Computer Interface
The output from GPS receiver is available at two seconds interval, whereas a fathometer can take soundings at much higher frequencies, as high as 20 times per second. The computer program monitors the GPS serial port for incoming data, and every time a GPS data string is received, the program immediately retrieves a depth reading from the second serial port. The survey software has many other capabilities. They are listed below.
- Fixing of grid line on screen.
- Display of boat position, speed, direction, heading angle
etc. on screen in real time.
- Interfacing with bathymeter and DGPS and writing of outputs in X, Y, Z format
- Display of depth and depth chart in real time.
- Selection of base line for survey.
- Display of coordinates in chosen coordinates system.
- Display of chainage being covered.
- Display of offset from the selected line.
Personnel
Qualified and experienced personnel are essential for an efficient and productive field operation. The key personnel include a ydrographic crew chief experienced in all phases of the field operations and knowledgeable about the computation and report needs of the study and a crew member or members capable of assisting in the operation and maintenance of the field instruments. GPS system allow collection of data by one person, but for safety purposes and assistance, a minimum of two field personnel are recommended. For larger reservoirs, one or two additional crew members may be necessary to support the survey and for the safety of the operation.
Methodology
The development of the present collection system has made the contour method the preferred method for data collection and analysis. Prior to these collection system the surveys were scheduled when the reservoir were near empty. Now they are usually scheduled when near full capacity to reduce the collection time and cost. If the reservoir areas are not aerial surveyed, then the bathymetric survey should be scheduled when the reservoir is as full as possible.
Brief description of the method adopted for topographic and hydrographic survey is given below.
Hydrographic survey
The survey is conducted in a rapid and efficient manner. The basic equipment required for a GPS hydrographic survey are a boat, two GPS receivers, fathometer, and a laptop computer with data-logging software. The boat is equipped with the bathymetric equipment, the GPS system mounted on board and a lap-top computer while its reference station is positioned on a known geographical benchmark. The survey software enables fixing of grid lines and interfacing of bathymeter and DGPS and taking X, Y, Z values at required interval/ grid. Boat navigation is also controlled by the software so that boat tracks the grid line accurately. The surveys can also be carried out at random mode, rather than attempting to locate and survey pre-defined sediment range lines. The GPS receivers output position information is available at every two seconds for hydrographic survey works. Survey grade fathometers take soundings at much higher frequencies, as high as 20 times per second. Therefore, the survey software monitors the GPS serial port for incoming data, and every time a GPS data string is received the program immediately retrieves a depth reading from the second serial port. Alternatively, to collect the data simultaneously each port can be monitored continuously and data values time-stamped. The programme then uses interpolation to define simultaneous data from each port.
Topographic survey
For accurate land survey, control points were established using DGPS all along the periphery of the reservoir. Land survey covered the entire area between water spread area and MWL. The survey is carried out using Total Station having internal memory to store up to 3000 points and facility to transfer data on PC.
Data Reduction and Analysis
The bathymetric data is first transferred to reduced levels format, after removing all collected points without differential correction. Entire data from Total Station available in digital format is then merged with bathymetry data, after doing required formatting. Contours for the reservoir areas are computed from the compiled data using the TIN (Triangular irregular network) surface modeling package. A TIN is a set of adjacent, non-overlapping triangles computed from irregularly spaced points with x, y coordinates and z values. In this method triangles are formed between all collected data points including all boundary points preserving all collected survey points. The contour surface areas for the reservoir are computed from the TIN at selected elevation intervals for the complete vertical elevation of the reservoir. Grid interpolation method can also be used for working out contour surface areas. The basic elevation verses surface area data developed from the survey become the input data for the computation of revised detailed area and capacity tables. The following outputs can be obtained using suitable software. There are several software packages available for this.
- Cross-sections at required locations.
- Area-elevation capacity table at any required interval from the lowest bed level to MWL.
- Contour maps with contours at any required interval.
- Longitudinal section of the reservoir.
Further, analysis of data for obtaining average annual sedimentation and sedimentation in various zones of the reservoir and mathematical modeling studies etc. are also carried out.
Advantages of GPS survey over other hydrographic survey techniques
The conventional hydrographic survey methods are based on typical land survey techniques and are labour intensive. The survey vessel is required to be brought to stationary position every time observation is taken. In DGPS hydrographic surveying the observations are taken from a moving survey vessel, which allows faster data acquisition with better accuracy. The line-of sight from the base station to the boat is not necessary, as required in the conventional survey. A GPS survey can be completed between control points (even on opposite side of a mountain) without having to traverse or even see the other point. A large number of reference stations/ range monuments and Bench Mark pillars are required to be erected, all along the periphery of the reservoir, for conducting conventional survey, whereas in GPS survey only a few control stations are required. Other advantages are the ability to achieve greater accuracy and the ability to efficiently collect large amounts of data. For data collection, the grid generally adopted for a very large reservoir in a conventional survey is 1000x100m, and in GPS survey a 50x50m grid is adopted ie. the intensity of data collection in GPS survey is 50 times in a conventional survey. The hydrographic survey can be carried out at random mode without specifying range lines. The data collecting system with GPS is compact and can be accommodated in smaller boats.
References
- Report on Capacity Survey of Ghataprabha Reservoir (2000), Central Water Commission, Consulting Engineering Services (India) Ltd. and Tojo-Vikas International Pvt. Ltd., New Delhi.
- Report on Capacity Survey of Jayakwadi Reservoir (2000), Central Water Commission and Rail India Technical & Economical Services Ltd. New Delhi.
- Compendium on Silting of Reservoirs in India, Central Water Commission, Publication No. 82/90, Jan. 1991.
- “Experience in Sedimentation of Reservoirs in India”, Central Water Commission, New Delhi. Proceedings of the International Conference on Reservoir Sedimentation, held at Colorado, USA from 9-13, September,1996.
- A.K. Shangle, “Reservoir Sedimentation Surveys (using high technology)”. Lecture notes of regional training course on reservoir sedimentation and control held at New Delhi from 9-22, December’1991
- Scot A. Sullivan “DGPS and GIS Improve Lake Sedimentation Survey Procedures.” Proceedings of the International Conference on Reservoir Sedimentation, held at Colorado, USA from 9-13, September, 1996.
- James D. Schall and George A. Fisher “Hydrographic Surveying Using Global Positioning Techniques.” Proceedings of the International Conference on Reservoir Sedimentation, held at Colorado, USA from 9-13, September,1996.
- Ronald Ferrari and Wayne Dorough ‘Measuring Deposited Sediment”. Proceedings of the International Conference on Reservoir Sedimentation, held at Colorado, USA from 9-13, September,1996.