Levelling survey

There are many special and non-special terms commonly used in levelling. Some of these are highlighted in this section without emphasis and detailed explanation. Other terms will be explained in each section they appear. These terms are;

1. Instrument station: is the point where instrument is set up for observations
2. Station: is the point where levelling staff is held. It is the point whose elevation is to be determined or the point that is to be established at a given elevation.
3. Height of instrument (HI): is the elevation of the line of sight with respect to the assumed datum. It is also referred to as height of collimation (HC). In levelling it does not mean the height of the telescope above the ground level where the level is set up.
4. Back sight (BS): is the first sight taken on a levelling staff held at point of known elevation. It ascertains the amount by which the line of sight is above or below the elevation of the point. Back sight enables the surveyor to obtain the height of the instrument.
5. Fore sight (F.S): is the sight taken on a levelling staff held at a point of unknown elevation to ascertain the amount by which the point is above or below the line of sight. Fore sight enables the surveyor to obtain the elevation of the point. It is also generally known as minus sight as the foresight reading is always subtracted from the height of the instrument (except when the staff is held inverted) to obtain the elevation.
6. Change point (CP): is a point on which both the fore sight and back sight are taken during the operation of levelling. Two sights are taken from two different instrument stations, a fore sight to ascertain the elevation of the point while a back sight is taken on the same point to establish the height of the instrument of the new setting of the level.
7. Intermediate sight (IS): is the fore sight taken on a levelling staff held at a point between two change points or a benchmark and a change point, to determine the elevation of that point. It may be noted that are stations sighted between BS and FS.
8. Bench mark (BM): permanent and fixed reference point of refers to a relatively known elevation above the assumed datum.
9. Line of collimation (LC): refers to the line passing through the optical centre of the objective and the point of intersection of the cross hairs stretched in front of the eye piece and its continuation.
10. Line of sight (LS): refers to a line passing through the optical centre of the objective, traversing the eye-piece and entering the eye.
11. Vertical datum: refers to any level surface to which elevations are referenced. Means sea level is one of the vertical datum assigned on elevation or reduced level of zero.
12. Mean sea level datum (M.S.L): is a mean sea level obtained by making hourly observations of the tides at any place over a period of 19 years. The M.S.L datum adopted by the survey of India for determining the elevations of different points in India is that of Mumbai. In East Africa, the datum line is situated in Mombasa, while in Nigeria it is in Lagos and for South Africa it is in Cape Town.
13. Reduced Level (R.L): is the height or depth of a point above or below the assumed datum. It is also known as elevation of the point. Elevations of the points below the datum surface are known as negative elevations.
14. Vertical line: is a line that follows the local direction of gravity as indicated by a plumb line.
15. Level surface: is a curved surface that at every point is perpendicular to the local plumb line (i.e., the direction in which gravity acts).
16. Level line: is a curved line on a level surface.
17. Horizontal place: is a plane perpendicular to the local direction of gravity and tangential to the level surface.
18. Horizontal line: is a line on a horizontal which is perpendicular to the vertical line.
19. Vertical control: is a series of benchmarks or other points of known elevation established throughout an area.

Traditional levelling survey requires three major equipments for its operations. The equipments includes; a level, tripod stand and a levelling staff. A level is an instrument with a telescope and bubble tube and levelling head used for sighting targets, particularly, levelling staff . A telescope is an optical instrument mounted on levels to magnify and view distant objects. It provides a line of sight. A bubble tube is used together with the levelling head to make the line of sight horizontal by bringing the bubble to the center of its run.

Components of a level instrument

There are three basic types of level instruments, namely: Dumpy levels, engineers' or tilting levels and automatic levels. The first two are spirit bubble levels. The dumpy level is a very simple basic instrument, while the tilting level has certain modifications which give it greater convenience of operation and a possibility of greater clarity. However, the most often used and "quick set" level is a tilting level without foot-screws. The automatic type of levels which gives a horizontal line of sight automatically, are the most favorable to use, but they are relatively expensive.

1. Dumpy level: is characterized by their telescope being rigidly attached to the vertical spindle. The levelling of the instrument is done by means of three-foot screws separating their two plates. The upper plate with the vertical spindle on which the telescope and bubble tube are mounted has to be levelled with the foot screws.

   

2. The engineer's level: also known as tilting level has a telescope which is not rigidly fixed to the vertical spindle . Instead, the telescope is capable of tilting slightly in the vertical plane about a point just below the telescope. This vertical movement of the telescope is made by rotating a tilting screw below the eyepiece. They have two bubbles, circular bubble on the upper plate is used to achieve approximate levelling by means of the three foot screws and telescope bubble (tube bubble) to be levelled for each sighting by the tilting screw only. This type of levels exists in two features, the "quickest" and normal tilt levels. Contrary to the "normal" tilting level the "quickset" level does not have foot screws in the levelling head. A ball-and-socket joint is provided to level the instrument quickly, but only approximately. Accurate levelling of the instrument must be completed with the tilting screw for each sighting.

   

3. The automatic level: automatic levels are fairly advanced than dumpy and tilting levels. They are provided with either foot screws or a ball-and-socket joint. However, they do not have a bubble tube for precise levelling, instead they use compensators to automatically to make the line of sight horizontal once an approximate levelling has been achieved. Setting up and approximate levelling is done with the circular bubble in the same way as for a tilting level.

   

4. Tripod stand: is an important component in any levelling process for it carries a level instrument during levelling survey. The level is mounted on a tripod stand which consists of three solid wooden or aluminum framed legs. At the lower ends, the legs are provided with pointed iron shoes for driving them on the ground during the levelling process.

   

5. Levelling staffs: are wooden or metallic rods, graduated into metre or feet and further smaller divisions of 10 mm intervals and marked in red, black and white for easy reading . The staffs are available in 3 m to 5 m long. The bottom of the levelling staff represents the zero reading. The reading given by the line of sight on a levelling staff held vertically is the vertical distance above the point on which the staff is held. Levelling staffs may be grouped into two classes; self-reading staffs and target staffs.

   

6. Theodolite and total station: although levels are still highly in use, it is not because they are the only equipment that can be used for levelling. Technological development has provided options to other advanced and sophisticated equipment like Theodolites and Total stations.

   

Electronic theodolite

Total station

Different books classify levelling differently based on different perspectives and context. All levelling practice can be divided into direct levelling which involves all methods which take vertical distances direct from the field and indirect levelling in which vertical distances are not measured directly in the field. Direct levelling is of two types namely

1. differential levelling
2. profile levelling.

Differential levelling is carried out to determine the elevation of a distant point that cannot be determined with a single set up of the instrument. It involves setting up a level several times to take reading along the route between the benchmark and the distant point. At each instrument set up, only two staff readings, a back sight and a fore sight are observed.

Profile levelling is another category of direct levelling which determines elevations of a series of points along a line before shifting a level to another station. In this type of levelling, an instrument can determine elevation of several points between a back sight and fore sight. As such a back sight, a fore sight and as many intermediate sights as possible are taken in profile levelling . Profile levelling is particularly considered important in drainage and terrace layout.

Levelling skills and knowledge are of great demand in our daily life. Apart from the general problem of determining the difference in levels between two points, the main applications of levelling are in contouring, preparation of longitudinal and cross-sectional profiles and setting out of levels in construction works.

Contouring is the process of creating contours. Contours are imaginary lines connecting points of equal elevation. Contouring can be done using

1. direct method
2. indirect method.

In the direct method of contouring, points defining a contour line are pegged out on the ground and their horizontal positions determined. The contouring method involves the following procedure:

1. A level is set at a convenient position in the area and a BS reading is taken at an appropriate bench mark.
2. The height of collimation is found and the required staff reading for a contour line is calculated. The required staff reading is determined by subtracting a contour on the height of collimation staff reading (IS or FS) = HC – contour value.
3. The instrument man asks staff man to move up and down in the area till the required staff reading is found. In Figure 3.81, a staff reading of 1.3 m is needed to mark a contour of 101.5 m. The staff was first held at point 1 and its reading was higher than 1.3 m, thus it was moved to points of higher elevations until a staff reading of 1.3 m was taken.

Then, a surveyor determines the horizontal coordinates of that point using chain surveying, compass surveying, plane table surveying or other advanced surveying. The method is very accurate but, slow and tedious.

In the indirect contouring method, staff readings are taken at some selected points and their levels are reduced. That is the horizontal position is established first and then the levels of those points are found. After locating the points on the plan, reduced levels are marked and contour lines are interpolated between the selected points. During the interpolation it is assumed that the ground is uniform between any two spot heights. The indirect method is further classified into grid and radiation methods based on the approach used to select points for staff reading.

Grid method

Grid method is best suited to gently sloping undulating land and fairly small areas. In this method the area to be surveyed is divided into grid or series of squares. The grid corners are marked on the ground and their spot heights are determined through levelling. The grid is plotted to the scale and the spot heights of grid corners are entered. The contours of desired values are then located by interpolation.

Radiation method

Radiation method is useful for areas located in small hills. The method establishes a central position on a hill and set out radiating lines using the prismatic compass, theodolite or total station. Points are then marked on ground at a regular interval along the radiating line and their spot heights are determined through levelling. The radiating lines and the marked points are plotted to scale and the spot heights of the marked stations are entered. The contours of the desired values are then located by interpolation

In leveling, errors can happen during measurement or recording. Here are common errors in leveling explained in simple terms:

1. Incorrect Setup of the Level Instrument

   If the level is not set up on stable ground or is not properly leveled, the measurements will be wrong. Always make sure the tripod is steady and the bubble is centered.

2. Mistakes in Reading the Staff

   Misreading the numbers on the leveling staff (too high or too low) can lead to errors. Always check the reading carefully.

3. Parallax Error

   This happens when the crosshairs of the level are not aligned correctly with the staff. Move your eye until the crosshairs and numbers line up.

4. Human Errors in Recording

   Writing down the wrong number or mixing up the backsight and foresight readings can cause mistakes. Double-check your notes.

5. Incorrect Position of the Level Instrument

   Placing the level too far from the staff or in a location where the line of sight is blocked can cause errors. Always position the level where you have a clear view.

6. Staff Not Held Upright

   If the leveling staff is leaning and not vertical, the reading will be inaccurate. Use a bubble level on the staff to keep it straight.

7. Errors Due to Weather Conditions

   Heat or wind can cause slight movement in the level or the staff, leading to errors. Work during calm weather when possible.

8. Failure to Perform Check Levels

   Not re-checking measurements at the end of leveling can cause unnoticed errors. Always repeat some readings to ensure accuracy.