Circular No. 27/2011/TT-BTNMT stipulates on testing and calibration of certain marine surveying and mapping equipment.

CIRCULAR

Regulations on Testing and Calibration of Certain Surveying Equipment for Marine Mapping

_____________________

 

MINISTER OF NATURAL RESOURCES AND ENVIRONMENT

BASED ON THE LAW ON ENACTMENT OF LEGAL DOCUMENTS;

Pursuant to Decree No. 25/2008/NĐ-CP dated March 4, 2008 of the Government stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment, amended and supplemented by Decree No. 19/2010/NĐ-CP dated March 8, 2010 and Decree No. 89/2010/NĐ-CP dated August 16, 2010 of the Government;

Pursuant to Decree No. 12/2002/NĐ-CP dated January 22, 2002 of the Government on surveying and mapping activities;

Considering the proposal of the General Director of the Vietnam Administration of Seas and Islands, the Head of the Science and Technology Department, and the Head of the Legal Department.

c) Enterprises may be granted permission for no more than one block out of the total three dual-frequency blocks (FDD) B

PART I

GENERAL PROVISIONS

Article 1. Scope of Regulation

This Circular stipulates the testing and calibration of certain equipment used in marine surveying, including:

1. Positioning devices;

2. Digital compasses;

3. Wave sensors;

4. Sound speed meters;

5. Single-beam echo sounders;

6. Multi-beam echo sounders;

7. Single-beam echosounder systems;

8. Multi-beam echosounder systems.

Article 2. Applicability

This Circular applies to organizations and individuals when using the equipment specified in Article 1 of this Circular.

Article 3. Explanation of Terms

In this Circular, the following terms shall be understood as follows:

1. Fix is the marking or taking data from a system or device that is continuously measuring at a specific point in time.

2. Satellite Compass is a device using positioning satellite data obtained by two antennas to calculate the true direction of the line connecting the centers of the two antennas.

3. Transducer (English: Transducer) is the part of an echo sounder that emits and receives sound waves for measurement. The distance measured is calculated from the surface of this transducer to the surface reflecting the sound wave.

4. Draft (English: Draft or Draught) is the distance from the still water surface to the surface of the transducer.

5. The depth index of an echo sounder is the systematic error in the measurement data of the echo sounder, mainly caused by signal processing delay in the machine.

6. Longitudinal tilt angle is the angle deviation of the device along the longitudinal tilt of the survey vessel.

7. Lateral tilt angle is the angle deviation of the device along the lateral tilt of the survey vessel.

8. Heading deviation is the angle deviation of the device along the main axis of the survey vessel.

9. A wave sensor is a device that determines and provides the effects of water waves such as lateral and longitudinal tilts of the vessel and heave for other machines and equipment to correct their measurement results.

Article 4. General Requirements for Testing and Calibrating Marine Surveying Equipment

1. All devices and entire surveying systems must be tested and calibrated according to the provisions of this Circular before being put into use. Only equipment with satisfactory test and calibration results may be used.

2. Only personnel with professional expertise in geodesy or related fields, having at least three years of work experience, who have attended safety training courses, and equipped with all necessary safety tools may perform testing and calibration work on marine surveying equipment.

3. On-site testing and calibration must be carried out in the area specified in the detailed technical design of the project. Testing and calibration work can only be conducted when environmental, weather, and climatic conditions meet the working requirements of all machines and equipment used during the testing and calibration process.

4. Test results must be confirmed by technical supervisors.

5. Data, results, and reports of testing and calibration must be stored together with the original product documentation of the project.

Chapter II

TESTING OF SURVEYING EQUIPMENT FOR MARINE MAPPING

Article 5. Calibration of Positioning Devices

1. Each positioning device shall be calibrated using one of the following two methods:

a) Calibration at a reference point;

b) Calibration after installation on a survey vessel.

2. Calibration at a reference point shall be carried out as follows:

a) Place the positioning antenna on a known coordinate point (equivalent to the base point for surveying);

b) Connect the positioning device to a computer;

c) After the positioning device has completed its startup process and reached stable operation, use a data logging program to continuously record data for one hour, with intervals between recordings of 10 seconds. The positioning data must be converted to flat coordinates (X, Y, H) according to the conversion parameters specified in the technical requirements of the project;

d) Prepare a calibration result report according to the model prescribed in Appendix No. 01 issued together with this Circular after completing the calibration. The report shall include the following contents:

- Table of results for calculating deviations:

DX = X_origin - X_collected 

DY = Y_origin - Y_collected

DH = H_origin - Central agencies of political-social organizations;_collected

- Horizontal positioning accuracy: , (n being the number of data recordings)

- Vertical positioning accuracy:

- Conclusion: the device meets the requirements for production if the calculated horizontal and vertical accuracies do not exceed the accuracy specified in the technical specifications of the device; or the device does not meet the requirements and shall not be put into production.

3. Calibration after installation on a survey vessel shall be carried out as follows:

a) The survey vessel must be securely moored at a pier where there are high-precision original coordinate points with control points having an accuracy of at least that required for surveying;

b) The positioning device must be installed according to technical requirements;

c) The electronic total station used for calibration must have been tested and adjusted for errors and must have the following minimum technical features:

- Angle measurement accuracy of 6";

- Slope angle measurement accuracy of 10";

- Distance measurement accuracy of ±(3+3ppm)mm x D (D being the length of the measured distance);

d) The distance from the location of the electronic total station used for calibration to the orientation point and the GPS antenna placement point must ensure that the position error of the measurement point does not exceed 10 cm;

đ) Before calibration measurements, the positioning device and the data logging program must be connected and started up; synchronize the clocks of the data logging system with the clock of the measurer;

e) The calibration measurement process begins when the person in charge of the measurement issues a command to the measurement team via walkie-talkie or signal. Prior to each measurement command, the person in charge must request all participants to prepare adequately. Once everyone is ready, the person in charge issues a command to simultaneously measure the GPS antenna with the electronic total station and fix the GPS positioning data. Each measurer records the measurement data in a logbook according to the model, with the measurement time recorded to the second. In cases where the electronic total station has real-time coordinate calculation capability, the recorded data will be coordinates;

g) The number of calibration measurements shall not be less than 20 times, with a minimum interval of 3 minutes between each measurement;

h) After calibration measurements, the coordinate data of the antenna at each measurement time shall be filtered from the fixed coordinate data file on the vessel and the total station measurement data to be included in the table for the calibration result report;

i) Prepare a calibration result report with the contents as stipulated in Clause 2 of this Article.

Article 6. Calibration of Compasses

1. Calibration of Satellite Compass

a) The calibration of the satellite compass is carried out by placing the two antennas of the device on two known direction points (or points with known coordinates to calculate the standard direction).

b) After the device operates stably, a computer program records the direction data provided continuously for one hour, with intervals between recordings of 10 seconds.

c) Compare the data with the standard direction.

d) Prepare a calibration report according to the form prescribed in Appendix No. 02 issued together with this Circular, containing at least the following information:

- Direction deviation Δa = a_LB - a_goc , where a_LB is the compass direction, a_goc is the initial direction;

- Compass accuracy:  , where n is the number of measurements.

- Conclusion: the device meets the requirements for production if the calculated accuracy does not exceed the accuracy specified in the technical specifications of the device; otherwise, it does not meet the requirements and shall not be put into production.

2. Calibration of Compass Installed on Survey Vessels

a) The vessel must be securely moored at the port, ensuring that the vessel's direction does not fluctuate more than 1⁰ during the calibration process;

b) Turn on the compass and the software recording the compass direction data. The clocks of the computers recording the data and those of the personnel conducting the calibration must be synchronized, accurate to 0.5 seconds;

c) Use a total station and a coordinate network in the calibration area, ensuring that the azimuth accuracy of the two points measuring the vessel's direction does not exceed 10';

d) Use two mirrors placed on two points indicating the vessel's direction;

đ) Once the system recording the compass data on the vessel operates stably, and the total station and mirrors are ready for calibration, begin the calibration process under the command of the team leader through a walkie-talkie or by signal;

e) Each measurement is conducted as follows:

- The total station operator aims at the mirror placed at the bow of the vessel and reports to the team leader when ready. The team leader issues a command to simultaneously measure the mirror and fix the compass direction data;

- The total station operator quickly changes the aiming direction to the mirror placed at the stern and reports to the team leader when ready. The team leader issues a command to simultaneously measure the mirror and fix the compass direction data;

g) Each calibration session consists of 20 measurements. The duration of each measurement should not exceed 2 minutes;

h) The measurement results are calculated using the following table:

- The actual direction of the vessel at each measurement is calculated based on the coordinates of the two mirror points at the stern and bow. The compass direction is calculated as the average value of the recorded values within each measurement time interval;

- The direction deviation for each measurement is calculated as: actual direction - compass direction;

- The correction factor is calculated as the average deviation value.

Article 7. Calibration of Wave Sensors

1. Equipment used in the calibration of wave sensors includes:

a) A water level gauge that has been calibrated and adjusted according to regulations, and must have an accuracy of determining height difference from 1mm/1km upwards;

b) A ruler with millimeter markings;

c) A calibration stand with a base to place the wave sensor, this base having marks for the vertical and horizontal tilt directions of the machine, and the center of the machine. The stand has parts that can tilt the base along the vertical and horizontal tilt axes of the wave sensor.

2. The calibration procedure is as follows:

a) Install the wave sensor on the base correctly centered and aligned with the marked vertical and horizontal tilt directions;

b) Start the machine and connect it to the accompanying software to record the measured data. Use the water level gauge to position the stand horizontally. Use the software to read the initial offset values of the sensors (Roll, Pitch, Heave);

c) Change the tilt angle of the base in the horizontal direction in increments across the full range of horizontal tilt of the machine, using the water level gauge to accurately determine the tilt angle. Record the tilt angle measurement results and the horizontal tilt value in the logbook. If the machine's horizontal tilt range is small, and the number of calibration measurements (n) is less than 10 times, reduce the tilt angle increment of the calibration base to achieve n≥10 measurements;

d) Change the tilt angle of the base in the vertical direction in increments across the full range of vertical tilt of the machine, using the water level gauge to accurately determine the tilt angle. Record the tilt angle measurement results and the vertical tilt value in the logbook. If the machine's vertical tilt range is small, and the number of calibration measurements (n) is less than 10 times, reduce the tilt angle increment of the calibration base to achieve n≥10 measurements;

đ) Change the height of the machine in steps of 0.2m with an accuracy of 1cm, record the height changes and the wave measurement data provided by the machine at these heights in the logbook;

e) Calculate the horizontal tilt error, vertical tilt error, and wave measurement errors using the formula , where ∆ is the difference between the actual value and the corrected value provided by the wave sensor;

g) Prepare a calibration result report according to the form prescribed in Appendix No. 03 issued together with this Circular. The machine meets the requirements if the calculated error does not exceed the accuracy specified in the technical specifications of the machine.

Article 8. Testing of Sound Speed Measurement Devices

The testing process for sound speed measurement devices shall be carried out as follows:

1. Use a testing device to measure the sound speed in a container filled with distilled water while simultaneously measuring the temperature of the water using an accurate thermometer. Record the sound speed and water temperature data every minute over a period of 15 minutes.

2. Calculate the sound speed for each measurement according to the formula:

V = 1449,2 + 4,67xT - 0,0569xT²+ 0,00029xT³+ (1,39 - 0,012xT)(S - 35) + 0,01625xD

Where:

V is the speed of sound transmission

T is the water temperature

S is the salinity, which is zero in this case.

D is the submergence depth of the sound speed measuring device.

3. Create a comparison table between the two measured sound speeds.

The deviation is calculated using the formula: ΔV = V_{Technology and Environment on the establishment of the Journal of Standards} - V (VND/year): is the total investment capital allocated annually for the usable area of social housing for rent, ensuring the preservation of capital, calculated according to the following formula:_calculation

Accuracy is calculated using the formula

where n is the number of measurements.

4. Prepare a test result report according to the model specified in Appendix No. 04 issued along with this Circular. The device meets quality standards if the accuracy obtained does not exceed the accuracy of the sound speed measuring device.

Article 9. Testing of Single Beam Echo Sounders with a Sound Speed Measuring Device

The Ministry of Education and Training takes the lead together with the Organizational Structure Department of the provincial government to develop the examination plan to be submitted to the Chairman of the Provincial People's Committee for decision.

a) A sound speed measuring device that has passed technical quality testing;

b) The test disc is securely fixed to a non-stretchable steel cable. On the cable, there are markings at intervals of one meter from the surface of the disc. The error of these markings must not exceed 0.5 cm;

c) Determine and set the submergence depth of the acoustic transducer on the echo sounder;

d) The marking for lowering the disc must be marked on the survey vessel with an accuracy of 5 mm relative to the surface of the acoustic transducer. The markings must facilitate the lowering of the test disc in increments from the surface of the disc to the surface of the acoustic transducer;

đ) Record the depth deviation index of the existing device (if any) in the logbook.

2. The testing process is carried out as follows:

a) Use the sound speed measuring device to measure the sound speed passing through the column of water at the test site. The depth interval for taking readings is 0.5 meters;

b) Lower the test disc to the minimum depth the device can measure, enter the measured sound speed data for the water column from the acoustic transducer to the test disc into the echo sounder. Record the measured data by the echo sounder in the logbook;

c) Calculate the depth deviation measured by the echo sounder (D_ds) compared to the actual submergence depth of the test disc (D_dia). Use this deviation to correct the initial depth deviation index. After this step, the depth deviation between (D_ds) and (D_dia) must be zero. For machines without a separate section to enter this depth deviation index, this deviation is added to the submergence depth of the acoustic transducer;

d) Continue lowering the test disc in 2-meter increments while changing the sound speed measured for the water column from the acoustic transducer to the test disc in the echo sounder. Record the measured depth (D_ds) and submergence depth (D_energy) in the logbook. Calculate the depth deviation between(D_ds) and (D_energy). Δ_{ANNEX I.A[31]} Proportion of domestic currency (Vietnamese Dong) loan capital in total loan capital as specified in the Appendix attached to this Circular (%);_dsi - D_ngi, where D_dsi - is the measured depth at the inspection level i, D_ngi - is the submergence depth of the test disc at the inspection level i. This step must be performed up to the maximum possible depth and must be repeated if the number of measurements (n) has not exceeded 10;

đ) Calculate the depth measurement error of the machine using the formula: ;

e) Prepare a test result report according to the model specified in Appendix No. 05 issued along with this Circular. The machine meets quality standards for use if the measurement error obtained falls within the machine's accuracy range.

Article 10. Testing of Single Beam Echo Sounders Without a Sound Speed Measuring Device

The Ministry of Education and Training takes the lead together with the Organizational Structure Department of the provincial government to develop the examination plan to be submitted to the Chairman of the Provincial People's Committee for decision.

a) The test disc is securely fixed to a non-stretchable steel cable. On the cable, there are markings at intervals of one meter from the surface of the disc. The error of these markings must not exceed 0.5 cm;

b) Determine and set the submergence depth of the acoustic transducer on the echo sounder;

c) The marking for lowering the disc must be marked on the survey vessel with an accuracy of 5 mm relative to the surface of the acoustic transducer. The markings must facilitate the lowering of the test disc in increments from the surface of the disc to the surface of the acoustic transducer;

d) Record the depth deviation index of the existing device (if any) in the logbook.

2. The testing process is carried out as follows:

a) Set the sound speed for the echo sounder to the most appropriate value known in the testing area. Determine and calibrate the initial depth deviation index of the machine;

b) Lower the disc in 2-meter increments with an accuracy of 0.5 cm, change the sound speed until the measured depth to the disc matches the actual depth of the disc. Record the sound speed at that depth in the logbook. This step must be performed up to the maximum possible depth and must adjust the disc lowering steps to ensure the minimum number of measurements (n) is 10 times;

c) Raise the disc to each lowered depth with an accuracy of 0.5 cm, set the sound speed recorded for each depth. Read and record the measured depth data by the echo sounder in the logbook;

d) Calculate the depth deviation between the two depths when lowering the disc and raising it back up at each inspection mark using the formula: Δ_{ANNEX I.A[31]} Proportion of domestic currency (Vietnamese Dong) loan capital in total loan capital as specified in the Appendix attached to this Circular (%);_ix - D_il. In which D_ix - is the depth measured at inspection level i when lowering the disc. D_il - is the depth measured at inspection level i when raising the disc;

đ) The depth measurement error of the machine is calculated using the formula: , where n is the number of inspection levels conducted;

e) If the depth measurement error obtained ensures the technical performance of the machine, conclude that the machine meets quality standards and can be put into production. If the obtained error exceeds the error stated in the machine's technical performance, then repeat the testing steps from point b to point đ of this clause two more times. If both subsequent tests yield a depth measurement error that ensures the machine's technical performance, conclude that the machine meets quality standards and can be put into production. If at least one further test does not meet the standard, conclude that the machine does not meet quality standards for production.

Article 11. Testing Multi-beam Echo Sounder

1. The testing shall be conducted for the central beam.

2. The testing process is carried out as follows:

a) Use a sound speed meter to measure the sound speed at the testing location, then enter the measured data into the echo sounder;

b) Lower the test plate to the minimum depth that the device can measure. The depth of the test plate is determined with an accuracy of 0.5 cm;

c) Measure the depth of the test plate using the echo sounder (50 Fix with intervals of 5 seconds/1 fix);

d) Calculate the system depth deviation index (index) by subtracting the depth of the test plate measured by ruler (D1) from the average depth of the test plate measured by the echo sounder (after excluding gross errors) (D2);

đ) Enter the calculated correction index (index) according to the formula: I = D1 - D2 into the echo sounder. After this step, D1 must equal D2;

e) Lower the test plate down by 2-meter increments with an accuracy of 0.5 cm until the maximum allowable depth under the measurement conditions (depth, flow, waves) is reached. At each depth, use the echo sounder to measure 25 fixes with intervals of 5 seconds; calculate the average depth after excluding gross errors.

3. Prepare a report on the testing results according to the model specified in Appendix No. 06 issued together with this Circular, including the following main contents:

a) Calculate the depth deviation: Δ_{ANNEX I.A[31]} Proportion of domestic currency (Vietnamese Dong) loan capital in total loan capital as specified in the Appendix attached to this Circular (%);_dsi - D_ngi , where D_dsi - is the depth measurement data at the testing level i, D_ngi - is the submergence of the test plate at the testing level i;

b) Calculate the depth measurement error of the machine according to the formula: ;

c) Conclusion: the machine meets the quality requirements for use if the obtained error falls within the machine's accuracy range.

Chapter III

TESTING AND CALIBRATION OF SINGLE-BEAM ECHO SOUNDER SYSTEM AND MULTI-BEAM ECHO SOUNDER SYSTEM

Section 1

TESTING AND CALIBRATION OF SINGLE-BEAM ECHO SOUNDER DEPTH MEASUREMENT SYSTEM

Article 12. Single-beam Echo Sounder Depth Measurement System

1. A complete single-beam echo sounder depth measurement system consists of the following machines connected together:

a) Positioning machine;

b) Compass machine;

c) Single-beam echo sounder;

d) Wave sensor;

đ) Survey software installed computer.

2. After installation, measure eccentricities, set necessary parameters, and conduct a full system test to determine calibration indices for the entire system or for data processing.

Article 13. Testing the Submergence of the Transducer Head Due to Ship Movement

1. To determine the submergence of the transducer head when the ship is moving, the geometric water level method must be used, measuring from a station on shore to a water level gauge placed at the position of the transducer head on the ship. If the tide level changes significantly during testing, an additional gauge should be used to measure the tide level.

2. The testing area must ensure:

a) Convenience for measuring the water level gauge on the ship;

b) Safety when the ship reaches the required stable speed and runs towards the shore;

c) No obstacles affecting the ship's movement in the measurement area.

3. The measurement process is carried out as follows:

a) Move the ship to the testing area, let it float statically. Measure the water level gauge on the ship and the tide level gauge;

b) Run the ship in the direction from shore with a stable speed (number of engine revolutions), when reaching the testing area, read the gauge on the ship, record in the column Running Downstream. Then turn the ship around, run in the opposite direction towards the shore with the previous speed, when reaching the testing area, read the gauge on the ship, record in the column Running Upstream;

c) Measure the tide level gauge.

4. The measurements according to point b, Clause 3 of this Article are performed for speeds of 2 Knt, 3 Knt, 4 Knt, 5 Knt, 6 Knt, 7 Knt, and 8 Knt (nautical miles/hour).

5. The recorded measurement data follow the format:

Table 1: Sample table for testing the submergence of the transducer head

Submergence = H₀ - (H_ix - Central agencies of political-social organizations;_{3.2.5. Medical examination and treatment establishments, laboratories, and vaccination centers at the provincial level} )/2 + TT₀ - TT_i

Where:

H₀ - Reading of the submergence check when the ship is stationary

H_ix - Reading of the submergence check when the ship is running downstream at speed i

H_{3.2.5. Medical examination and treatment establishments, laboratories, and vaccination centers at the provincial level} - Reading of the submergence check when the ship is running upstream at speed i

No.₀ - Reading on the tide level gauge when the ship is stationary

No._{ANNEX I.A[31]} - Reading on the tide level gauge when the ship is running at speed i

6. These calibration indices obtained are entered into the survey software to automatically calibrate the measurement data according to the ship's speed if the software has such a function. If the software does not have the function to calibrate submergence according to speed, this calibration index table will be used for subsequent data processing.

Article 14. Determining Position Delay

1. Select a relatively smooth terrain area with a slope of approximately 10 to 20 degrees, depth under 100 meters, design a test route running perpendicular to the contour lines, heading uphill. The slope must be long enough (500 to 1000 meters) to obtain good samples and must be even (no horizontal slope, no unevenness).

2. Conduct two measurements along the designed route at two different ship speeds. The difference in speed between the two runs must be at least 9 km/h (Figure 1)

Figure 1: Diagram of ship operation for determining position delay on a sloping surface

Position delay is calculated using the formula

BB 1.1₂ where Vfast is the speed of the faster run;

BB 1.1₁ Vslow is the speed of the slower run;

Δx is the horizontal position offset between two adjacent soundings near the peak.

If the testing site has a flat terrain, it can be conducted over a recognizable object (such as a sand dune) with the ship operation diagram as described in Figure 2.

Figure 2: Diagram of ship operation for determining position delay on a flat surface.

Section 2

TESTING AND CALIBRATION OF MULTIBEAM SONAR DEPTH MEASUREMENT SYSTEM

Article 15. Requirements for Testing and Calibration of Multibeam Sonar Depth Measurement System

1. Conducted after the complete installation of the system and completion of measurements to determine the offsets of the equipment: sonar sensor unit, compass, transducer head of the multibeam echo sounder, and the transducer head sag due to ship movement. Necessary parameters (coordinate system, height, transfer function parameters, positioning error limits, depth measurement, ship heading measurement, lateral tilt angle measurement, longitudinal tilt angle measurement, wave height measurement) must be fully set up for the system. Sound velocity profile data from the measurement area must be obtained and incorporated into the system.

2. This testing and calibration can only be performed when weather conditions are favorable and waves are less than 1 meter to ensure high-quality depth measurements with minimal wave disturbance.

3. The positioning device used in the system must have a horizontal accuracy better than ±3 meters.

4. On the survey vessel, there must be at least one computer installed with software for processing test data and survey data. This software must have the capability to calculate correction factors for position delay, longitudinal tilt offset, compass heading offset, and lateral tilt offset based on test data.

5. At least two pairs of test routes must be measured.

6. Prepare a test report according to the template specified in Appendix 07 issued together with this Circular.

Article 16. Testing Transducer Sag

Testing of transducer sag due to ship movement is carried out as specified for the single-beam echo sounder system in Article 13 of this Circular.

Article 17. Full System Testing

1. Full system testing is conducted by "calibration" to identify the following errors: position delay (for systems without synchronized data devices); longitudinal tilt offset; heading offset and lateral tilt offset.

2. Determining position delay: carried out as specified for the single-beam echo sounder system in Article 14 of this Circular.

3. Determining longitudinal tilt offset:

a) Select a relatively smooth terrain area with a slope (the steeper the better), depth under 100 meters, design a test route running perpendicular to the contour lines. The length of the route must be at least 500 to 1000 meters;

b) Conduct two measurements along the designed route at the same ship speed. The direction of the two runs must be opposite as described in Figure 3;

Figure 3: Diagram of ship operation for determining longitudinal tilt offset

c) After correcting for position delay, the longitudinal tilt offset is determined using the formula where δz is the longitudinal tilt offset required, and Δd is the separation of two profiles at depth z. 4. Determining heading offset:

a) Heading offset is the integrated effect of the following offsets: compass heading, transducer head heading relative to the ship's axis;

b) To determine the remaining offset, select an area with distinct features (for example, a sand dune). The measurement must be conducted along two routes in opposite directions such that the overlap of the edge beams is between 10 to 20% of the swath width and covers the feature completely;

c) To avoid interference from position delay and longitudinal tilt offset, these delays must be corrected before calculating the heading offset;

d) The heading offset is calculated using the formula

as described in Figure 4 Figure 4: Diagram of ship operation for determining heading offset

5. Determining lateral tilt offset:

a) This lateral tilt offset is caused by the lateral tilt of the sonar sensor and the transducer head offset along the lateral axis of the survey vessel. To measure this offset, choose a flat seabed area and conduct measurements along one route in both directions. The speed of the two runs must be the same;

b) To avoid interference, the system must be corrected for position delay, longitudinal tilt, and heading offset;

c) The lateral tilt offset is determined by measuring the vertical displacement of the depth data from the edge beams of the measurement routes and is calculated using the formula

as described in Figure 5. Figure 5: Determination of lateral tilt offset

This Circular takes effect from September 5, 2011.

Chapter IV

IMPLEMENTATION

Article 18. Effective Date

The Director of the Vietnam Marine and Island Administration is responsible for guiding, monitoring, and inspecting the implementation of this Circular. Any issues encountered during implementation should be promptly reported to the Ministry of Natural Resources and Environment for study, amendment, and supplementation as necessary.

Any issues encountered during implementation should be promptly reported to the Ministry of Natural Resources and Environment for study, amendment, and supplementation as necessary./.

Nguyen Van Duc