What are regulations on measurement of the vertical gradient at the national gravity landmark in Vietnam?

What are regulations on measurement of the vertical gradient at the national gravity landmark in Vietnam?

Pursuatn to Article 9 of the Circular 11/2020/TT-BTNMT stipulating measurement of the vertical gradient at the national gravity landmark in Vietnam as follows:

1. The measurement of vertical gradient at the gravity datum is applied in the case of gravity measurement by absolute gravity method. The vertical gravity gradient Δγ at the gravity landmark is determined by measuring the difference in the gravitational force acceleration between the concrete surface of the landmark and the vertical points at a height of 0.25 m, 0.50 m, 0.75 m and 1.0 m. Measurement positions are at the landmark and 4 points at the four corners on the concrete surface of the landmark. Summary table of measurement data and vertical gradient calculation according to Form No. 09 issued with this Circular.

2. The vertical gradient value at the measuring point is calculated by formula (1).

In which: δg is the difference of the acceleration due to gravity between the two measuring positions in the vertical direction; δH is the vertical distance between two measuring positions.

3. The accuracy of determining the vertical gradient at the landmark of the national gravity origin must be ≤ ± 0.003 mGal.

4. Accuracy of determining vertical gradient at baseline gravity landmarks must be ≤ ± 0.005 mGal.

5. The accuracy of determining vertical gradients at grade I gravity landmarks must be ≤ ± 0.010 mGal.

What are regulations on determination of coordinates and altitude for the national gravity net in Vietnam?

Pursuatn to Article 10 of the Circular 11/2020/TT-BTNMT stipulating determination of coordinates and altitude for the national gravity net in Vietnam as follows:

1. Determine coordinates.

a) Points in the national gravity network are determined with an accuracy equivalent to level 1 basic grid points using GNSS technology. The process of measurement and calculation shall be carried out in accordance with the provisions of Articles 10, 11 and 12 of the Circular No. 68/2015/TT-BTNMT dated December 22, 2015 of the Minister of Natural Resources and Environment stipulating on techniques of direct topographic measurement for the establishment of topographic maps and geographic base databases at scale 1:500, 1:1,000, 1:2,000, 1:5,000;

b) In case the gravity points built in the house cannot be directly determined by GNSS technology, it is allowed to measure the connection by measuring means which is an electronic total station, but it must be clearly stated in the technical design and estimates.

2. Determine the altitude.

a) For the basic gravity points: the design, measurement and calculation shall be carried out according to the grade II altimeter process. National elevation points as starting points are grade I, class II elevation points. The margin of error between the two high-ranking points does not exceed ± 4√L mm for the plains and ± 5 √L mm for the mountains (L is the length of the line measured in km);

b) For grade I and class II gravity points: the design, measurement and calculation shall be carried out according to the grade III altimeter measurement process. National elevation points as starting points are grade I, class II, and III elevation points. The margin of error between the two high-grade points does not exceed ± 10√L mm for the plains and ± 12√L mm for the mountains (L is the length of the line measured in km).

What are regulations on determination of the gravitational acceleration at the origin of national gravity, the base point of gravity in Vietnam?

Pursuatn to Article 11 of the Circular 11/2020/TT-BTNMT stipulating determination of the gravitational acceleration at the origin of national gravity, the base point of gravity in Vietnam as follows:

1. Preparation work.

a) Prepare the means of measuring absolute gravity, means of measuring gradients (or means of measuring relative gravity in service of measuring gradients) which have been tested and calibrated to meet the requirements as prescribed in Article 16 of this Circular. Gravity measuring instruments must be transported by special-use vehicles, avoiding vibrations and impacts from affecting the verified and calibrated gravimeters;

b) Prepare the room (or movable room) to place the gravimetric measuring device to ensure that it is closed; fully prepare battery, dehumidifier, thermometer, air conditioner (to stabilize temperature, humidity, pressure, power source during measurement);

c) Prepare computers, data backup devices, and measurement books according to Form No. 04a issued together with this Circular;

d) Determine the measurement time to ensure the best external conditions.

2. Measure the vertical gradient according to the provisions of Article 9 of this Circular.

3. Measurement of absolute gravity at the origin of national gravity:

a) Install means of measuring absolute gravity, centering, balancing the machine, checking the synchronization of measuring means;

b) Determine the height of the absolute gravity measuring device with an accuracy of 1mm;

c) Input coordinate values, altitude into the absolute gravity measuring device to serve the calculation of correction numbers;

d) Measure absolute gravity according to the measuring SET. The minimum measurement time of one measurement is 72 hours, corresponding to 72 measurement SETs, each measurement SET obtains a value of acceleration due to gravity. At the end of each measurement, the results obtained at the measurement point must be checked, in case the square error of the gravitational acceleration values ​​> ± 0.005 mGal, the measurement must be repeated;

dd) During the measurement of absolute gravity, the temperature and pressure at the beginning and the end of the measurement must be determined; The information about the measuring person, measuring shift, measuring time, and weather conditions must be fully recorded in the measuring book according to Form No. 04a issued together with this Circular.

4. Measure the absolute gravity at the base gravity point.

The procedure for measuring the acceleration of gravity at the base point of gravity is the same as for the measurement of the acceleration of gravity at the origin of the national gravity specified in Clause 3 of this Article. In which the minimum measurement time of one measurement is 24 hours corresponding to 24 SET measurements, in case the square error of the gravity acceleration values ​​> ± 0.010 mGal, the measurement must be repeated.

5. At the end of the measuring shift, the measurement results must be backed up to the data storage device enclosed with the measuring book for the calculation and delivery of the results.

Best regards!