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.

Surveying Principles and Methods in Civil Engineering

Surveying Principles and Methods in Civil Engineering

Surveying is the process of determining relative positions of different objects on the surface of the earth by measuring horizontal distances between them and preparing a map to any suitable scale.

Measurements are taken in horizontal plane only. Surveying is the art of determining the relative positions of points on, above or beneath the surface of the earth by means of direct or indirect measurements of distance, direction and elevation.

Leveling is a branch of surveying the object of which is to find the elevations of points with respect to a given or assumed datum and to establish points at a given elevation or at different elevations with respect to a given or assumed datum.

Survey is performed to prepare a map of relative positions on surface of the earth. It shows the natural feature of a country such as towns, villages, roads, railways, etc. They may also include details of different engineering works such as bridge, canal, dam etc.

Before commencement of any major engineering project, survey is carried out to determine the location of site and its surrounding. To understand the topography of the site and natural resource availability. Survey is carried out to prepare the cadastral map showing property boundaries.

Principles of Surveying

The fundamental principle upon which the various methods of plane surveying are based can be stated under the following two aspects.

Location of a point by measurement from two points of reference

According to this principle, the relative position of a point to be surveyed should be located by measurement from at least two points of reference, the positions of which have already been fixed.

If P and Q are the two reference points on the ground, any other point, such as R, can be located by any of the direct methods shown in the above figures. But, although a single method is sufficient to locate the relative position of ‘R’ with respect to reference points P and Q, it is necessary to adopt at least any two methods to fix the position of point ‘R’.

While the measurements made in the either of the first method or second method will be helpful in locating the point ‘R’, the measurements made in the other method will act as a check.

Working from whole to part

According to this principle, it is always desirable to carryout survey work from whole to part. This means, when an area is to be surveyed, first a system of control points is to be established covering the whole area with very high precision. Then minor details are located by less precise methods.

The idea of working this way is to prevent the accumulation of errors and to control and localize minor errors which, otherwise, would expand to greater magnitudes if the reverse process is followed, thus making the work uncontrolled at the end.

Method of Surveying in Civil Engineering

Method of Surveying in Civil Engineering

Primary types of Surveying are:

Plane surveying

Geodetic surveying

1. Plane surveying

Plane surveying is conducted by state agencies as well as private agencies. As we know earth is spherical in shape but its diameter is big enough to consider plane in small dimensions. It is that type of surveying in which the mean surface of the earth is considered as a plane and the spheroidal shape is neglected. All triangles formed by survey lines are considered as plane triangles. The level line is considered as straight and plumb lines are considered parallel. Plane surveying is done of the area of survey is less than 250 km².

2. Geodetic surveying

Geodetic survey is conducted by survey department of the country. It is that type of surveying in which the curved shape of the earth is taken in to account. The object of geodetic survey is to determine the precise position on the surface of the earth, of a system of widely distant points which form control stations in which surveys of less precision may be referred. . Line joining two points is considered as curved line and angles are assumed as spherical angles. It is carried out if the area exceeds over 250 km².

Secondary classification of Surveying

Surveys may be classified based on the nature of the field of survey, object of survey and instruments used.

1) Surveying based on Nature of Survey

a) Topographical Surveys

They are carried out determine the position of natural features of a region such as rivers, streams, hills etc. and artificial features such as roads and canals. The purpose of such surveys is to prepare maps and such maps of are called topo-sheets.

b) Hydrographic Survey

Hydro-graphic survey is carried out to determine M.S.L. (Mean Sea Level), water spread area, depth of water bodies, velocity of flow in streams, cross-section area of flow etc.

c) Astronomical Survey

The Astronomical Survey is carried out to determine the absolute location of any point on the surface of earth. The survey consists of making observations to heavenly bodies such as stars.

d) Engineering Survey

This type of survey is undertaken whenever sufficient data is to be collected for the purpose of planning and designing engineering works such as roads, bridges and reservoirs.

e) Archeological Survey

This type of survey is carried out to gather information about sites that are important from archeological considerations and for unearthing relics of antiquity.

f) Photographic Survey

In this type of survey, information is collected by taking photographs from selected points using a camera.

g) Aerial Survey

In this type of survey data about large tracks of land is collected by taking photographs from an aero-plane.

h) Reconnaissance Survey

In this type of survey, data is collected by marking physical observation and some measurements using simple survey instruments.

2) Surveying based on Type of Instruments

a) Chain Surveying

Chain surveying is the basic and oldest type of surveying. The principle involved in chain survey is of triangulation. The area to be surveyed is divided into a number of small triangles. Angles of triangles must not be less than 30 degree and greater than 120 degree. Equilateral triangles are considered to be ideal triangles. No angular measurements are taken, tie line and check lines control accuracy of the work.

This method is suitable on level ground with little undulations and area to be survey is small.

b) Compass Surveying

Compass survey uses the principle of traversing. This method does not requires the need to create triangles. It uses a prismatic compass for measuring magnetic bearing of line and the distance is measured by chain. A series of connecting lines is prepared using compass and measuring distances using chain. Interior details are located using offset from main survey lines.

They suitable for large area surveying crowded with many details. It can be used to survey a river course.

c) Plane Table Surveying

The principle of plane table survey is parallelism. They are plotted directly on paper with their relative position. The rays are drawn from station to object on ground. The table is placed at each of the successive station parallel to the position of the last station.

They are basically suitable for filling interior detailing and is recommended when great accuracy is not required.

d) Theodolite Surveying

The theodolite is an instrument used mainly for accurate measurement of the horizontal and vertical angles. They are accurate to measure up to 10″ or 20″ angles.

Theodolite can be used to measure:

Horizontal angles

Vertical angles

Deflection angle

Magnetic bearing

Horizontal distance between two points

Vertical height between two points

Difference in elevation

Nowadays theodolite is shadowed and replaced by the use of Total Station which can perform the same task with greater ease and accurate results

e) Tacheometric Surveying

Tacheometry is a branch of surveying in which horizontal and vertical distances are determined by taking angular observations with an instrument known as a tacheometer. Tacheometer is nothing but a transit theodolite fitted with a stadia diaphragm and an anallatic lens. There is no need for chaining in such survey. The principle of Tacheometer is based on property of isosceles triangle, where ratio of the distance of the base from the apex and the length of the base is always constant.

Different form of stadia diaphragm commonly used:

f) Photographic Surveying

Photographic survey is based on technique of taking photographs from different angle to prepare topographic details with relative high speed.