undefined | unit 6 introduction to geodetic survey hydrograph survey aerial photogrammetry

Unit - 6

Introduction to Geodetic Survey, Hydrograph Survey & Arial Photogrammetry

Q1) What are the objective of Geodetic surveying?

A1) Following are the important object or purpose of geodetic surveying:

Geodetic surveying is used to precisely determine the relative positions of a system of widely separated points on the surface of the earth.

It is also used to find the absolute positions of the various points on the surface of the earth.

The relative positions of various points are found out in the term of the azimuths and the lengths of the lines joining the various points and absolute positions in terms of longitude, latitudes and elevation above mean sea level.

The geodetic points which are determined, gives the most precise control for a more detailed survey of the intervening country.

To establish accurately located control points for a plane and geodetic survey of large areas.

To accurately locate the center lines, terminal points of various projects and shaft for long tunnels. Also, to locate the center lines and abutments for long span bridges.

To establish accurately located control points in connection with aerial surveying.

Q2) What is triangulation system?

A2)

"The method of measuring the chain angles of a framework of triangle being formed by marking the station on the surface of earth is called as triangulation".

A chain of triangles is very rapid and economical if a narrow strip of terrain is to be surveyed. For example, highway, valley, river possesses the feature of narrow strip.

Fig.: Well conditioned triangle

Note that angles less than 30° or more than 120° are treated as ill-conditioned triangles and not to be taken in survey work. Angles more than 30° or less than 120° are treated as well-conditioned triangles and to be considered or to be taken in surveying work.

The triangulation system consists of triangles which are inter connected at apexes called as triangulation stations. Length of one line of triangle in a triangulation system is called as base line. In triangulation system, the angles of triangles are measured accurately.

Inter-connected triangles form a system is called as triangulation system as shown in Fig.

Fig.: Quadrilaterals

Q3) What are the objectives of triangulation survey?

A3) Following are the various uses of triangulation in geodetic surveying.

Object of triangulation is the find the size and shape of the earth surface by performing the various observations for latitude, longitude and gravity.

It is used fix the center line, terminal points and shafts for long tunnels.

Triangulation form a net work or a frame work. The cadastral, hydrographical, engineering survey, topographical survey and other survey are based on triangulation.

It provides most accurate and precise system of horizontal control.

Triangulation system provides the desired checks on the computations and usually better triangles can be selected.

It is used to fix the center line of abutments for long bridges over large rivers.

It is used to transfer the control points across wide sea channels, large water bodies etc.

It provides a simple, rapid and economical system.

Q4) Explain the classification of Triangulation system?

A4) Based upon the degree of accuracy required, length of the base, length of the sides, the extent of the area for which the survey to be done, the triangulation system is classified into three divisions, namely-

Primary triangulation or First order triangulation

Secondary triangulation or Second order triangulation

Tertiary triangulation or Third order triangulation

1. Primary triangulation or first order triangulation

The primary triangulation or first order triangulation system is the highest order triangulation.

This system is used for very large areas like earth's figure. Primary triangulation is done for larger area for obtaining the most precise and accurate control in mapping of the area and for small scale mapping.

In this system, precise and advanced instruments are probably used for taking observation or reading and surveying work is well exercised so as to obtain every possible refine work.

This system includes well-conditioned triangles of large area.

Specification for primary triangulation system:

The common and general specifications for the primary triangular system or first order triangulation are given below:

Average triangle of closure error is less than 1".

Actual error of the base line is 1 in 500000.

Probable error of the base line is 1 in 1000000.

Length of base line is 15 km to 20 km.

Length of sides of triangle varies from 30 km to 150 km.

Probable error in astronomical is 0.5".

Maximum triangle of closure error is not more than 3".

Probable error of computed distance is 1 in 50000 to 1 in 250000.

2. Second order triangulation or secondary triangulation

A second order triangulation or secondary triangulation in which number of points with in the frame work of primary triangulation are fixed at short intervals so as to connect two primary series. In this way control points are obtained, which are closer together.

Since the stations are fixed at close intervals, the sizes of triangles formed have smallest size in comparison with the primary triangulation.

It consists of well-conditioned triangle of smaller sizes as compared to primary triangulation with less precise instruments. But in case of primary triangulation instruments are more precise and accurate.

Specification of secondary triangulation or second order triangulation are given below:

Average triangle of closure error is 3".

Actual error of the base line is 1 in 250000.

Probable error of the base line is 1 in 500000.

Length of base line varies from 2 km to 5 km.

Length of sides of triangles varies from 10 km to 50 km.

Maximum triangle of closure error is 8".

Probable error of computed distance is lin 20000 to 1.

3. Third order triangulation and tertiary triangulation

The third order triangulation or tertiary triangulation in which the number of points with in the frame work of secondary triangulation are fixed so as to provide the control points between stations of secondary and primary order series.

In third order triangulation, size of triangles is small as compared to secondary triangulation and instruments having moderate precision can be used.

This system provides the horizontal control points on a topographic survey.

It forms the immediate control for topographical surveys on various scales.

Specification of third order triangulation and tertiary triangulation:

The common and general specification triangulation are given below:

Average triangle of closure error is 6".

Actual error of base line is 1 in 100. 23

Probable error of the base line is 1 in 250.

Length of base line varies from 0.5 km to 2 km.

Length of sides of triangles varies from 2 km to 10 km.

Maximum triangle of closure error is 12".

Probable error of computed distance is 1 in 5000 to 20000 distances.

Probable error in astronomical is 5".

Q5) Explain Braced Quadrilateral figure?

A5)

In the triangulation system, the braced quadrilateral figure is more suited and hence it should be adopted whenever it is feasible and possible.

When the quadrilateral is near about square or if the triangle is near about equilateral, then it gives the maximum strength with a minimum of essential geometric conditions.

Fig.: Braced Quadrilateral

From Equation (ii), we know,

C = (n'-s'+1) + (n - 2s + 3)

From braced quadrilateral we know,

n= Total number of lines (including known side) = 6,

S = Total number of stations = 4

s' = Number of stations occupied = 4

n' = number of lines observed in both directions including known side 6

∴C = (6-4+1) +(6-2x4+ 3) = 4

D = Number of directions observed (forward and backward) excluding those along known side = 5× 2 = 10

=0.6

Q6) Explain Four-sided central point figure without diagonals?

A6)

Fig. shows four-sided central point figure without diagonals. This type of figure is probably used when both the diagonals are obstructed.

Fig.: Four-sided central point figure without diagonals

From Fig. 1.9.9, excluding one known side we get, number of forward directions = AB + BC + CD + CE + BE +AE + DE = 7, (provided that side AD is known)

Number of backward directions=BA+CB + DC + EC+ EB + EA + ED= 7

D = 7+7= 14, Or D = 2x7= 14

We know,

n = 8, n'=8, s=5, s' = 5

C = (n'-s'+1) + (n +2s + 3) = (8-5+1) + (8-2x5+ 3) = 5

(D-C)/D= (14-5)/14=0.64

Q7) Explain study and use of one second theodolite?

A7) Construction:

Micro optical theodolite has optical machine construct all through manufacturing.

This optical machine faceplates analyzing of each horizontal and vertical attitude exactly.

The least be counted number of this theodolite is 1”

It has an automated repayment of vertical circle index.

It has 30 X vivid and sharp telescope.

It gives clean and vivid show with backlight illumination and additionally gives the show for each the sides.

Uses:

This tool is maximum appropriate for harsh surroundings and intense climate conditions (i.e., warm or bloodless temperatures)

For specific dimension of horizontal attitude.

For specific dimension of deflection attitude.

For specific dimension of vertical attitude.

Alignment of transmission towers wearing electric powered cable may be exactly done.

For finding alignment of tunnel.

Q8) Explain study and use of electronic total station?

A8)

An electronic total station means an E.D.M. and a digital theodolite built as one unit.

The total station is powered by small rechargeable batteries and built into its memory are a number of self-contained functions which include the measurement of slope, distance, horizontal distance, vertical distance.

There is a data recording module which is also called as electronic field book. This data recording module records the data and additional information.

In total station, field survey data consists of the angles, distance and related information is stored in electronic field book.

Advantages of Total Station over Level and Theodolite:

With total station, distance is measured by Instrument itself as total station consists features of EDM, thus accuracy increases.

It is digital instrument which gives the direct reading personal mistake can be eliminated.

Use of Total Station:

With the help of electronic total station, measurement of slope distance, horizontal and vertical distance can accurately and precisely found out.

Electronic total station gives the complete basic surveying exercise in order to appreciate how land survey measurements can be used in support of engineering construction and environmental restoration activities.

Vertical angle and horizontal angle are accurately measured by electronic total station. Total station for levelling classified as the indirect levelling method and since it is judged that the method can maintain the considerable accuracy, now it has been increasingly used for many public works as road, airport and city etc.

Q9) What are the objectives of hydrography?

A9) Following are the various purposes or objects of hydrographic survey of the bodies containing still or running water:

With the help of hydrographic survey, bed depths by soundings can be determined. The depth of bed is needed for navigation and it is also useful to find the location of rocks, sand bars, buoys, navigation lights etc.

Determination of bed depths are also helpful to find the location of underwater works, also helpful to compute the volume of underwater excavation and also useful for the purpose of planning of drainage work and irrigation work.

2. With the help of hydrographic survey, various characteristics and behaviour of bottom of bodies of water can be found out.

3. With the help of hydrographic survey, intensity of tides at sea coast can be measured.

4. With the help of hydrographic survey, areas subjected to scouring and silting can be rectified by determining the direction of current.

5. Hydrographic survey is also useful to measure the velocity, discharge and various characteristics of the flow of water. In such case, measurement of discharge is helpful to water supply schemes, power development, flood control etc.

6. Hydrographic survey is also useful to determine the direction of currents of water. Determination of direction of currents is useful for navigation and for location of sewer outfalls.

Q10) What are the applications of hydrographic survey?

A10) Following are the various applications of hydrographic surveying:

Hydrographic surveying can be used to determine the quantities of subaqueous excavations.

It can be used to measure areas subjected to scouring or silting in docks and harbours.

It is used to develop water resources for power, irrigation and recreation.

It can be used to locate rocks and other objects like buoys. lighthouses etc. for safe navigation.

It can also be used to control floods and to plan water supply and storage from rivers.

It can also be helpful to prepare navigation charts exhibiting the depths available for navigation.

Q11) Explain Shore line survey?

A11) Following are the various objects or purpose of the shoreline survey:

To determine the location of shorelines.

To determine high and low tides.

To locate various distinct or prominent features on the shoreline.

To locate the points of high-water line by contouring.

Shoreline is located by running a traverse and taking offsets from the traverse lines to the various shoreline points.

In case of narrow river, an open traverse on one bank is sufficient to locate the shorelines on both banks whereas, in case of wide river, the traverses should be run on the both banks to determine shorelines.

Fig. shows the shoreline survey for wide river.

Fig.: Shore line survey

Q12) What is the use of soundings?

A12) Following are the various uses of soundings in hydrographic surveying:

Soundings is used to determine the bed profile.

It is used to prepare the charts for navigation.

It is used to determine the quantity of material to be dredged.

It is also used to find the area for dredging and dumping.

It is used to locate the areas subjected to scouring or silting.

It is also used to collect subaqueous information which can be used further for the design of construction development and improvement of ports and other related structure like break waters, sea walls, wharves etc.

Q13) Explain Sounding Rods?

A13)

For a depth about 4 m to 6 m, the sounding rods or poles are more suitable and convenient in shallow and smooth waters.

These rods are made from well seasoned tough and strong timber. It is circular in cross section having 50 mm diameter. It is normally 5m to 8m in length. A rod of about 5 m long is required for sounding depth upto 4 m.

For obtaining the samples of material at bottom, rods are provided with shoe at one end with cup-shaped cavity. Tallow or grease is applied to the inner surface of cavity so as to adhere the bottom sample material.

Sounding rods or poles are painted with white oil paint and graduations are marked as m and cm on the opposite faces for taking proper reading. Zero graduation is started from the bottom of shoe. Fig. shows the sounding rod or pole with provision of cup-shaped cavity.

Fig.: Sounding rod with cup shaped cavity

Q14) Explain Fathometer in brief?

A14)

Fathometer or echo-sounder is an instrument by which sounding at higher depths can be found. It is an electric device which consist of two units like is transmitted units and a receiving unit. Supersonic impulse is transmitted through the transmitter and the echo or return impulse is catched by receiver. It records time interval between transmission and reception.

Fig.: Operation of fathometer

The depth is indirectly found from the time of travel of the sound waves from a point very close to the surface water upto the bottom of the ocean or sea and again back to the water surface as shown in Fig.

In short fathometer is an electric instrument which measures the time required for the sound impulses travels to the bottom and comes back to water surface.

The following expression shows the relation with respect to depth (h)

∴h=vt/2

Where,

V = speed of sound in water

T=time interval held between the transmission and reception of the signals.

Note that small correction is needed to apply because distance (D) between transmitter and receiver is small.

Q15) What are the objectives of aerial photogrammetry?

A15) Following are the various objects of aerial photogrammetry:

To construct or prepare the planimetric and topographic maps.

To make reconnaissance survey.

For acquisition of military intelligence.

To interpret the geology and soil details.

To make survey for inaccessible regions, forbidden properties, unhealthy regions such as malarial affected areas.

To make surveys of buildings.

To make survey of mountainous and hilly areas having less number of trees.

Q16) What are the classification of aerial photogrammetry?

A16) There are two types of photogrammetry which are as follows:

Terrestrial photogrammetry

The photogrammetry in which the photographs are taken by means of a special instrument provided with a camera supported on the ground and a thedolite is called as terrestrial photogrammetry.

In this type, the photographs are taken with the help of a camera supported on the grounds and then maps are compiled from the photographs.

The basic principle of terrestrial photogrammetry is quite similar to that of plane table surveying.

Aerial photogrammetry

Aerial photogrammetry is the advanced technology and gives the quick result, hence the terrestrial photogrammetry has been replaced by aerial photogrammetry.

It is most suitable for small scale mapping probably on flat country.

It requires highly qualified, trained and experienced technitian.

Aerial photographic surveying work is very elaborative and more expensive. Hence the work is usually done by Government department of survey of India.

Q17) What are the application of aerial photogrammetry?

A17)

Aerial photography is extensively used in military during war so as to get topography of the area from which information can be collected and it becomes easy to attack over the enemies.

Aerial photography is used in cartography particularly in photogrammetric surveys, which are often the basis for topographic maps.

Aerial photography is also used in land-use planning, archeology, movie production, environmental studies, espionage, commercial advertising, converyancing and other fields.

Aerial photography is used in perspective correction.

It is also used in the process of stitching. Stitching creates an aerial photograph of a large area by taking multiple photographs. In the process of stitching, multiple photographs are joined together so that they form a single large photograph. This process creates "seamless" imagery.

Q18) Explain comparison of Map and Aerial photograph?

A18)

Sr.No.	Maps	Aerial Photograph
1	Maps are plotted by actually taking the linear and angular measurements of the ground and terrain.	A photograph in which a photograph of terrain or ground at the time of exposure and showing all the details of the terrain is called as aerial photographs.
2	There is no distortion caused by tilt or relief displacement because these maps are prepared after taking the necessary and required measurements.	There is distortion of details of terrain or ground caused by tilt and relief displacement.
3	Contour map which is plotted by carrying actual measurements can be used to determine the elevation of desired objects directly.	Elevation of the different objects or points cannot be taken directly.
4	Plotting of map and its reading is common and more familiar to users. Various details of various objects like roads, pond, river, valley, railway crossing, rail-lines, fence etc. can be shown by conventional signs and symbols.	The photo-views so obtained by aerial photographs are not familiar to the eyes of user and it needs a special technique to read the photo-views.
5	Plotting of map is economical.	Preparation of maps from aerial photography is uneconomical due to costly automatic plotting machines.
6	Maps can be prepared by conventional methods of surveying.	Aerial photographs are prepared by aerial surveying which is a highly skilled job and also need highly skilled technician.
7	Preparation of map is a time-consuming process and need more time.	Preparation of aerial photographs require less time.
8	Maps prepared by conventional methods do not depict confusing details and easily recognizable.	Aerial photographs depict confusing details which are not easily identified.

Q19) What is mean by Crab?

A19) When the camera is not square with respect to the direction of the flight at the time of exposure, then it is called as crab. Fig.(a) and (b) shows the crab and drift respectively.

Fig.: Crab

Fig.: Drift

Q20) How we can calculate no. of photograph?

A20)

Net length covered by each photograph (L) = (1-P₁) Sl

L = (1-P₁) SI

Where, P=Longitudinal overlap

1 = Length of the photograph in the direction of flight

S = Scale of a photograph

Net width covered by each photogram (w) = (1-Pw) Sw

Where, P = Side overlap

S = Scale of a photograph

W =Width of the photograph normal to the direction of flight

If the dimensions of the area A (L 1x B1) are given, the number of photographs required is determined by calculating the number of photographs in each strip and the number of strips.

Consider N₁ =Number of photographs in each strip

N₂ = Number of strips required

N₁ =+1

Where, L₁= Length of the area

P₁ = Longitudinal overlap

S = Scale of a photograph

1 = Length of the photograph in the direction of flight

N2=+1

Where, B₁= Width of the area

Pw= Side overlap

S = Scale of photograph

W= Width of the photograph normal to the direction of flight

Hence number of photographs required =N₁ X N₂

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