Measuring instruments must be calibrated and within their calibration period before being put into use. This is crucial for ensuring project quality, accelerating project progress, reducing labor intensity, and creating conditions for the automation of construction machinery. With the development of the construction industry, the scale of projects is expanding, the height of buildings is increasing, and the degree of mechanization and automation in construction is constantly improving. As a result, laser measuring instruments have developed rapidly and are widely used in various construction surveys, placing higher demands on surveying work.
I. Commonly Used Measuring Instruments and Their Functions
Commonly used surveying instruments in water conservancy and hydropower engineering construction include level instruments, theodolites, electromagnetic distance measuring instruments, total stations, and global positioning systems (GPS).
(I) Classification and Function of Leveling Instruments
Leveling instruments can be classified into ordinary levels and precision levels according to their accuracy. Domestically produced levels are further categorized by accuracy, including models such as DS05, DS3, and DS10. In engineering surveying, the DS3 type micro-tilt ordinary level is generally used. "D" and "S" represent the first letters of the Chinese pinyin for "geometry" and "leveling instrument," respectively, and the number 3 indicates the instrument's accuracy, specifically a random mean error of ±3mm for the mean height difference measured per kilometer. Other types include automatic leveling instruments and digital levels.
A level is used for leveling. Leveling is a surveying process that uses a horizontal line of sight provided by a level to measure the elevation difference between two points on the ground with the help of a ruler with graduations. Then, based on the measured elevation difference and the elevation of a known point, the elevation of the other point is calculated.
(II) Classification and Functions of Theodolites
Theodolites can be classified according to their accuracy, such as DJ07, DJ1, DJ2, DJ6, and DJ10. D and J are the first letters of the Chinese pinyin for "geometry" and "theodolite," respectively, and the numbers 07, 1, 2, 6, and 10 indicate the instrument's accuracy. They can also be classified into two categories based on their reading devices: micrometer scale reading devices and single-plate glass micrometer reading devices.
Theodolites are the primary instruments for angle measurement. They include horizontal and vertical angle measurement; horizontal angles are used to determine the planar position of points on the ground, while vertical angles are used to determine the elevation of points. Additionally, theodolites can also be used for stadia measurement in low-precision surveying.
(III) Classification and Functions of Electromagnetic Wave Distance Meters
Electromagnetic rangefinders can be classified according to the carrier wave they use: microwave rangefinders that use microwave radio waves as carrier waves; laser rangefinders that use lasers as carrier waves; and infrared rangefinders that use infrared light as carrier waves. The latter two are collectively referred to as photoelectric rangefinders.
Electromagnetic distance measuring instruments use electromagnetic waves (light waves or microwaves) as carrier waves to transmit distance measurement signals in order to measure the distance between two points. They are generally used for small-area control surveying, topographic surveying, cadastral surveying, and engineering surveying.
(IV) Total Station and its Functions
A total station is an automated, digital, and intelligent three-dimensional coordinate measurement and positioning system that integrates automatic distance measurement, angle measurement, calculation, and automatic data recording and transmission functions.
The total station is used to measure horizontal angles, zenith distance (vertical angle), and slope distance. With the help of the built-in software, it can perform a variety of measurement functions, such as calculating and displaying horizontal distance, elevation difference, and the three-dimensional coordinates of the mirror station, and performing eccentricity measurement, suspension height measurement, side measurement, and area calculation.
(v) Global Positioning System (GPS)
The Global Positioning System (GPS) is a next-generation satellite navigation and positioning system with all-weather, real-time, three-dimensional navigation and positioning capabilities across sea, land, and air. GPS is characterized by its all-weather operation, high precision, automation, and high efficiency. It is widely used in geodesy, urban and mining control surveying, building deformation measurement, and underwater topographic surveying.
II. Use of Common Measuring Instruments
(I) Use of a Leveling Instrument
1. The steps for using a tilting level include setting up the instrument and rough leveling (referred to as rough leveling), focusing and aiming, precise leveling (referred to as fine leveling), and reading the value.
(1) Setting up the level and coarse leveling. First, select a flat and firm ground as the setting point for the level. Then, spread out the tripod to a suitable height, with the tripod head roughly horizontal. Then, use the connecting screws to fix the level to the tripod head and firmly press the toes of the tripod legs. Adjust the three leveling screws to center the circular bubble level, which is called coarse leveling.
(2) Focusing and aiming. After leveling the instrument, point the telescope at a bright background and rotate the eyepiece focusing screw to make the crosshairs clear. Use the telescope's front and rear sights to aim at the leveling rod, and then tighten the brake screw to fix the telescope. Rotate the objective lens focusing screw until the leveling rod image is clear, and then rotate the horizontal fine adjustment screw to align the vertical crosshairs with the leveling rod. After aiming at the target, you can move your eye up and down through the eyepiece. If you find that the crosshairs and the target image move relative to each other, and the reading changes with the movement of your eye, it indicates parallax. The cause of parallax is that the target image does not coincide with the crosshair reticle, which will affect the accuracy of the reading. Parallax must be eliminated. The method is to first adjust the eyepiece focusing screw to see the crosshairs clearly, and then continue to carefully rotate the objective lens focusing screw until the image of the rod coincides with the plane of the crosshairs.
(3) Fine leveling. Rotate the micro-tilt screw while observing the bubble observation window of the level tube. When the bubble image matches, it indicates that the leveling has been precise.
(4) Reading. When the level bubble is centered, immediately read the value on the leveling rod according to the middle crosshair. Regardless of whether the level instrument used is upright or inverted, the reading is always read from the smaller end of the marking to the larger end. Usually, the reading is retained to four digits.
2. Operating Procedures of a Precision Level. The procedure is basically the same as that of a standard DS3 level, except that the precision level uses an optical micrometer to measure values less than one division. During operation, first rotate the micro-tilt screw to make the images of the two ends of the coincidence bubble tube on the left side of the telescope's field of view coincide, ensuring the line of sight is horizontal. Then rotate the micrometer wheel to precisely clamp the wedge wire on the crosshair onto the entire division, and read the reading for that division.
3. Automatic leveling instrument operation procedure: coarse leveling - aiming - reading.
4. The operating procedure of a digital level is basically the same as that of an automatic level, but a digital level can automatically observe and record, and display the measurement results in digital form.
(II) Use of theodolite
The use of a theodolite involves four operational steps: centering, leveling, aiming, and reading.
1. Centering and leveling
There are two methods: one using a plumb bob for centering and the other using an optical centering device for centering and the other using an optical centering device for leveling.
(1) Method of centering with a plumb bob and leveling with a theodolite
Centering the plumb bob: First, unfold the tripod and place it on the measuring station. The tripod length should be appropriate for easy observation; the tripod head should be roughly horizontal. Place the connecting screw on the tripod head in the center position, hang the plumb bob, and move the tripod so that the tip of the plumb bob is roughly aligned with the measuring station, while keeping the tripod head roughly horizontal. Take the instrument out of the case and place it on the tripod, tightening the connecting screw to connect the instrument to the tripod. Now carefully observe whether the plumb bob deviates from the center of the mark. If it does, slightly loosen the connecting screw, move the instrument horizontally on the tripod head, and accurately align the tip of the plumb bob with the measuring station, then tighten the connecting screw again.
Leveling. First, rotate the instrument's aiming head so that the level tube is parallel to the line connecting any two leveling screws. Rotate these two leveling screws to center the bubble. Then, rotate the instrument's aiming head 90° and rotate the third leveling screw to center the bubble. Repeat this process several times until the bubble is centered no matter which position the instrument is in.
(2) Methods of centering with an optical centering device and leveling with a theodolite
Visually assess the initial centering and ensure the tripod head is roughly horizontal;
Rotate and push/pull the eyepiece of the centering device to focus, making the ground markers clear and the center circle on the reticle clearly visible;
Rotate the instrument's leveling screws until the image of the ground marker is centered within the circle;
Adjust the tripod legs with telescopic adjustment to center the bubble in the circular spirit level;
Perform precise leveling using the leveling method for placing the instrument with a plumb bob;
Check the optical centering device. If the mark is located in the center of the circle, the centering and leveling are complete. If there is still a deviation, loosen the connecting screw slightly and move the instrument on the stand to make it accurately centered. Then, perform precise leveling again until the centering and leveling meet the requirements.
2. Aim
(1) Eyepiece focusing: Point the telescope at a bright background and rotate the eyepiece focusing screw to make the crosshairs clear.
(2) Coarse aiming at the target: Loosen the horizontal and vertical brake screws of the telescope, aim at the target through the coarse aiming device on the telescope, and then tighten the brake screws.
(3) Objective lens focusing: Rotate the objective lens focusing knob of the telescope to make the target image clear. Pay attention to eliminating parallax.
(4) Aim accurately at the target: Rotate the horizontal and vertical micro-motion screws to make the vertical crosshair bisect the single line of the target image or clamp the double crosshair, and make the intersection of the crosshairs aligned with the bottom of the target.
3. Reading
Open the reflector and adjust its position to ensure bright and uniform light enters the reading window. Then, focus the reading microscope to ensure clear divisions within the reading window before taking the reading. Electronic theodolites allow direct readings on the screen.
(III) Use of Electromagnetic Distance Meters
1. To measure the distance D between points A and B, first set up the theodolite at point A, center and level it, and then place the rangefinder above the theodolite's telescope.
2. Place the reflector at point B.
3. Aim at the reflector.
4. Set the unit, prism type, and scale correction switch to the desired position.
5. Distance measurement.
6. In addition to measuring distances as described above, the keyboard can also be used to calculate horizontal distance, elevation difference, and coordinate increments by inputting relevant data.
(iv) Use of total station
The total station's stakeout mode has two functions: determining stakeout points and setting new points using known coordinate data stored in memory. If the coordinate data is not stored in memory, it can also be entered from the keyboard. The stakeout steps are as follows:
1. Select the data acquisition file to store the collected data in that file.
2. Select the coordinate data file to call up the station coordinate data and backsight coordinate data.
3. Set up test sites.
4. Set the backsight point and determine the azimuth angle.
5. Enter the required layout coordinates and start the layout.
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