The following are the main sources of error in levelling:

• Instrumental Errors.
• Errors of Manipulation.
• The Errors due to settlement of Level and Staff.
• Errors due to Natural causes.
• Common mistakes in levelling.

Read more about Compass surveying in full detail

Instrumental Errors ( Error in levelling )

Imperfect Adjustment.

The essential requirement in levelling is that the line of collimation must be horizontal when the instrument is level. If the level is in perfect adjustment, the line of collimation is parallel to the bubble axis so that it becomes horizontal when the bubble is centre. If the adjustment is not correct, the line of collimation will be inclined even though the bubble is central. The error due to the inclination of the line of collimation is proportional to the distance of the staff from the instrument. This is the most common and serious source of error. It can be eliminating (a) by testing and adjusting the instrument, and (b) by equalising the back sight and foresight distances.

Defective Level Tube

If the bubble is sluggish, it may remain in centre even though the bubble axis is not horizontal. And if it is over sensitive, much time is wasting in levelling the instrument. Therefore the level tube fitted with the level should be only of the required sensitiveness.

Shaky Tripod.

A shaky tripod renders the instrument unstable and thus leads to erroneous readings. The tripod should therefore be examined and loose joints be tightened.

Incorrect Graduations of the staff

When the staff is new one, the graduations should be checked up with an Invar tape as there is every possibility of error in graduating the staff. It is however negligible in ordinary work.

Errors of Manipulation ( Error in levelling )

• Careless levelling-up of the Instrument. The instrument should be accurately level and the hand should not rest on the telescope or tripod while taking the readings.
• The Bubble not being central at the time of taking Readings. This is the most serious and common source of error. The position of the bubble should be check just before and after each reading is taking. The bubble should be brought in the centre of its run, if necessary, by means of a levelling screw most nearly in line with the telescope.
• The Parallax not removed properly. This error is due to imperfect focusing of the eye-piece and of the object-glass. The parallax should be entirely eliminating by proper focusing before taking any reading.
• The Staff not being held vertical. If the staff is not held perfectly vertical, the reading obtained will always be too great. This error is much pronounced for greater readings of staff, the greater the staff reading, greater is the error. Great care should, therefore, be taken when taking larger readings. This error can be avoiding by keeping the staff vertical by having a spirit level or a pendulum plumb-bub attach to the staff or by waving the staff and noting the smallest reading.

Errors due to settlement of level and staff ( Error in levelling )

Settlement of level

If the level is set up on soft ground it may gradually settle down in the interval between taking a back sight and the next fore sight reading. It will make the fore sight reading smaller than it should be. The level should, therefore, be set up as far as possible on solid ground and the legs of the tripod firmly pressed into the ground. Also the foresight reading should be taken as soon after taking the back sight reading as possible. In exceptionally soft ground, the legs of the tripod may be set over long wooden stakes driven firmly into the ground.

Settlement of Staff

At change points, if the staff is held on soft ground, the ground may settle down during the time the instrument is shifted and set up on the next point. The back sight will become greater than it should be. This is a serious source of error and can be eliminated by avoiding soft ground as a change point. In case the soft ground is unavoidable, a long peg driven into ground or a steel foot plate should serve as a change point.

Errors due to Natural Causes ( Error in levelling )

• Curvature and Refraction. The effect of curvature of the earth is to cause the objects sighted lower than they actually are while that of the refraction is to make them appear higher than they actually are. In ordinary levelling, the error due to curvature and refraction need not be taken into consideration, since it is very small and hence negligible (only 0.003 m for 200 m sight-length). For long sights, the correction for curvature and refraction has to be applied.
• Wind. It is difficult to perform accurate work in a wind storm because of the vibration of the instrument. If levelling is necessarily to be performed during wind, adequate arrangements should be made to shelter the instrument and larger readings should be avoided due to difficulty in holding the staff vertical.
• Sun. On hot sunny days, during the mid-day hours, the staff appears trembling near the ground and taking the correct reading becomes impossible. The work should therefore be suspended during hot hours. If this is not possible, the line of sight should be kept as much above the ground as possible and the length of sight kept short. Also ray-shade should be used if the Sun shines on the object-glass.

Common Mistakes in Levelling. ( Error in levelling )

• The levelling staff is not being fully extending.
• The staff held upside down.
• Reading the staff upwards instead of downwards.
• Taking reading coincident with one of the stadia hairs (top or bottom) instead of central-hair.
• Omitting an entry.
• Entering a reading in the wrong column.
• Reading the wrong number of metres and decimetres.

Remote sensing is broadly defined as the science and art of collecting information about objects. The area or phenomena from distance without being in physical likewise contact with them.

Human eye is therefore most familiar example of a remote sensing system. In fact, sight, smell and hearing are all rudimentary forms of remote sensing. However, the term remote sensing is restrict to methods that employ electromagnetic energy ( such as light, heat, microwave ) as means of detecting and measuring target characteristics.

Air craft and satellites are the common platforms use for remote sensing. Collection of data is usually carry out by highly sophisticated sensors ( i.e. camera, multi spectral scanner, radar etc. ). The information carrier, or communication link is the electromagnetic energy.

Classification of Remote sensing

• Passive R.S.
• Active R.S.

Passive remote sensing

It uses sun as a source of EM energy. It’s records the energy that is naturally radiate and/ or reflect from the objects.

Active remote sensing

It uses its own source of EM energy. It’s is direct towards the object and return energy is measure.

Idealize Remote sensing system

• Energy source
• Propagation of energy through atmosphere.
• Energy interaction with earth’s surface features.
• Airborne/ space borne sensors receiving the reflect and emit energy.
• Transmission of data to earth station and generation of data produce.
• Multiple data users

Principles of Remote sensing

It employ electromagnetic energy. Moreover, to a great extent relies on the interaction of electromagnetic energy with the matter. It refers to the sensing of EM radiation, which is reflect, scatter or emit from the object.

Electromagnetic Energy

It is a form of energy that moves with the velocity of light ( 3 x 10^8 m/sec ) in a similarly harmonic pattern. Electro magnetic Energy consisting of sinusoidal waves, equally and repetitively space in time. It has therefore two fields : ( i ) electrical field and ( ii ) magnetic field. Both are however orthogonal to each other. In which the electric components are vertical as well as magnetic components are horizontal.

Electromagnetic energy consists of photons having particle like properties such as energy and momentum. The EM energy is characterized in terms of velocity c ( = 3 x 10^8 m/s ), wave length and frequency f. These parameters are relate by the equation :

wavelength = c/f

The amplitude is the maximum value of the electric field. It is a measure of the amount of energy that is transport by the wave.

Wave theory concept explains how EM energy propagates in the form of a wave. However, this energy can only be detect when it interacts with the matter. This interaction suggests that the energy consists of many discrete units refer as photons whose energy (Q) is given by :

Q = h.f = h.c/ wavelength

h = Plank’s constant = 6.6252 x 10^-34 J-s

Electromagnetic Spectrum

Although visible light is the most obvious manifestation of EM radiation, other forms also exist. EM radiation can be produce however at a range of wave lengths. It can be categorize according to its position into discrete regions which is generally refer to electromagnetic spectrum. Thus the electromagnetic spectrum is the continuum of energy that ranges from meters to nano meters in wave length travels at the speed of light. It propagates through a vacuum likewise the outer space.

All matter radiates a range of electromagnetic energy, with the peak intensity however shifting toward progressively shorter wave length at an increasing temperature of the matter. In general, the wave lengths and frequencies vary from shorter wavelength high frequency cosmic waves to long wave length low frequency radio waves .

Remote sensing observation platforms

• Air borne platforms
• Space based platforms

Air borne platforms

Remote sensing of the surface of the earth has a long history, however dating from the use of cameras carried by balloons and pigeons in the eighteenth and nineteenth centuries. Later, air craft mount systems were developed for military purposes during the early part of 20th century.

Air borne remote sensing was used in 1960’s and as well as 1970’s. Air craft were however mostly used as RS platforms for obtaining photographs. Aircraft carrying the RS equipment should therefore have maximum stability, free from vibrations and fly with uniform speed.

Space based platforms

Space borne remote sensing platforms, such as a satellite, offer several advantages over airborne platforms. It provides synoptic view ( i.e. observations of large area in a single image ), systematic and repetitive coverage.

Types of Space borne platform

• Low altitude near polar orbiting satellites.
• High altitude Geo-stationary satellites.

Applications of Remote sensing

• Agriculture
• Forestry
• Land use and soils
• Geology
• Urban Land use
• Water resources
• Coastal Environment
• Ocean Resources
• Watershed
• Environment
• Street network based applications
• Land parcel based application
• Natural resources based applications
• Facilities management
• Disasters
• Digital elevation models

A total station is an electronic theodolite and an electronic distance meter ( EDM ). This combination makes it possible to determine the coordinates of a reflector by aligning the instrument’s cross hairs on the reflector and simultaneously measuring the vertical and horizontal angles and slope distances. 

A microprocessor in the instrument takes care of recording, readings and the necessary computations. The data is easily transferred to a computer where it can be use to generate a map. Wild, ‘ Tachymat ‘ TC 2000, and it is manufactured by M/s Wild Heerbrugg.

As a teaching tool, a total station fulfills several purposes. Learning how to properly use the total station involves the physics of making measurements, the geometry of calculations, and statistics for analysing the results of a traverse. In the field, it requires teamwork, planning, and careful observations.

 If the total station is equipped with a data collector it also involves interfacing the data-logger with a computer, transferring the data, and working with the data on a computer. The more the user understands how a total station works, the better they will be able to use it.

Fundamental Measurements

• The rotation of the instrument’s optical axis from the instrument north in a horizontal plane : i.e. horizontal angle
• Inclination of the optical axis from the local vertical i.e. vertical angle.
• Distance between the instrument and the target i.e. slope distance.

Horizontal Angle

The horizontal angle is measure from the zero direction on the horizontal scale. When the user first sets up the instrument the choice of the zero direction is made – this is Instrument North. The user may decide to set zero ( North ) in the direction of the long axis of the map area, or choose to orient the instrument approximately to True, Magnetic or Grid North.

The zero direction should be set so that it can be recover if the instrument was set up at the same location at some later date. This is usually do by sighting to another benchmark, or to a distance recognizable object. Using a magnetic compass to determine the orientation of the instrument is not recommended and can be very inaccurate.

 Most total stations can measure angle to at least 5 seconds, or 0.0013888 degree. The best procedure when using a Total Station is to set a convenient ” north ” and carry this through the survey by using back sights when the instrument is move.

Vertical Angle

The vertical angle is measure relative to the local vertical ( plumb ) direction. The vertical angle is usually measure as a zenith angle ( 0 degree is vertically up, 90 degree is horizontal, and 180 degree is vertically down ), although one is also given the option of making 0 degree horizontal. The zenith angle is generally easier to work with. The telescope will be pointing downward for zenith angles greater than 90 degree and upward for angles less than 90 degree.

Measuring vertical angles require that the instrument be exactly vertical. It is very difficult to level an instrument to the degree of accuracy of the instrument. Total stations contain an internal sensor ( the vertical compensator ) that can detect small deviations of the instrument from vertical.

Electronic in the instrument then adjust the horizontal and vertical angles accordingly. The compensator can only make small adjustments, so the instrument still has to be well level. If it is too far out of the level, the instrument will give some kind of ” tilt ” error message.

Slope Distance

The instrument to reflect distance is measure using an Electronic Distance Meter ( EDM ). Most EDM’s use a Gallium Arsenide Diode to emit an infra light beam. This beam is usually modulate to two or more different frequencies. The infra beam is emit from the total station, reflect by the reflector and received and amplify by the total station. 

The receive signal is then compare with a reference signal generate by the instrument ( the same signal generate that transmits the microwave pulse ) and the phase-shift is determine. This phase shift is a measure of the travel time and thus the distance between the total station and the reflector.

Basic Calculations

Total Stations only measure three parameters :

• Horizontal Angle
• Vertical Angle
• Slope Distance

Horizontal Distance

Let us use symbol I for instrument ( total station ) and symbol R for the reflector. In order to calculate coordinates or elevations it is first necessary to convert the slope distance to a horizontal distance. The horizontal distance is –

Hd = Sd cos ( 90* – Za ) = Sd sin Za

where Sd is the slope distance and Za is the zenith angle. The horizontal distance will be use in the coordinate calculations.

Vertical Distance

We can consider two vertical distances. One is the Elevation Difference ( dZ ) between the two points on the ground. The other is the Vertical Difference ( Vd ) between the tilting axis of the instrument and the tilting axis of the reflector. For elevation difference calculation we need to know the height of the tilting-axis of the instrument, that is the height of the center of the telescope, and the height of the center of the reflector ( Rh )

The way to keep the calculation straight is to imagine that you are on the ground under the instrument. If you move up the distance Ih, then travel horizontally to a vertical line passing through the reflector then up ( or down ) the vertical distance ( Vd ) to the reflector, and then down to the ground. This can be write as

dZ = Vd + ( Ih – Rh )

The quantities Ih and Rh are measure and recorded in the field. The vertical difference Vd is calculate from the vertical angle and the slope distance

Vd = Sd sin ( 90* – Za ) = Sd cos Za

where dZ is the change in elevation with respect to the ground under the total station. We have chosen to group the instrument and the reflector heights. Note that if they are the same then this part of the equation drops out. If you have to do calculations by hand it is convenient to set the reflector height the same as the instrument height.

If the instrument is at a know elevation, Iz , then the elevation of the ground beneath the reflector, Rz , is

Rz = Iz + Sd cos Za + ( Ih – Rh )

Q1 : Different grades are joined together by a …..

(A) Compound curve

(B) Transition curve

(C) Reverse curve

(D) Vertical curve

Answer: Option D

Q2: The line of collimation method of reduction of levels, does not provide a check on …..

(A) Intermediate sights

(B) Fore sights

(C) Back sights

(D) Reduced levels

Answer: Option A

Q3: Which of the following methods of contouring is most suitable for a hilly terrain …..

(A) Direct method

(B) Square method

(C) Cross-sections method

(D) Tachometric method

Answer: Option D

Q4: The size of a plane table is …..

(A) 750 mm × 900 mm

(B) 600 mm × 750 mm

(C) 450 mm × 600 mm

(D) 300 mm × 450 mm

Answer: Option B

Q5: The line normal to the plumb line is known as …..

(A) Horizontal line

(B) Level line

(C) Datum line

(D) Vertical line

Answer: Option B

Q6: After fixing the plane table to the tripod, the main operations which are needed at each plane
table station are …..

(A) Levelling, Orientation, Centering

(B) Levelling, Centering, Orientation

(C) Centering, Levelling, Orientation

(D) Orientation, Centering, Levelling

Answer: Option B

Q7: The two point problem and three point problem are methods of …..

(A) Resection

(B) Orientation

(C) Traversing

(D) Resection and orientation

Answer: Option D

Q8: The desired sensitivity of a bubble tube with 2 mm divisions is 30″. The radius of the bubble tube
should be …..

(A) 13.75 m

(B) 3.44 m

(C) 1375 m

(D) None of these

Answer: Option A

Q9: Volume of the earth work may be calculated by …..

(A) Mean areas

(B) End areas

(C) Trapezoidal

(D) All the above

Answer: Option D

Q10: Chain surveying is well adopted for …..

(A) Small areas in open ground

(B) Small areas with crowded details

(C) Large areas with simple details

(D) Large areas with difficult details

Answer: Option A

Plane table surveying also refer as Plane tabling, is a method of surveying in which field-work and office-work are do same time on the plane table. The field observations are taken and record side by side on the sheet fix upon the plane table and a map of the area is obtain.

The use of field-book is altogether eliminate. It is most suitable for the survey of details between stations which have already been fixed by theodolite or any other accurate method of surveying.

It is commonly employ for small and medium scale mapping of comparatively large areas where great accuracy is not the main consideration such as for topographical surveys.

The equipment essentially need for a plane tabling is a plane table or drawing board which carries a drawing sheet.

It is mount on a tripod stand and an alidade which provides line of sight and a straight graduate edge.

Advantages of Plane Tabling

• It is one of the most rapid methods of surveying.
• Field-notes are not require, and thus the possibility of mistakes in booking is eliminate.
• Measuring of lines and angles is mostly dispense with since they are obtain graphically.
• Since the map is plot in the field, there is no chance of omitting necessary measurements.
• The surveyor is fully confident about the true representation of the area since he can always compare his work with the actual features on the ground and cannot, therefore, over-look any essential detail.
• The surveyor can check the accuracy of his work more frequently and from any position he may desire, thus eliminating all errors at the spot.
• It is particularly suitable for filling in details in hilly areas and in magnetic areas where chain and compass, surveys are not suitable.
• Contours and other irregular objects may be accurately represent on the map since the tract is in the view.
• It is less costly than theodolite survey.
• No great is require in making a satisfactory map and the work can be entrust even to a subordinate.

Disadvantages of Plane Tabling

• It is unsuitable for work in a wet climate and is difficult in high wind.
• It is not useful for large scale surveys and accurate work.
• Not suitable for surveying a densely wooded area.
• The instrument is heavy and cumbersome and the various accessories, being loose, are likely to be lose.
• The absence of field-notes is sometimes inconvenient if the survey has to be replotted to a different scale.
• Only day time can be avail of for the field and plotting work whereas in other methods of surveying, day time can be use for field-work and nights or even hot and rainy days can be utilise for plotting.