Level and Leveling

LEVELLING

Levelling (or Leveling) is a branch of surveying, the object of which is: i) to find the elevations of given points with respect to a given or assumed datum, and ii) to establish points at a given or assumed datum. The first operation is required to enable the works to be designed while the second operation is required in the setting out of all kinds of engineering works. Levelling deals with measurements in a vertical plane.

Level surface: A level surface is defined as a curved surface which at each point is perpendicular to the direction of gravity at the point. The surface of a still water is a truly level surface. Any surface parallel to the mean spheroidal surface of the earth is, therefore, a level surface.

Level line: A level line is a line lying in a level surface. It is, therefore, normal to the plumb line at all points.

Horizontal plane: Horizontal plane through a point is a plane tangential to the level surface at that point. It is, therefore, perpendicular to the plumb line through the point.

Horizontal line: It is a straight line tangential to the level line at a point. It is also perpendicular to the plumb line.

Vertical line: It is a line normal to the level line at a point. It is commonly considered to be the line defined by a plumb line.

Datum: Datum is any surface to which elevation are referred. The mean sea level affords a convenient datum world over, and elevations are commonly given as so much above or below sea level. It is often more convenient, however, to assume some other datum, specially, if only the relative elevation of points are required.

Elevation: The elevation of a point on or near the surface of the earth is its vertical distance above or below an arbitrarily assumed level surface or datum. The difference in elevation between two points is the vertical distance between the two level surface in which the two points lie.

Vertical angle: Vertical angle is an angle between two intersecting lines in a vertical plane. Generally, one of these lines is horizontal.

Mean sea level: It is the average height of the sea for all stages of the tides. At any particular place it is derived by averaging the hourly tide heights over a long period of 19 years.

Bench Mark: It is a relatively permanent point of reference whose elevation with respect to some assumed datum is known. It is used either as a starting point for levelling or as a point upon which to close as a check.

Methods of levelling

Three principle methods are used for determining differences in elevation, namely, barometric levelling, trigonometric levelling and spirit levelling.

Barometric levelling

Barometric levelling makes use of the phenomenon that difference in elevation between two points is proportional to the difference in atmospheric pressures at these points. A barometer, therefore, may be used and the readings observed at different points would yield a measure of the relative elevation of those points.

At a given point, the atmospheric pressure doesn’t remain constant in the course of the day, even in the course of an hour. The method is, therefore, relatively inaccurate and is little used in surveying work except on reconnaissance or exploratory survey.

Trigonometric Levelling (Indirect Levelling)

Trigonometric or Indirect levelling is the process of levelling in which the elevations of points are computed from the vertical angles and horizontal distances measured in the field, just as the length of any side in any triangle can be computed from proper trigonometric relations. In a modified form called stadia levelling, commonly used in mapping, both the difference in elevation and the horizontal distance between the points are directly computed from the measured vertical angles and staff readings.

Spirit Levelling (Direct Levelling)

It is that branch of levelling in which the vertical distances with respect to a horizontal line (perpendicular to the direction of gravity) may be used to determine the relative difference in elevation between two adjacent points. A horizontal plane of sight tangent to level surface at any point is readily established by means of a spirit level or a level vial. In spirit levelling, a spirit level and a sighting device (telescope) are combined and vertical distances are measured by observing on graduated rods placed on the points. The method is also known as direct levelling. It is the most precise method of determining elevations and the one most commonly used by engineers.

Levelling Instruments

The instruments commonly used in direct levelling are:

A level

A levelling staff

Describe the different surveys to be carried out for the highway projects.

Describe the different surveys to be carried out for the highway projects.

i) Reconnaissance

ii) Preliminary Survey

iii) Location Survey

i) Reconnaissance:

During the reconnaissance survey the following factors have to be taken into consideration.

Ø Obstructions along the route.

Ø Gradients and length of curves.

Ø Cross drainage works.

Ø Soil type along the route.

Ø Sources of construction materials and

Ø Type of terrain.

ii) Preliminary Survey:

The preliminary survey in a highway project is done with the main objectives

Ø Various alternate arrangements

Ø Estimate the quantity of earth work materials and other construction aspects

Ø Compare different proposals.

The following surveys are constructed

Ø Primary transverse

Ø Topographical surveys

Ø Levelling work

Ø Hydrological data

Ø Soil surveys.

iii) Location Survey:

The final alignment decided after the preliminary survey is to be first located on the field by

establishing the centre line. Next the detailed survey should be carried out.

The detailed survey involves:

Ø Fixing temporary bench marks along the route for every 300 m.

Ø The cross sectional details are taken for 30 m on either side of the central line.

Ø All details of cross drainage works are taken.

Ø Topographical details are taken.

Ø Detailed soil survey is carried out.

Explain the methods of transferring reduced levels from surface to underground in a tunnel setting out work.

Explain the methods of transferring reduced levels from surface to underground in a tunnel setting out work. 

i) Setting out central line of tunnel

ii) Setting out inside tunnels

iii) Transferring of alignment through shafts

i) Setting out central line of tunnel:

The centre-line of tunnels are fixed on the surface along with shaft locations.

Generally the surface control points of the tunnels are not visible from each other. However, by the method of reciprocal ranging points on the summit can be established which can be joined to get the central line. The measurements should be made accurately. Linear measurements are made using invar substance bars with an accuracy of 1 in 10000. Angular measurements are made using 1 second theodolite with an accuracy 0f 15√ N where N is the number of angles. In case of N tunnels in hilly regions it is neither feasible to align the tunnel ends by direct ranging or reciprocal ranging. In such cases precise triangulation has to be used.

The figure shows a scheme of triangulation network with QR as base line for a tunnel

project. Here all the angles are measured accurately by one second theodolite. Usual corrections for length, temperature, terrain, sag and reduction of levels with respect to sea level are all followed in arriving at the values of the coordinates. The traverse is adjusted for angles and coordinates. The proposed tunnel axis is shown in figure as HR.

ii) Setting out Inside Tunnels:

After the coordinates of portals and shafts are finalized, setting out is started. Centre line of

tunnel is done as shown in figure from various portals and shafts.

Back sighting on the pillar, aligned and constructed as far as practicable on the extended

centre line such as pillar C and then by transiting. Reference points are constructed on the roof of tunnels or slightly below the invert for every 300 m.

iii) Transferring of alignment through shafts:

Transfer of alignment is done through shafts by adopting any one of the following methods:

i) By hanging two or more plumb lines down the shaft.

ii) By lighting directly from edge of shaft where shaft diameter to depth ratio is high.

Co-planning is done by hanging two or more plumb lines down the shaft and determining

the bearing of the plumb planes so formed which are connected to the surface. The plumb lines

should be well apart as for as possible. The plumb lines are of special type. The line shall be of

fine steel wire and carrying a symmetrical weight of 35 kg or more. The wire should be well

stretched to keep it tight. In order to keep the wires vertical, the bob should be contained in a

canister with a hood. This arrangement will shield the bob and will reduce oscillations set up by air currents or by water dropping down the shafts. The canister can be filled with water or oil to reduce the vibrations. The bearing of the plumb plane underground is assumed same as at the surface.

This forms the starting direction for the underground survey work.

The following procedure is adopted for transferring the centerline from top.

Ø A theodolite is set up on top of the hill at a suitable position to maintain the centre line of theshaft.

Ø The RLs of both the ends of the shaft are determined by a level. Knowing the bottom RLs ofthe ends the depth of the shaft is found.

Ø Excavation of the shaft is started and verticality is maintained with the help of the plumb-bob which is suspended from wires from top through pulleys.

Ø The excavation is continued until the required bottom level is reached. The depth of the shaft is measured by measuring the length of suspended wire.

Ø The centre line inside the tunnel is maintained by a precise theodolite. This type of

theodolites are provided with an artificial illumination system to enable work at night and in

the darkness of the tunnel.

Ø It should be properly taken care to see that the centre line is maintained from both ends and one transferred from top coincide.

Types of GIS

Types of GIS 
The following GIS types are not
necessarily mutually exclusive and a GIS application can be always classified under more
than one type.
2.1 Four-dimensional GIS
While spatio-temporal georepresentations can handle two dimensions of
space and one of time, four-dimensional GIS are designed for three dimensions of space and one
of time.
2.2 Multimedia/hypermedia GIS
Multimedia/hypermedia GIS allow the user to access a wide range of georeferenced
multimedia data (e.g., simulations, sounds and videos) by selecting resources from a
georeferenced image map base. A map serving as the primary index to multimedia data in a
multimedia geo-representation is termed a hypermap. Multimedia and virtual georepresentations
can be stored either in extended relational databases, object databases or in
application-specific data stores.
2.3 Web GIS
Widespread access to the Internet, the ubiquity of browsers and the explosion of
commodified geographic information has made it possible to develop new forms of multimedia
geo-representations on the Web.
Many current geomatics solutions are Web-based overtaking the traditional Desktop
environment and most future ones are expected to follow the same direction.
2.4 Virtual Reality GIS
Virtual Reality GIS have been developed to allow the creation, manipulation
and exploration of geo-referenced virtual environments, e.g., using VRML modelling
(Virtual Reality Modelling Language). Virtual Reality GIS can be also Web-based. Applications
include 3D simulation for planning (to experiment with different scenarios).

Different Types of Levels Used for Leveling in Surveying

Different Types of Levels Used for Leveling in Surveying

There are various types of levels for leveling in surveying. The process of measuring vertical distances in surveying is called leveling.

To perform leveling, we need some level instruments to focus or to read the object. Nowadays, the technology also introduced in surveying and so many easy measuring instruments are designed. Here we discuss about the different levels used in leveling.

Types of Levels Used in Leveling

Following are the types of different levels used for leveling in surveying:

Dumpy level

Y level

Cushing’s level

Tilting level

Cooke’s reversible level

Automatic level

Dumpy Level

Dumpy level is the most commonly used instrument in leveling. In this level the telescope is restricted against movement in its horizontal plane and telescope is fixed to its support. A bubble tube is provided on the top of the telescope.

But however, the leveling head can be rotated in horizontal plane with the telescope. The telescope is internal focusing telescope is a metal tube contains four main parts as given below.

Objective lens

Negative lens

Diaphragm

Eye-piece

Objective Lens

Objective lens should be made as the combination of crown glass and flint glass. Because of this some defects like spherical aberration and chromatic aberration can be eliminated. A thin layer coating which has smaller refractive index than glass is provided on the objective lens to reduce the loss due to reflection.

Negative Lens

Negative lens located co axial to the objective lens. So, the optical axis for both lenses is same.

Diaphragm

Diaphragm is fitted inside the main tube which contains cross hairs (vertical and horizontal) and these are adjusted by capstan headed screws. The cross hairs are made of dark metal as filament wires which are inserted in diaphragm ring in exact position. For stadia leveling purposes, extra two horizontal cross hairs are provided above and below the horizontal wire.

Eyepiece

Eyepiece lens enable the ability to sight the object together with cross hairs. The image seen through eye piece is magnified and inverted. Some eyepieces erect the image into normal view and those are called as erecting eyepieces.

Y Level

Y level or Wye-level consists y-shaped frames which supports the telescope. Telescope cane be removed from the y-shaped supports by releasing clamp screws provided. These y-shaped frames are arranged to vertical spindle which helps to cause the rotation of telescope.

Compared to dumpy level, adjustments can be rapidly tested in y- level. But, there may be a chance of frictional wear of open parts of level.

Cushing’s Level

In case of Cushing’s level, the telescope is restricted against rotation in its longitudinal axis and it is non-removable. But, the object end and eye piece end can be interchangeable and reversible.

Tilting Level

Tilting level consist a telescope which enabled for the horizontal rotation as well as rotation about 4 degree in its vertical plane. Centering of bubble can be easily done in this type of level. But, for every setup bubble is to be centered with the help of tilting screw.

The main advantage of tilting level is it is useful when the few observations are to be taken with one setup of level.

Cooke’s Reversible Level

Cooke’s reversible level is the combination of dumpy level and y-level. In this instrument, the telescope can be reversed without rotation the instrument. Collimation error can be eliminated in this case because of bubble left and bubble right reading of telescope.

Automatic Level

Automatic level is like the dumpy level. In this case the telescope is fixed to its supports. Circular spirit can be attached to the side of the telescope for approximate leveling. For more accurate leveling, compensator is attached inside the telescope.

Compensator can help the instrument to level automatically. Compensator is also called as stabilizer which consists two fixed prisms and it creates an optical path between eye piece and objective.

Due to the action of gravity, the compensator results the optical system to swing into exact position of line of sight automatically. But before the process of leveling, compensator should be checked.

To check the compensator, just move the foot screws slightly if the leveling staff reading remains constant then compensator is perfect. If it is not constant, then tap the telescope gently to free the compensator. Automatic level is also called as self-adjusting level.

Types of Leveling Methods used in Surveying

Types of Leveling Methods used in Surveying

There are various types of leveling used in surveying for measurement of level difference of different points with respect to a fixed point. This is useful in various civil engineering construction works where levels of different structures need to be maintained as per drawing.

What Is Leveling?

Leveling is a branch of surveying in civil engineering to measure levels of different points with respect to a fixed point such as elevation of a building, height of one point from ground etc.

Types of Leveling in Surveying

Direct leveling

Trigonometric leveling

Barometric leveling

Stadia leveling

Direct Leveling

It is the most commonly used method of leveling. In this method, measurements are observed directly from leveling instrument.

Based on the observation points and instrument positions direct leveling is divided into different types as follows:

Simple leveling

Differential leveling

Fly leveling

Profile leveling

Precise leveling

Reciprocal leveling

Simple Leveling

It is a simple and basic form of leveling in which the leveling instrument is placed between the points which elevation is to be find. Leveling rods are placed at that points and sighted them through leveling instrument. It is performed only when the points are nearer to each other without any obstacles.

Differential Leveling

Differential leveling is performed when the distance between two points is more. In this process, number of inter stations are located and instrument is shifted to each station and observed the elevation of inter station points. Finally difference between original two points is determined.

Fly Leveling

Fly leveling is conducted when the benchmark is very far from the work station. In such case, a temporary bench mark is located at the work station which is located based on the original benchmark. Even it is not highly precise it is used for determining approximate level.

Profile Leveling

Profile leveling is generally adopted to find elevation of points along a line such as for road, rails or rivers etc. In this case, readings of intermediate stations are taken and reduced level of each station is found. From this cross section of the alignment is drawn.

Precise Leveling

Precise leveling is similar to differential leveling but in this case higher precise is wanted. To achieve high precise, serious observation procedure is performed. The accuracy of 1 mm per 1 km is achieved.

Reciprocal Leveling

When it is not possible to locate the leveling instrument in between the inter visible points, reciprocal leveling is performed. This case appears in case of ponds or rivers etc. in case of reciprocal leveling, instrument is set nearer to 1^st station and sighted towards 2^nd station.

Trigonometric Leveling

The process of leveling in which the elevation of point or the difference between points is measured from the observed horizontal distances and vertical angles in the field is called trigonometric leveling.

In this method, trigonometric relations are used to find the elevation of a point from angle and horizontal distance so, it is called as trigonometric leveling. It is also called as indirect leveling.

Barometric Leveling

Barometer is an instrument used to measure atmosphere at any altitude. So, in this method of leveling, atmospheric pressure at two different points is observed, based on which the vertical difference between two points is determined. It is a rough estimation and used rarely.

Stadia Leveling

It is a modified form of trigonometric leveling in which Tacheometer principle is used to determine the elevation of point. In this case the line of sight is inclined from the horizontal. It is more accurate and suitable for surveying in hilly terrains.

TOTAL STATION –OPERATION, USES & ADVANTAGES

TOTAL STATION –OPERATION, USES & ADVANTAGES

  

What is a Total Station?

Total station is a surveying equipment combination of Electromagnetic Distance Measuring Instrument and electronic theodolite. It is also integrated with microprocessor, electronic data collector and storage system. The instrument can be used to measure horizontal and vertical angles as well as sloping distance of object to the instrument.

Capability of a Total Station:

Microprocessor unit in total station processes the data collected to compute:

Average of multiple angles measured.

Average of multiple distance measured.

Horizontal distance.

Distance between any two points.

Elevation of objects and

All the three coordinates of the observed points.

Data collected and processed in a Total Station can be downloaded to computers for further processing.

Total station is a compact instrument and weighs 50 to 55 N. A person can easily carry it to the field. Total stations with different accuracy, in angle measurement and different range of measurements are available in the market. Figure below shows one such instrument manufactured by SOKKIA Co. Ltd. Tokyo, Japan.

Fig: Parts of total station

Brief Description of Important Operations of Total Station:

Distance Measurement:

Electronic distance measuring (EDM) instrument is a major part of total station. Its range varies from 2.8 km to 4.2 km. The accuracy of measurement varies from 5 mm to 10 mm per km measurement. They are used with automatic target recognizer. The distance measured is always sloping distance from instrument to the object. Angle Measurements: The electronic theodolite part of total station is used for measuring vertical and horizontal angle. For measurement of horizontal angles any convenient direction may be taken as reference direction. For vertical angle measurement vertical upward (zenith) direction is taken as reference direction. The accuracy of angle measurement varies from 2 to 6 seconds.

Data Processing :

This instrument is provided with an inbuilt microprocessor. The microprocessor averages multiple observations. With the help of slope distance and vertical and horizontal angles measured, when height of axis of instrument and targets are supplied, the microprocessor computes the horizontal distance and X, Y, Z coordinates. The processor is capable of applying temperature and pressure corrections to the measurements, if atmospheric temperature and pressures are supplied.

Display:

Electronic display unit is capable of displaying various values when respective keys are pressed. The system is capable of displaying horizontal distance, vertical distance, horizontal and vertical angles, difference in elevations of two observed points and all the three coordinates of the observed points.

Electronic Book:

Each point data can be stored in an electronic note book (like compact disc). The capacity of electronic note book varies from 2000 points to 4000 points data. Surveyor can unload the data stored in note book to computer and reuse the note book.

Use of Total Station

The total station instrument is mounted on a tripod and is levelled by operating levelling screws. Within a small range instrument is capable of adjusting itself to the level position. Then vertical and horizontal reference directions are indexed using onboard keys. It is possible to set required units for distance, temperature and pressure (FPS or SI). Surveyor can select measurement mode like fine, coarse, single or repeated.

When target is sighted, horizontal and vertical angles as well as sloping distances are measured and by pressing appropriate keys they are recorded along with point number. Heights of instrument and targets can be keyed in after measuring them with tapes. Then processor computes various information about the point and displays on screen.

This information is also stored in the electronic notebook. At the end of the day or whenever electronic note book is full, the information stored is downloaded to computers.

The point data downloaded to the computer can be used for further processing. There are software like auto civil and auto plotter clubbed with AutoCad which can be used for plotting contours at any specified interval and for plotting cross-section along any specified line.

Advantages of Using Total Stations

The following are some of the major advantages of using total station over the conventional surveying instruments:

Field work is carried out very fast.

Accuracy of measurement is high.

Manual errors involved in reading and recording are eliminated.

Calculation of coordinates is very fast and accurate. Even corrections for temperature and pressure are automatically made.

Computers can be employed for map making and plotting contour and cross-sections. Contour intervals and scales can be changed in no time.

However, surveyor  should check the working condition of the instruments before using. For this standard points may be located near survey office and before taking out instrument for field work, its working is checked by observing those standard points from the specified instrument station.