Hydrographic Mapping on the Colorado River in the Grand Canyon
The GCMRC Survey Department hydrographic mapping program was established for the purpose of producing a sub-aqueous channel map of the Colorado River between Glen Canyon Dam and Lake Mead. Hydrographic mapping supports several GCMRC scientific initiatives including aquatic habitat monitoring, channel geomorphology monitoring, sediment budget and transport analysis, and sand bar evolution and fluvial processes modeling.
Hydrographic technology is used in the Grand Canyon primarily to create a base map and to measure changes in the river channel. Primarily, changes that occur are due to the movement of sediment. GCMRC Survey Department monitors these changes with hydro-acoustic measurements that are accurately positioned over the course of the river channel. The hydrographic data collection method uses multi-beam technology which offers swath coverage requiring few passes up and down the channel for a complete map of a river section. Multi-beam systems require extremely accurate positioning and motion compensation to successfully map the river bottom.
Hydrographic Surveying System
The GCMRC multi-beam system consists of a boat equipped with a multi-beam depth sounder mounted at the base of the mast and positioned approximately 0.5m below the surface of the water. A laser tracking diode, mounted on the top of the mast, allows a robotic total station system to track the boat and geographically position the transducer. A gyrocompass and Motion Reference Unit (MRU) are mounted on the boat to resolve the boat’s heading, and heave, pitch and roll motion during collection of the multi-beam data. Geographic positioning of the transducer requires the robotic total station tracking system (Geodimeter ATS-PT) to be set up on a shore control point visible from the boat on the stretch of river to be mapped. Geographic coordinates and a reference azimuth for the shore station are derived from the control network through GPS occupations and conventional surveying methods. The robotic total station, on the fixed control point, automatically tracks the boat and sends positional data via radio telemetry to the computer on board the boat. A computer combines the positional data, multi-beam data, and motion compensation data using specialized software.
Photo: Orange total station: set up over control point and robotically tracks boats position (reflector on top of mast), then telecommunicates the positions to the computer onboard the boat. The boat collects multibeam data (depth readings) from a transducer mounted at bottom of mast (~0.5m below water surface). The depth and multibeam data are combined with the positional coordinates on the computer using specialized software (in the crossbox under the white pad).
Inset photo: View onboard boat from boat drivers seat of computer monitors in crossbox. The two forward monitors display real time integration of position and depth (in software package). The overhead monitor assists boat driver in maintaining his course.
Note: Solar panels on bimini supply most of power for this operation (sunny days)
Data Processing
Hydrographic data analysis is a complex series of steps requiring editing, adjusting, and processing the data to calculate coordinate positions and elevations used to create surface maps. Typically, the data is processed to create 0.5m resolution maps, but 2cm to 6cm resolution maps can be produced from the raw data. The channel surface maps are integrated with terrestrial base maps, produced as part of the FY2000 terrestrial mapping project (i.e., the LiDAR mapping) to provide the most accurate three-dimensional canyon geometry obtained thus far.
Approximately 78 miles of the Colorado River have been mapped, so far. The channel mapping project requires four more years of hydrographic mapping to acquire complete coverage of the additional 177 river miles in the CRE. An average of three channel mapping river trips are needed per year to accomplish the hydrographic mapping as planned where roughly 1.5 miles to 2.5 miles of river are mapped per day.