Hydrographics surveying

 Introduction, shore line survey, soundings methods,
gauges,
 Equipment required for hydrographic surveying,
sounding.
 Methods of locating soundings.
 Reduction of soundings and plotting of Soundings
 Problems related to hydrographic surveying.
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 It is the branch of surveying which deals with water bodies
e.g. Lake, river etc.
 Hydrographic Surveying includes all types of hydrological
observations which are necessary for design of hydraulic or
marine structure.
 For design of dams barrages, and weirs , it is essential to
know amount of rainfall on catchment area and discharge
of water in rivers.
 The usual fundamental principles of surveying and leveling
are adopted for acquiring data for determination of :
1. Water volume
2. Rate of flow
3. To determine the shape of the area underlying the water
surface etc.
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1. To determine the quantities of subaqueous
excavations.
2. Measure areas subjected to scouring or silting in
harbours or docks.
3. Locate rocks and other objects such as buoys, lights
etc to aid safe navigation.
4. To prepare navigation charts exhibiting the depths
available for navigation.
5. Control floods, and to plan water supply and storage
from rivers.
6. To develop water resources for power, irrigation and
recreation
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1. The measurement of depth of water at various points
is termed as sounding.
2. Depth of Sounding is referred to the water level at
the time it is made.
3. A number of benchmarks (B.M.) are established at
frequent intervals along the shorelines, and gauges
are set on them.
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1. The field work consists of both horizontal as well as
vertical control.
2. The horizontal control is established by traversing or
triangulation.
3. For vertical control, the tide gauges are kept in
operation continuously since the water level at the
gauge must also be known when soundings are
recorded.
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1. These are used to determine the exact water surface
level.
2. The movement of tides during the time soundings are
made.
3. The gauges are read at regular intervals, varying from 10
min to 30 min.
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1. These gauges may be non-registering or self
registering types.
2. Non registering types requires an observer to record
the water level.
a) Staff gauge.
b) Float gauge
c) Weight gauge
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10
1. Simplest type of gauge.
2. It consists of a graduated board, 150 to 250 mm wide
and 100 mm thick, fixed in vertical position.

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1. The float gauge is designed to overcome the difficulty
in reading a staff gauge when the intensity of tides is
high and the variations of water level is more.
2. It consists of a float to which a graduated vertical
staff is attached.

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1. The weight gauge consists a weight attached to brass
chain or wire.
2. The chain passes over a pulley, and is laid horizontal
along the side of a graduated scale.
3. The weight is lowered to touch the water surface,
and the reading is taken on the graduated scale
against an index attached to the chain.

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1. Self registering tide gauges automatically record the
variation of water level with time.

 The process of determining depth below water
surface is called as sounding.
 The step before undergoing sounding is determining
the mean sea level.
 Depth of Sounding is referred to the water level at the
time it is made.
 The reduced level of any point of a water body is
determined by subtracting the sounding from mean
sea level, Which it is analogous to levelling.
 A number of benchmarks (B.M.) are established at
frequent intervals along the shorelines, and gauges
are set on them.
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1. Preparation of accurate charts for navigation.
2. Determination of the quantities of the material to be filled.
3. Obtaining information for design of breakwaters, sea wells
etc.
4. Material that to be dredged has to be determined early to
facilitate easy movement in project without any confusion.
5. Material dredging should also accompany where filling has
to be done. Material dumping is also measured
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1. The sounding points should be selected keeping in mind
that all the important irregularities are recorded.
2. The soundings are thus made along a series of straight
lines at right angles to the shoreline.
3. The spacing between the sounding lines and between the
sounding points depends upon the nature of submarine
surface as well as on the object of the survey.
4. Usually Spacing between sounding lines is kept 30 m and
spacing between sounding points is kept 7.5m to 15 m.
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20
Shore line
Range line
Sounding point

A range or a range line is the line along which soundings are
made.
These are usually fixed perpendicular to the shoreline and are
parallel to each other.
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The essential equipment and instrument employed for
taking the sounding may be grouped as :
1. Shore signals and buoys.
2. Sounding equipment.
3. Angle measuring instruments.
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 Shore signal are required to mark range lines.
 Each range line is marked with two signals, at some distance
apart, along it on each shore.
 The signals are usually wooden tripods with a white and
colored flag on the top.
 A buoy is a float made of light wood or hollow air tight vessel
properly weighted at the bottom, and is anchored in a vertical
position by means of wires.
 In deep waters, the range lines are marked by a signal at shore
and buoys in water.
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1. Sounding boat :
 The sounding operation is carried out from a flat bottom boat
of low draft.
 The boats are generally provided with opening, called wells
through which soundings are taken.
 The motor should have adequate control both for low speed
and for rough waters
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2. Sounding Pole or rod
 These are made of strong well seasoned timber usually 5 to 10 cm in
diameter and 5 to 8 m in length.
 The sounding rods consists of two or three lengths screwed together
so that unnecessary length may be removed when not required in
shallow water.
 A lead shoe of sufficient weight is fitted at the bottom to keep the
rod vertical in flowing water, and to avoid sinking in mud or sand.
 The graduations on the rod are marked from bottom upwards.
 Thus, the reading corresponding to the water surface, is directly the
depth of water.
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3. Lead line
It consists of a graduated line or chain to which a lead is attached.
Under ordinary tension, when wet, the line should not change its
length.
Every one feet of the lead is marked with a cloth.
The mass of the lead is generally between 5 to 10 kg, depending upon
the strength of current and depth of water.
A correction is required to be applied to the measured length to get the
true depth when using lead line.
Due to drag, the measured length will be greater than the true depth.
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5. Echo sounding machine / Fathometer
 Where the depth of water is too much, an echo sounding
machine known as fathometer is used.
 It measures the depth below the boat on which it is installed.
 It works on the property echo property.
 It consists of a transmitter and a receiving oscillator, recorder
unit and a power unit.
 The sound waves emitted at the surface of the water are
recorded back after these return from the underwater surface.
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 Since the velocity of sound waves in water is known, the
distance travelled by sound waves can be calculated.
 D = ½ V t
 This method is very accurate and can be used in strong winds.
 It is more sensitive than other methods, and produces
continuous record in the form of curves when boat is in
motion
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Advantages of Fathometer :
1. It is more accurate than the lead line.
2. It can be used in strong currents or streams.
3. It is more sensitive than the lead line method.
4. It can be used on days, or in any weather, when the ordinary
lead line method would be impossible.
5. It is much more rapid in use than the ordinary method
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Most common angle measuring instruments are :
1) Theodolite
2) Prismatic compass
3) Sextant.
The theodolite and prismatic compass are not suitable for angle
measurements from sounding boats due to instability of rowing
boats.
Sextant has been found to be most suitable for measuring angle
in any plane.
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Navigators and surveyors measure angles from sounding boat
by sextant only.
When observations are made from the shore, theodolite and
prismatic compass are used.
The sextant used in hydrographic surveying is known as the
sounding sextant.
It slightly differs in construction from the astronomical sextant.
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Points to be kept in mind while using sextant :
1) Angle measured with a sextant is an oblique unless three
points sighted lie in horizontal plane.
2) The size of the angles and length of sights affect the
precision of angle measurement.
3) A sextant is not recommended for angle measurements
when the angles are less than 15 degrees and the sights are
less than 300 m.
4) Vertical angles can also be measured with a sextant in a
similar manner to that of horizontal angles.
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Hydrographics surveying

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