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QUY HOẠCH DI DỜI HỆ THỐNG CẢNG B

 

ACCURACY AND EFFICIENCY OF BATHYMETRIC SURVEY TO BE IMPROVED WITH RTK TECHNIQUE

Dr. Nguyen Ngoc Lau (1), Dr. Truong Ngoc Tuong (2), Eng. Nguyen Tan Son (3) and
Eng. Nguyen Quang
(3)

(1)              Geodesy and  GIS Department  – HCMC University of Technology

(2)              Ports and Waterways – HCMC University of Technology

(3)              Geomatics Department - Portcoast Consultant Corporation

 

(The first Scientific Conference of Geodesy and Mapping and Land Management in Hanoi, Dec.2004)

 

       Abstract: Measuring water level near shore to determine sea/river – floor height in hydrographic surveying has provided low precision in limited areas. In this paper, we present an application of RTK GPS technique at PortCoast Consultant to experimentally survey a part of Soai Rap river, Ho Chi Minh City, the results showed that the use of traditional method can cause an error in height about three times bigger and much less efficiency than RTK technique.

 

1.      Introduction:

Measuring bathymetry is formerly a complicated procedure and it could give us low accuracy and efficiency. This procedure has been improved so much thanks to Global Positioning System.

 

Figure 1. DGPS Technology

 

One of GPS techniques used widely is DGPS (Differential Global Positioning System). The technique can be described briefly in Figure 1. The first receiver located at a fix location onshore whose coordinate is known clearly. It is called base station or reference station transmitting distance correction into rover station via radio links. The second one located on a boat called rover station will receive distance correction from base station and adjust distance measurements from it to satellite to process and supply coordinate and height of its antenna in real time. Measurements used to process are pseudo-range measurements C/A-code or P-code with accuracy 3m and 0.3m respectively. In case of dynamic measuring, the measurements can get accuracy of coordinate positioning form 0.5m to 3m. Accuracy of height determined by GPS technology is 3 times smaller than that of coordinate positioning so that it does not meet requirements and GPS technology is not used for height determination. Depth of seabed is determined in measuring water level method as the following figure:

Figure 2. Measuring water level with leveling machine and target.

 

Above figure height of water level near river bank is calculated by formula:

                                                    hw = h0 – b + a – c                                               (1)

The principle of determination of riverbed height in measuring water level assumes that water level at points near river bank equals to that at location of a boat. At that time height of riverbed is calculated by:           

            h = hw – d                                                            (2)

In this report, we would like to present the proceed of test measuring with RTK technology on Soai Rap river in Ho Chi Minh City at the same time with the proceed of water level measuring. Analysis results will give information about accuracy of depth determination with water level method in the area. Thanks to the experiment, advantage of accuracy and efficiency of RTK technology for bathymetric survey is also emphasized.

 

2.      What is RTK Technique?

 

RTK is the abbreviation of Real-Time Kinematic (i.e. dynamic survey in real time). RTK technology is almost similar to DGPS technology. However, in the case of RTK technology, base station will transmit phase measured value to rover station. Rover station establishes phase measured value in the form of double difference for processing. Therefore, the technology is also called DGPS technology using phase-measured value. Because phase-measured value has the accuracy in a few millimeters so that the positioning accuracy can get from a few centimeters to decimeters and that depends on to finding the integer of ambiguity resolution number in phase measured value. If 100 percent of multi-value parameters are solved, we will get the fix-solution with the highest accuracy. If multi-value parameters can be solved partially, we will get the float-solution.

Solution of finding out multi-value parameters only based on  phase measured values at the measuring time is a big challenge to GPS equipment including theirs hardware and software. Besides that depends on distance between base station and rover station, the number of satellite observed simultaneously and velocity of the boat.

Figure 3 - RTK method (Real Time Kinematic)

Above figure describes principle of determining the river floor height by using RTK method, where:

Dh: different height between two antennas of the base station and the rover station, determined by RTK with cm accuracy.

a: antenna height at the base station, measured by tape with about 5mm accuracy.

ho: The point’s height compared with the reference ellipsoid (assume no error).

f: antenna height of the rover station compared with water level at a boat position, measured by a tape with cm accuracy.

d: river-bottom depth, measured by an echo sounder.

h: river-bottom height, calculated by formula:

                 h = hw – d                                                             

hw is the instant height of water level and hw = ho + a - Dh – f.

Apparently, RTK provides both exact height and position of antenna at measuring time. It helps avoid shortcomings of bathymetric process by measuring water level and enhances the accuracy of bathymetric surveying process.

3.      Equipment and a sample of measuring area:

 

GPS receiver used in this case is dual frequency Topcon Legacy H for base station and Topcon Hiper for rover station. Radio links with 38400 Baud frequency and RTCM format of transmitted data. According to [4], accuracy with RTK technique is 10mm+1.5ppm in coordinate positioning and 20mm+1.5ppm in height determination. It means that if distance between base station and rover station is 3 kilometers, accuracy of coordinate positioning and height determining is 14.5mm and 24.5mm respectively.

             

        Topcon Legacy H               Topcon Hiper                        Odom Echotrac MKIII

Figure 4. Hardware for bathymetric survey

 

Odom Echotrac MKIII dual frequency Echo sounder is used with its accuracy:

m=0.01m+ 0.1% X depth (m) at the frequency 200 kHz.

Text Box: Tide measure Station

 

 

Figure 5. Depth measuring area and measuring line.

 

Mean square error of determining level water is calculated by:

 (m)

If maximum depth of river bed is 50m, mean square error of determining river bed depth is    (m)

      If we use the equipment, accuracy of determining water level at boat location and river bed height is about 27mm and 66mm respectively.

Topcon Hiper and Topcon Legacy are connected to two ports RS232 of a laptop, where Hypack Max software, specializing in bathymetric survey, is installed. This software enables users pre-design surveying lines on map in form of DXF [5]. In the operation process, boat’s location shall be regularly updated on the base map. This allows the boat to be operated to follow design lines with errors of few metres even in offshore areas, in condition of un-definable directions and being influenced by outside factors (Figure 5). Data of GPS locations in form of NMEA 0183 version 3.0 and bathymetry shall be automatically saved in computers’ hardware.

The surveying area is a bend of Soai Rap river – Ho Chi Minh City with maximum depth of approximately 30m. Determined location of base station is the cadastral bench mark, which is performed with a triangular sign in Figure 5. Measuring the water level at this location is implemented in interval of 15 minutes. Bathymetric lines are designed horizontally along the river and in interval of 100m. The recording interval of GPS receivers and Odom Echotrac MKIII shall be 2 seconds.

The transfer of coordinates from WGS84 to local coordinate system is a complicated process, which shall not be mentioned in the report. Therefore, WGS84 shall be used in the study to avoid errors arisen from the transfer.

      After the period of approximately 45 minutes necessary for installing base station and rover station onto the boat, the boat shall be operated following the design lines from upstream to downstream areas. The maximum distance from base station to rover station is about 4.5km. The number of GPS maintained during the survey shall not frequently be less than 7. With the speed of the boat approximately 10km/h, it only takes 6 hours to finish the survey.

 

4.      Transferring elevation from ellipsoid into chart datum:

 

       Elevation of sea/riverbed is practically performed upon chart datum different from Ellipsoid used in GPS, to which these 2 following solutions based on Hypack Max Software can be applied:

·        In case of small distance from the rover station to the base station, Ellipsoid plane and standard plane of chart datum in the surveying area shall be considered as 2 parallel planes, of which the distance between them shall be assumed as the difference of 2 levels at the base station (Ellipsoid – Chart datum). This difference shall be utilized by Hypack Max to transfer elevation of points automatically.

·        At the site, control points of which Ellipsoid datum and chart datum have been available shall be input into Hypack Max software before measurement. In the measuring phase, difference of two levels at the point shall be calculated for adjustment, which is considered as the measurement for the height at the points for adjustment. This measurement can be applied at large areas or stretched areas, where two Ellipsoids and standard plane of chart datum are not parallel.

To this experiment, the first measurement shall be applied. However, the second measurement can be applied because along the river there are base cadastral points, of which chart datum has been determined by geometric measurement.

 

5.      Processing and analysis of results:

 

The post-processing of depth measuring data is totally implemented by Hypack Max software. There is approximately 8000 bathymetric points, 100% of which is root fix. This shows the high reliability of RTK Technique. The height of water level is calculated with formula (1) at base station and formula (4) at rover station as shown in the following diagram:

Figure 6. Height of water level at base station and rover station

 

According to the diagram, water level is performed by “*” sign at base station and “.” sign at rover station with RTK Technique in interval of 2 seconds. Water level changes complicatedly in periods because the antenna of rover station on boat at calm water area is influenced by impacts of wind, tide or current, which is unavoidable in bathymetric survey. However, with thick density of data (in interval of 2 seconds or more), calm water level at the point can be determined by filtering method. An interfering filter has been used to collect value in average interval of 5 minutes (equivalent to 150 data points). The result is dark points “.”as in figure 6. These results subtracted by values collected by water level measurement near shore shall obtain differences as shown in the following table:

 

Time

Difference

(cm)

Time

Difference

(cm)

Time

Difference

(cm)

01:15

01:30

02:00

02:15

02:30

23

27

23

19

18

02:45

03:00

03:15

03:30

03:45

20

18

18

18

17

04:15

04:30

05:00

06:00

06:30

15

15

12

8

10

      As performed in the above table, differences of two water levels vary from 10 cm to 30cm and increase along with increasing distance between base station and rover station (Figure 7), which well suits the hydraulic regime at narrow river sections.

      In such case, when replacing water level at boat with that near shore, an average error 0.2m of height should be accepted. This value is approximately 3.3 times different from depth measuring error (0.06m) and hence, mean square error for determining the height of bottom shall be

 

Figure 7. Correlation of level difference and distance of stations

 

6.      Conclusions and Recommendations:

 

Measurement of water level near shore was demonstrated to create errors up to 3 times different from those of depth measurement. However, it is still the result of surveying on a small river section. Errors could be bigger when measurement is implemented at seas, where impacts of tide and wind are much bigger.

      RTK technique is the new instrument that shall provide more accurate and efficient results for bathymetric survey. With the above instrument, it shall take much less time to post-process with Hypack Max software for bathymetric map and DEM.

 

      As stated in chapter 2, accuracy and reliability of RTK Technique mainly depends on the successful solution of multivalue parameter in phase measured value, which is affected by different factors such as distance between base station and rover station, quantity of simultaneously monitoring satellites and moving speed of boat. RTK Technique has fully satisfied proposed requirements of this study. And RTK Technique shall be applied at complicated areas, namely coastal areas, in which the distance between base station and rover station shall be up to kilometers.

 

Reference

[1]. Chris Rizos, 1996, Principles and Practice of GPS Surveying, University of NewSouth Wales, Australia.

[2]. Department of the US Army, 2002, Hydrographic Surveying.

[3]. Tran Minh Quang, 2000, Viet Nam National University Ho Chi Minh City.

[4]. User guide of using Topcon Legacy H and Topcon Hiper

[5]. User guide of using software Hypack Max.

 

 
 

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