Site Surveying and Levelling
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Because of this, if the horizontal crosshair of the instrument is lower than the base of the rod, the surveyor will not be able to sight the rod and get a reading. The rod can usually be raised up to 25 feet high, allowing the level to be set much higher than the base of the rod. The curvature of the earth means that a line of sight that is horizontal at the instrument will be higher and higher above a spheroid at greater distances.
The effect may be insignificant for some work at distances under meters. The line of sight is horizontal at the instrument, but is not a straight line because of atmospheric refraction. The change of air density with elevation causes the line of sight to bend toward the earth. The combined correction for refraction and curvature is approximately: . For precise work these effects need to be calculated and corrections applied. For most work it is sufficient to keep the foresight and backsight distances approximately equal so that the refraction and curvature effects cancel out.
Refraction is generally the greatest source of error in leveling. Assuming error-free measurements, if the Earth's gravity field were completely regular and gravity constant, leveling loops would always close precisely:.
In the real gravity field of the Earth, this happens only approximately; on small loops typical of engineering projects, the loop closure is negligible, but on larger loops covering regions or continents it is not. For precise leveling networks on a national scale, the latter formula should always be used.
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The dumpy level was developed by English civil engineer William Gravatt , while surveying the route of a proposed railway line from London to Dover. More compact and hence both more robust and easier to transport, it is commonly believed that dumpy levelling is less accurate than other types of levelling, but such is not the case. Dumpy levelling requires shorter and therefore more numerous sights, but this fault is compensated by the practice of making foresights and backsights equal. Precise level designs were often used for large leveling projects where utmost accuracy was required.
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They differ from other levels in having a very precise spirit level tube and a micrometer adjustment to raise or lower the line of sight so that the crosshair can be made to coincide with a line on the rod scale and no interpolation is required. Automatic levels make use of a compensator that ensures that the line of sight remains horizontal once the operator has roughly leveled the instrument to within maybe 0.
The surveyor sets the instrument up quickly and does not have to relevel it carefully each time he sights on a rod on another point. It also reduces the effect of minor settling of the tripod to the actual amount of motion instead of leveraging the tilt over the sight distance. Three level screws are used to level the instrument.
Surveying and Levelling Instruments 1 | Nature
This style is widely used in construction work but not for more precise control work. An advantage is that one person can perform the levelling independently, whereas other types require one person at the instrument and one holding the rod. From Wikipedia, the free encyclopedia. This article is about the surveying technique. Book this course now.
Who should attend Suitable for: trainee engineers graduate engineers construction managers site managers who would like to be able to supervise, recruit for, check or carry out levelling activities people with direct experience of the construction industry who would like to broaden their skills to include levelling and setting out by tape. Course content This course teaches the principles of levelling and setting out by tape which can be applied to all aspects of construction including earthworks, roads, buildings, foundations, piles, structural steel, structures, reinforced concrete, houses.
Actual construction drawings are used for all practical exercises. Levelling By the end of the course, delegates will be able to: Record work correctly and in the industry standard way Incorporate robust checks to all levelling List the sources of error in levelling Carry out a level survey existing features or as-built Set elements to a fixed level Measure the reduced level of ceilings and soffits Transfer TBMs install temporary benchmarks from scratch Check that the level is in correct calibration Set up profile boards for level excavation Set up profile boards for drainage Set up batter rails for cut and fill Taping By the end of the course, delegates will be able to: List the sources of error in taping Incorporate checks when setting out using a tape measure Set out on sloping ground Set out right angles and rectangles.
Mandatory Entry requirements You will need a reasonable level of mobility and the ability to lift and move loads. Please tell us beforehand if you have a medical ailment that may affect your ability to do manual work. Calculate their elevations as. Record all your measurements in a table. This table may also include plan-surveying information, such as azimuths and horizontal distances. You might also use two different tables as explained in step The first line of the table will refer to the known point X.
This point can be one of the perimeter points which you have already determined, or it can be a benchmark see step You find the position of point 0 from the azimuth of line OX and the horizontal distance OX. This method combines radiating with a closed traverse. You can use it to gather the information you need to make a topographical map of a land area such as a fish- farm site see Chapter 9. Using what you have learned so far about surveying, do the following: a With a closed traverse, plan survey the boundaries of the area ABCDEA. Find the lengths and directions of all of its sides see Section 7.
Survey the boundaries d Join all the selected levelling stations by straight lines to form a closed traverse.
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Survey it, using turning points as necessary, to fix the position of each station and to determine its elevation. Check for the closing error see Section 7. Use your magnetic compass and ranging poles or stakes. Mark on the ground the north-south line. You will call this the zero-degree line. Note: the fixed-angle interval you use depends on how accurate a survey you need. Smaller angles will help you make a more accurate map of the site.
You will need two additional columns in this table:. Topographical survey of partial area by composite radiating. You can also make topographical surveys along straight lines by using non-sighting level s , such as the line level see Section 5. You have already learned how to measure height differences by using the square-grid method with such levels see this section, step Remember , when you lay out your grid, that the distance between points cannot be more than the length of your level. Topographical survey with a line level 20 m.
Record the measurements in a table for each levelled section.
You will be measuring horizontal distances from one point to the next, and differences in elevation between one point and the next. At both the starting point and the last point, there is only one height measurement. The rear person will measure it on the starting point, and the front person will measure it on the last point. Find the cumulated distances from the starting point and the elevations of each point, as shown in the example. There are three possible checks , which you make at the bottom part of the table. As you have just learned, you will always start differential levelling surveys by measuring a height on a ground point of known or assumed elevation.
Levelling in surveying
This point becomes a bench-mark BM. The elevation of this bench-mark will form the basis for finding the elevation of the other points you need to survey in the area.
A bench-mark should be permanent. You should always establish at least one bench-mark near the construction site of a fish-farm to act as a fixed reference point or object. You may also use a bench-mark as a turning point during topographical surveys. A bench-mark should be a very well-defined poin t. You should be able to find and recognize it easily. It should be easy to reach, so that you can hold a levelling staff on it.