6.1 Introduction to Geodetic Surveying | unit 6 introduction to geodetic survey hydrograph survey aerial photogrammetry

Unit - 6

Introduction to Geodetic Survey, Hydrograph Survey & Arial Photogrammetry

6.1 Introduction to Geodetic Surveying:

Geodetic surveying is also called as trignometrical surveying and it differs from plane surveying. In geodetic surveying, curvature of earth surface is considered, and it involves the very large areas and very large distances.

In the geodetic surveying highly refined instruments and methods usually taken in meters, are employed. Geodetic work is usually done by state agency. Geodetic surveying is done by the survey of India in the nation 'India'.

Key Takeaways:

It is also called as trignometrical surveying and it differs from plane surveying. In geodetic surveying, curvature of earth surface is considered, and it involves the very large areas and very large distances.

6.2 Methods of Geodetic Surveying:

Geodetic surveying mainly involves two operations namely:

Triangulation

Precise leveling

6.3 Objects of Geodetic Surveying:

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.

Key Takeaways:

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.

6.4 Introduction to Triangulation:

"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.6.1: 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.6.2: Quadrilaterals

Key Takeaways:

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.

6.5 Introduction to Trilateration system:

In recent year with the introduction of electronic dist measure instrument (EDM) the entire, procedure of conventional triangulation is replaced by trilateration:

The trilateration consist of series of connected triangles are measured with high degree covering area to be surveyed in which length of all sides of triangle are measured by EDM.

Occcusionaly direction or angles of certain Survey lines are also observed to determine their azimuth.

The field procedure of trilateration is similar to triangulation with the difference that there is no angular measurement only linear measurement are carried with the help EDM instrument.

The accuracy obtained in linear measurements with an EDM instrument is much more as compared to triangulation System. Also, time required for completing survey work is much less as compared to triangulation.

The other advantage is that manpower required for this survey work is less, then atomatic recording of field data & also it is stored in data card.

However, the initial investment for the purchase of an EDM instrument is high and there are no effective easy checks for survey work.

Key Takeaways:

The trilateration consist of series of connected triangles are measured with high degree covering area to be surveyed in which length of all sides of triangle are measured by EDM.

6.6 Objective of triangulation survey:

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.

Key Takeaways:

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

6.7 Classification of Triangulation system:

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".

Key Takeaways:

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

6.8 Triangulation Figures:

Following geometrical figures are commonly used in triangulation system:

Well-shaped triangles

Quadrilaterals

Pentagons

Hexagons with central stations

Sometime, it is not possible to join the two points by a triangulation system, then a chain of single triangles as shown in Fig. may be used. It is simple and economical but less accurate.

Fig.6.3: A single chain of triangle

For greater area to be surveyed, a chain of hexagon may be used. Fig. shows a chain of hexagon which covers maximum area. This arrangement of a chain of hexagon gives accurate results rather than a single chain of triangle. A chain of hexagon also consists of a double row of single triangles as shown in Fig.

Fig.6.4: A chain of hexagon

A chain of quadrilateral may be used for very precise and accurate work. Fig. shows a chain of quadrilaterals used in the process of triangulation.

There is no station at the intersection of the diagonals. Geodetic quadrilateral is most accurate system because number of conditions involved in its adjustment is much more than the chain of hexagon and a single chain of triangles.

Gridiron system:

The primary network or primary triangulation consists of two series of chain of triangles. Out of these the series of triangles, one chain of triangles is run roughly in the direction of meridian i.e., north and south direction and the other chain is run roughly or approximately perpendicular to the meridian. Then area enclosed and surveyed with a grid or network of small triangles of second and third order of triangulation is called as gridiron system.

Gridiron system is used on very extensive surveys and this type of survey is mostly adopted in India.

Fig.shows a gridiron system.

Fig.6.5: Gridiron system

Central system:

Fig.6.6: Central system

It is the system in which the entire area to be surveyed is covered by a chain of triangles extending outward in all direction from the initial base line as shown in Fig. This type of system is used in U.K.

The best shaped triangle is equilateral and the best shaped quadrilateral is square.

In triangulation work, whenever possible, the station should be selected such that it forms an equilateral triangle and nearly so and whenever possible, the station should be selected such that it forms a square size quadrilateral.

Key Takeaways:

Following geometrical figures are commonly used in triangulation system:

Well-shaped triangles

Quadrilaterals

Pentagons

Hexagons with central stations

6.9 Strength of figures:

Note that, shape of figures in the triangulation system or network is very important so as to have better accuracy of a triangulation system. Hence the accuracy of triangulation system depends upon the two main points:

Methods and precision used in making observations.

Shapes of figures in the triangulation network.

The accuracy of shapes is measured in terms of the strength of figures. Strength of figures decides the layout of triangles in a triangulation system, which further gives a required degree of accuracy. In triangulation system, angles of triangles and length of one known side is used for. computations or calculation’s purpose. The other unknown sides are calculated by sine law.

Note that the sine of small angles changes more rapidly than large angles for a given change in the angles. The following examples gives the better idea:

sin 30º = 0.5

sin 30°, 30’ = 0.5075

Change: 0.5075-0.5 =7.5 x 10^-3

sin 70° = 0.9396

sin 70°, 30’ = 0.9426

Change: 0.9426-0.9396=3x10-3

From above examples it has been observed that, for a change of 30', the value of sin of 30° changes about twice the value of 70°.

Geodetic survey and US coast developed a more rapid and easy method to find the strength (R) of a triangle. The method is based upon an expression for the square of the probable error (L) which is seen at the sixth placed of the logarithm of any side.

For known side through a single chain of triangles, L² is given by the following expression,

L2=2)

Where, L² = Square of the probable error

d = Probable error of an observed direction in seconds

C= Number of conditions to be satisfied in each Figure

D = Number of directions observed which are forward and backward excluding the directions along the known side.

= Difference/second in the 6th place of logarithm of the sine of the distance angles A and B respectively (In this case, third angle is an azimuth angle) R represents the strength of figures.

If R = )

The Equation (i) can be written as follows,

L² =4/3 d2R

Note that, when figures of a triangulation network are more than single triangles and more than one route, then R, is used to indicate the strength of figures through the best route and R2 is used to show the second-best route. The value of R decides the strength of the figure.

For smaller value, greater will be the strength.

Factors on which the value of R depends:

Following are the various factors on which the value of R depends:

(i) Magnitudes of the distance angles A and B.

(ii) Number of observed directions (D)

(iii) Number of geometric conditions (C) To find the value of 'C', the following expression is used.

To find the value of ‘C’, the following expression is used

C = (n'-s'+1) +(n-2s + 3) … (2)

Where, C =number of geometric conditions

S = Total number of stations

s' = Number of stations occupied

n = Total number of lines including known side

n'= Number of lines observed in both directions including known side.

The relative strength of figures can be calculated in terms of the factor 'R'.

From the calculated values of R, alternative routes can be compared and finally best route can be selected.

Since strength of a figure is approximately equal to the strength of the strongest chain, the lowest value of R, is a measure of the strength of the figure.

Values of C and D for some simple figures:

Simple figures are:

(1) Braced quadrilateral figure

(2) Single triangle figure

(3) Four-sided central - point figure without diagonals.

(4) Four-sided central point figure with one diagonal.

1. Braced quadrilateral figure:

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.6.7: 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

2. Single triangle figure:

Fig. shows a simplest triangle

Fig.6.8: Single Triangle

From Fig., we know,

n=Total number of lines including known side = 3

S = Total number of stations = 3

n’=Number of lines observed in both directions including known side = 3

s’=Number of stations occupied= 3

C=(n'-s'+1) +(n-26+3)

= (3-3+1) (3-2x3+3)

∴C = 1+6-6=1

Forward directions AB + BC = 2, Backward direction =CB + BA = 2

D = 2+2= 4, (Excluding one known side)

(D-C)/D= (4-1)/4=0.75

3. Four-sided central point figure without diagonals:

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

Fig.6.9: 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

4. Four-sided central-point figure with one diagonal:

Fig. shows four-sided central point figure with one diagonal.

Fig.6.10: Four-sided central-point figure with one diagonal

From Fig.,

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

=2*8=16

n=9, n’=9, s=5, s’=5

C = (n'-s'+1) + (n -25+ 3) = (9-5+1) +(9-2*5+3) =7

(D-C)/D= (16-7)/16=0.56

Key Takeaways:

Simple figures are:

Braced quadrilateral figure

Single triangle figure

Four-sided central - point figure without diagonals.

Four-sided central point figure with one diagonal.

6.10 Study and Use of One second theodolite and Electronic Total station:

One second theodolite:

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.

Electronic Total Station:

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.

Key Takeaways:

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

6.11 Introduction to hydrographic Survey:

Hydrography in which hydrographic survey can be carried out on or near the body of water like lake, river; docks and harbour, bay; stream etc. Hydrography consists of the surveying work, (i) to determine the various positions or levels of the bottom of various bodies containing the natural resources of water, (ii) to determine the contours of submarine and (iii) to locate various features of topography of the large bodies of water.

Soundings: The measurement of depth below the water surface is called as soundings.

Key Takeaways:

Hydrography in which hydrographic survey can be carried out on or near the body of water like lake, river; docks and harbour, bay; stream etc.

6.12 Objects of hydrography:

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.

6.13 Applications of Hydrographic survey:

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.

6.14 Shore Line Survey:

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.6.11: Shore line survey

6.15 Sounding:

Definition: Soundings are the measurements of depth below the water surface.

Purpose of soundings is to determine the configuration of the bottom surface of water carrying body. Measurements of soundings at various points is done by a flat bottom boat. In hydrographic surveying, measuring the depth below the water surface i.e., soundings are most common and the various operations are similar to that of levelling. For tidal waters, round-bottomed boats are used. The sounding boat should have sufficient space and it should be more stable under various conditions. Note that, if the depth of water is not more than 25m, the soundings is done without stopping the boat.

Purpose of Sounding or uses of Soundings:

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.

Key Takeaways:

Soundings are the measurements of depth below the water surface.

6.16 Sounding Equipment and Methods of Sounding:

Soundings and their location can be done by the method’s direct method and indirect methods:

Sounding’s methods

1. Direct method

Sounding boat

Soundings rods or poles

Lead lines or Sounding cables

Sounding machine

Sounding chain

2. Indirect methods

Fathometer or Echo-sounder

1. Direct method

In this method, following equipments are commonly used, for taking soundings and their locations.

Sounding boat:

Sounding boat should be sufficiently spacies comfortable and stable such that there will not be any more difficulty while taking sounding.

In calm water, boat having flat bottom is preferably used and in case of rough water or little bit turbulent water, boat having round bottom is preferably used.

Power boats are most suitable for taking the soundings.

Sounding rods or poles:

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.6.12: Sounding rod with cup shaped cavity

Lead lines Or Sounding Cables:

Lead lines or also termed as sounding lines. Lead lines is generally a rope made of hemp and used usually when the depths over about 6 m.

Lead line is made of a line of hemp or cotton or brass chain or a metallic chain. One end of the lead line is attached by a weight called as a lead or sinker as shown in Fig.

Fig.6.13: lead lines

Fig.6.14: Lead lines

For better accuracy of reading, brass chain is much better than hemp lead line, because brass chain maintains its length.

If hemp lead line or cotton lead line are used for soundings; It get elongated due to wetting and hence to avoid this elongation, line is first properly stretched tightly between two posts or coiled tightly around a post or tree when wet and then it is dried. This process of stretching when wet and drying is repeated many times till the stretch is negligible.

This operation of wet stretching and drying is done before the line is graduated.

When it is observed and experienced that the line is free from stretch, then graduation is made on the line. Then line is soaked in water and graduated at every meter interval with the help of cloth tags or leather tags.

Fig.6.15: Sounding in deep and swift flowing water

Note that it should be tested at frequent interval by comparing it with length measuring steel tape. For deep and swift flowing water, the measured length of the line is more than the true length due to drag as shown in Fig. In such case, the correction should be applied to the measured length so as to get the true depth or correct depth.

Sounding machines:

Whenever the number of soundings to be taken are large and whenever depth of water is large, then sounding machine is more useful equipment. Sounding machine can be hand driven or power operated.

Fig.6.16: Hand driven sounding machine equipment

Hand-driven machine equipment consist of a piano wire and two dials. Piano wire is wound around a drum and it carries a lead having weight of 70 N. Out of two dials, outer dial shows the depth reading in metre (m) and inner dial shows the part of a metre (m). Two dials are connected to the drum with the help of gear system.

Fig. shows a hand-driven sounding machine equipment also called as Waddell's sounding machine.

Sounding machine is mounted on a sounding boat firmly. Sounding machine is useful for taking the maximum depth of 30 m.

Sounding chain:

Since the length of a brass chain remains constant in adverse atmospheric condition, it gives the better result for measuring the depth, hence for regular sounding works, a brass sash chain is preferably used.

The links brass chain is either brazed or welded. Leather or cloth tages are more suitable than brass tages, because brass tages may affect the injury to the hands of lead-mean. Tags are fixed to the chain at equal interval of 0.2m. Periodical checking and testing of chain should be done due to wearing of the links and tags.

Indirect methods:

It involves the sounding operations done by an electronics instrument known as fathometer or Echo-sounder.

Fathometer or Echo-sounder

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.6.17: 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,

Key Takeaways:

Soundings methods are as follows:

1. Direct method

Sounding boat

Soundings rods or poles

Lead lines or Sounding cables

Sounding machine

Sounding chain

2. Indirect methods

2. Fathometer or Echo-sounder

6.17 Stream gauging:

Stream gaging is a method used to degree the discharge, or the quantity of water transferring via a channel in keeping with unit time, of a flow.

The top of water withinside the flow channel, referred to as a degree or gage top, may be used to decide the dischage in a flow.

When used at the side of speed and cross-sectional region measurements, degree top may be associated with discharge for a flow.

If a weir or flume (devices, normally product of concrete, positioned in a flow channel which have a constant, regarded form and size) is used, mathematical equations primarily based totally at the weir or flume form may be used at the side of degree top, negating the want for speed measurements.

Key Takeaways:

Stream gaging is a method used to degree the discharge, or the quantity of water transferring via a channel in keeping with unit time, of a flow.

6.18 Three point problem:

Solution of three point problem is a special method for a plotting, when the soundings are located by two angles from the boat with respect to three well-defined known points on the shore.

In other words, the plotting can be done by adopting three point problem, when a sounding is located by two angles from the boat by observation to three known points on the shore.

Fig. shows the three shore A, B and C and angles

and

are known angles and it is required to plot the position of P.

Fig.6.18: Three point problem

Three point problem can be solved by three methods viz.

Mechanical method

Graphical method

Analytical method

Any one of the methods stated above is used to solve the three point problems.

Mechanical Method:

There are two ways of using mechanical method either by using a tracing paper or by using a station pointer.

Hence following are two mechanical methods preferably used.

1. Three point problem solved by a station pointer

Station pointer is a mechanical device by which solution to a three-point problem can be obtained.

It consists of three arms namely:

Middle arm

Two outer arms

Middle arm is fixed whereas two outer arms rotate about the centre of the instrument. Two outer arms are fitted with verniers, clamp and tangent screw. Vernier reads to 1 minute. Clamp and tangent screw facilitate the accurate arrangement. It also consists of fiducial edge which radiate to a common centre.

Procedure of using stational pointer

Initially arms are set by vernier in such way that it includes the observed angles i.e.,

and

Then instrument is moved over the paper until fiducial passes simultaneously through the three points a, b and c on the chart.

Then centre is marked with the help of pencil or pricker so as to obtain the point 'P'. This can also be done by drawing the rays along the edges of the arms. In this way, the point P is located or obtained.

Fig.6.19: Station Pointer

2. Three-point problem solved by a tracing paper

Take any point say 'o' on a tracing paper and angles o, and a are drawn from point 'o'. There are three shore point A, B and C.

The position of these shore points are plotted with respect to the shore line on a plain paper and then a tracing paper is taken over the plan in such way that the rays from point 'o' will pass through the points a, b and c. This process gives the apex 'o' of angles o, and a representing the position of required point 'P' which can be pricked with the helps of pin or can be marked with pencil on the tracing paper.

It is concluded that, the pin-point is marked as 'P' which is nothing but a sounding point'.

Graphical Method:

There are three graphical methods mentioned as follows:

1. First graphical method

2. Second graphical method

3. Third graphical method

1. First graphical method

Consider the position of the shore signals say A, B and C as shown in Fig. and let a, b and c are the plotted positions of A, B and C respectively.

Let angle a, and a made with respect to the point 'P' at the boat as shown in Fig.

Draw the Fig. for further simplification.

Fig.6.20: First graphical method

Procedure:

The position of point 'P' of the boat can be located by following steps:

Points a and c are joined as shown in Fig. A line 'ad' is drawn through 'a'. This line makes an angle a with line 'ac'.

A line 'cd' is drawn through 'c'. This line makes an angle a, with respect to 'ac' to meet the line drawn from c and d.

Then a circle is drawn through the points a, d and c.

Now the point 'b' is fixed along the diameter from 'd'.

Then points d and b are joined and extended further in such way that it meets the circumference of the circle at the point 'P' and this point 'P' shows the desired position of the point.

2. Second graphical method:

Consider the Fig. for better understanding. In order to obtain the required sounding point, the following graphical method can also be adopted as per the steps mentioned below:

Step 1: In Fig., points a and b and b and c are joined.

Fig.6.21: Second Graphical method

Step 2: The lines 'ao' and 'bo' are drawn from a and b. Each line i.e., 'no' and 'bo' makes an angle of (90°-) with 'ab' on the side towards point 'p'.

Extend the lines 'ao' and 'bo' such that it intersects at 'o' as shown in Fig.

Step 3: In same way, the line 'bo' and 'co' are drawn from b and e so as to meet at 'o'. Each line i.e., 'bo' and 'co' makes an angle of (90°) with 'be' on the side towards point 'p'. Extend the lines 'bo' and 'co' such that it intersects at 'o' as shown in Fig.

Step 4: Then a circle is drawn with centre as 'o' such that it passes through b and c.

Step 5: These two circles are drawn in such way that they intersect each other at a point 'p'. The intersection of both the circles at 'p' shows the required position of the boat.

3. Third graphical method

Consider the Fig. for better understanding its procedure. In order to obtain the required sounding point, third graphical method can also be used and its stepwise procedure is as follows:

In Fig., the points a and b and b and c are joined.

Two lines 'ad' and 'ce' are drawn perpendicular from a' and 'c' respectively as shown in Fig.

Line 'bd' is drawn through 'b' such that it makes an angle of (90°-a,) with respect to line 'ba' so as to meet the perpendicular through 'a' at 'd'.

In same way, line 'be' makes an angle of (90°- a) with respect to meet the perpendicular through 'c' at 'e' as shown in Fig.

Then points 'e' and 'd' are joined.

From point 'b', a perpendicular is drawn on 'de' so as to meet at 'p'. Thus, foot of perpendicular shows the required position of boat i.e., 'p' as shown in the Fig.

Fig.6.22: Third Graphical method

Key Takeaways:

Three point problem can be solved by three methods viz.

Mechanical method

Graphical method

Analytical method

6.19 Aerial Photogrammetry:

Aerial Photogrammetry is advance technique of obtaining the information (spectral, spatial, temporal) about material objects, area or phenomenon without coming into physical contact with objects or area, or phenomenon under investigation.

6.20 Objects of Aerial Photogrammetry

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.

6.21 Classification:

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.

Key Takeaways:

There are two types of photogrammetry:

Terrestrial photogrammetry

Aerial photogrammetry

6.22 Application of Aerial photogrammetry:

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.

6.23 Comparison of Map and Aerial photograph:

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.

6.24 Flight planning:

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Fig.6.23: Vertical profile through a flight line

Flight planning consists of the following information:

The area to be surveyed

Focal length of the camera

Scale of the photograph

Longitudinal and side overlap

Approximate ground speed of the aircraft in stable air

The above information is required to find the altitude of the aircraft above datum, time interval between the exposures, the area covered by each photograph, number of photographs number of strips and air base.

To cover the entire terrain to be surveyed, the sufficient photographs with suitable overlap should be taken.

1. Longitudinal overlap or forward overlap:

The overlap of the photographs occurs in the direction of flight line is called as longitudinal overlap or forward overlap. This type of overlap is kept at about 60%.

Vertical profile through a flight line can be easily understood by the Fig.

2. Side overlap:

The overlap between the adjacent flight lines is called as side overlap which is kept at about 30%.

Note that, there must be some overlap of the area covered by each aerial photograph so as to ensure a complete coverage of the area to be surveyed.

Following are the reasons for overlap:

Overlap is provided to orient prints so that there is a continuous flight strip formation.

Overlap is provided so as to discard the highly distorted outer portion of the photograph.

The overlapped portion is only useful for a stereoscopic vision.

The overlap is provided to prevent the possibility of gaps left because of the deviation of the aircraft from the flight line.

Other reasons to provide overlap are drift and crab.

3. Drift:

The failure of the aircraft to remain stay along with the flight line is called as drift when wind correction is not applied then drift is occurred.

4. Crab:

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.

Fig6.24: Crab

Fig.6.25: Drift

5. Altitude of aircraft:

Flying altitude (H) deals with the scale of the photograph, C factor and the contour interval and it can be determined by the following formula

H= (Counter interval) x (C-factor)

Where, H = flying height

C-factor varies from 500 to 1500

H can also be computed by the following formula

S = f/(H-h)

Where,

S = Scale of photograph;

F= Focal length of camera

H = Flying height;

h = Elevation of the ground above m.s.l.

6. Area covered by one photograph:

Area covered by one photograph can be determined provided the length and the exposed portion and the scale of the photograph are known.

Area covered by one photograph = [length x scale] [width x scale]

Key Takeaways:

Flight planning consists of the following information:

The area to be surveyed

Focal length of the camera

Scale of the photograph

Longitudinal and side overlap

Approximate ground speed of the aircraft in stable air

6.25 Calculation of No. of Photograph:

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