Hydrographic Survey (Marine Survey) By D.M Siddique

Date: 27/11/2012

Hydrographic SurveyingLecture 21: Hydrographic Survey

Marine Survey

Dr. Mohsin Siddique

NU-FAST Lahore Campus

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Hydrographic Survey

� Hydrographic survey includes the following

� 1. Rain gauging

� 2. River gauging

� 3. Marine survey

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Vertical Control

� According to the requirements of the particular survey, sounding will be reduced to one or two datum lines,

� 1. The land leveling datum

� This being the one generally used in civil engineering construction, because it enables the levels to be directly related to those of the adjoining shore installations.

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adjoining shore installations.

� 2. The tide datum

� It is generally used for navigational purposes. The usual level adopted is that the level of water surface below which the tide rarely falls

Tides

� Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth.

� Tide changes proceed via the following stages:

Sea level rises over several hours,

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� Sea level rises over several hours,

� The water rises to its highest level, reaching high tide.

� Sea level falls over several hours, revealing the intertidal zone; ebb tide.

� The water stops falling, reaching low tide.

Depth and Tide Measurement

� Depth measurement

� Similar like river depth measurements

� Tide Gauges

� Non-Recording

� Recording

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� Recording

� The tide gauges must allow the following measurements to be made simultaneously:

� (a). The level of undisturbed sea

� (b). The time at which this level occurs

Tide Gauges

� Non-Recording

� Also termed as Staff gauges

� Zero has been set at datum level

� Difficult to read due to

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� Difficult to read due to wave action

Tide Gauges

� Recording

� Termed as Float gauges if float mechanics is used

� Consist of stilling chamber to avoid fluctuations due to waves

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fluctuations due to waves

Tide Gauges

� Recording

� Contain pressure transducer which records water pressure over it and hence depth of water causing the pressure is

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causing the pressure is computed

� Auto-record and can create remote link to upload data

Horizontal Control: The shore framework

� Horizontal position fixing measurements may be carried out using:

� Hand-held optical square in conjunction with a float line

� Single theodolite in conjunction with a float line or twin

� Double sextant angles method

� Constant range tracking electronic positioning system (EPS); and

� Differential Global Positioning System (GPS).

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� Differential Global Positioning System (GPS).

Optical squares are simple sighting instruments used to set out right angles.


� Hand-held optical square in conjunction with a float line (Parallel line method);

� The hand-held optical square in conjunction with a float line method, also known as the parallel line method (Figure), is the most basic

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� It consists in setting out a straight baseline along the beach, say 100 metreslong or more, depending on the extent of the hydrographic survey, with a ranging rod placed at either end.

� At every 5 or 10 metres from either end (5 metres for irregular terrain and 10 metres for flat beach), a steel peg is driven into the ground and, by means of an optical square (or theodolite), a buoy is dropped offshore at right angles to each peg.

One end of the float line is anchored to the steel peg and the other to its

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� One end of the float line is anchored to the steel peg and the other to its corresponding buoy offshore.

� By tying in the baseline to the topographic survey, the depth readings may be plotted on paper in the right place.


� Single theodolite in conjunction with a float line or twin theodolites

� The single theodolite, in conjunction with a float line (ray method) or twin theodolite intersection (triangulation method), is the second most basic method of fixing positions offshore.

� In the past, positioning by these methods from baseline points onshore was often used to position vessels on near-shore projects

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� Although it is no longer commonly employed, this method is considered acceptable for all classes of surveys with a maximum offshore distance of 300 metres for the ray method and around 1 000 metres for the triangulation method


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� Double sextant angles method

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� In this method, simultaneously horizontal sextant angles are observed among three shore station.

� The accuracy of attained at distance from shore depends upon the operators experience.

� The vessel should be equipped with a plotting board and appropriate equipment for fixing the position of the sounding as soon as the two angles have been read by the observer.

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soon as the two angles have been read by the observer.

� Setting the angles between the three legs to match the observed sextant angles and then placing all three legs on the plotting sheet over the shore stations observed ensures that the position of the vessel is at the center of protractor of sextant.


� Constant range tracking electronic positioning system (EPS);

� The constant range tracking EPS used to be the most commonly employed positioning method.

� This method replaced the triangulation theodolites with electronic microwave ranging EPS which utilize range-range positioning techniques.

� This method has now been superseded by GPS techniques.

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� Differential GPS is the primary survey reference for all types of present-day engineering and construction activities.

� GPS is a continuous, all-weather, worldwide, satellite-based electronic positioning system.

� It is available to the general public and is known as a standard positioning service.

� Over the past several years, a technique has been developed to process

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� Over the past several years, a technique has been developed to process signals from two GPS receivers operating simultaneously to determine the 3-D line vector between the two receivers.

� This technique is known as “differential positioning” (DGPS) and can produce real-time positions of a moving vessel.


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Thank you

� Questions….

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Documents

Hydrographic Survey (Marine Survey) By D.M Siddique