ADCP for tracking (_cf._ Section 3.1.1). The R8 GNSS was set to collect raw data at 10 Hz, while the RiverSurveyor's internal GPS collects data at 1 Hz, concurrent with its flow measurements. Given that these measurements do not necessarily take place at the peak of each second and that, like many instruments, the specified 1-Hz interval of the ADCP is theoretical and exhibits slight variation about that value in reality, this made it necessary to collect the actual positioning data at 10 Hz.