ION GNSS 2010
Session A3: Next Generation GNSS Integrity for Aviation

Title: RTK Water Level Determination in Precise Inland Bathymetric Measurements
Author(s): D. Popielarczyk, University of Warmia and Mazury, Poland
Date/Time: Thursday, September 20, 2012, 11:26 a.m.

The paper presents application of Real Time Kinematic positioning integrated with classical Total Station technique for sub-centimetre determination of water level changes during single beam bathymetric measurements on the fragment of Vistula river - behind the dam and hydropower. A vertical datum used in hydrography is typically related to a physical surface, such as mean sea level (MSL) or lowest astronomical tide (LAT). In inland bathymetry these physical surface is usually stable and can only change slightly in time. Exception to the rule can be a reservoirs of a hydroelectric power station, where the water level changes significantly over the time and distance. The example is Wloclawek hydroelectric power plant located on one of the cross section on 674,85 km of Vistula river in Poland. It is the biggest flowing station in Poland (it was added into use in 1970). The station includes frontal dam, weir, fish pass and sailing lock. Hydroelectric power plant is placed on the left river bank between the navigable lock and weir form which is separated by the fish pass pillar. There are three sections with two hydro complexes each. The total power of the plant is 160,2 MW. Five hundred meters below the dam is located underwater threshold of damming and flow stabilization. Despite of the threshold the outflow swirled water causes significant bottom erosion. For that reason, the power station reservoir should be periodically investigated. It is crucial for proper and safe work of the power plant. The team of Chair of Satellite Geodesy and Navigation from University of Warmia and Mazury in Olsztyn in Poland has prepared and conducted integrated GPS and hydroacoustic measurements. The primary objective of the bathymetric surveys was to develop of the digital elevation models of the bottom with high accuracy. The power station work causes up to 1,7 m movement of vertical reference surface in aspect of local bathymetry surveys. That is why verified hydrographic data had to be brought to the common water level. To determine the final water level, data on the height of the RTK/GPS antenna positioned on the boat during sounding, Total Station data and the digital readings from the power plant water level indicators were considered. The resulting sets of adjusted elevation data points were used for construction of the bottom Digital Terrain Models (DTM). This paper describes the basic project stages and results of bathymetric measurements and water level determination using integrated Real Time Kinematic GPS positioning and Total Station Techniques. The classical precise geodetic surveying was used to analyse and confirm the RTK positioning. The bathymetric measurements were carried out on the bases of GPS/RTK satellite positioning and Simrad EA501P single beam hydroacoustic system (SBES) manufactured by Kongsberg. The local RTK base station was set up only for the dedicated project. The differential positioning system used two Ashtech Z-Xtreme GPS receivers. During bathymetric measurements the water level was changing significantly over time depending on the operational schedule of the power plant. The integrated bathymetric technology was used. The technology allows making a design of measurement profiles, navigation along the profiles, recording positions and bathymetric data, correlation of these both data and finally creating digital bathymetric maps. The Integrated Bathymetric System gives a great chance to study the underwater environment, to examine of bottom shape and compute of water volume. After conducting the field measurement campaign on Vistula river the bathymetric data recorded was processed. Data from hydroacoustic sounding along individual measurement profiles was recorded as echograms in the form of binary files containing the following data: echogram image recorded for printing under office conditions, depths, position and basic setup data of the echosounder. All echograms were used for verification of the numeric data recorded from the measurements: position with the depth. As a result of processing the raw measurement data the set of staff points of the reservoir bottom converted to one water level that formed the database for development of the Digital Elevation Model of the bottom. The RTK technique with 0,02-0,05 m of vertical accuracy controlled with the Total Station geodetic survey proved to be very useful and essential in engineering inland bathymetric measurements. The resulting set of elevation data points was used for creating and working with three-dimensional models. A new geo-data model was also used to edit, analyze, visualize and interpret results using 3D visualization environments. The spatial database allowed to request the relevant data required for a specific environmental, hydrogeological or other studies. Using historical data can be demonstrated how the power plant outflow water causes bottom erosion. The analyses of the RTK and total station vertical observations suggest that we can use the GPS kinematic methods to determine 3D hydrographic motorboat position. However, the surveyor should take under consideration that the satellite conditions (the number of satellites, PDOP), and the distance from the reference station are a key influence on the accuracy of the determinations.

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