Sounding in Hydrographic Surveying – Methods of Locating Sounding
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In hydrographic surveying, sounding is the measurement of depth below the water surface. Sounding is most important for any water body to improve its navigable properties, to know about silting and scouring etc.
Hydrographic surveying is branch of surveying which deals with the measurements of hydrographic bodies or water bodies.
Methods of Locating Soundings in Hydrographic Surveying
The soundings are located by the observations made from the boat or from the shore or from both.
There are four methods are there to locate the soundings by:
Conning the survey vessel
Observations with theodolite or sextant
Theodolite angles and EDM distances from the shore
Sounding by Conning the Survey Vessel
In this method, conning means keeping the boat at known course. This method is suitable for rivers, open sea up to 5 km off shore. The markers are fixed on the shore called as ranges along which vessel or boat is run. This method is again sub divided into two types as follows.
Location by cross rope
Location by range and time interval
Location by Cross Rope
In this method, a wire or rope with markings or tags at known distances is stretched across the channel. The starting point of rope at the shore is marked as reference point. Then using boat, the sounding at different distances of wire are determined by weighted pole. This method is more accurate. This is most suitable for rivers, narrow lakes and for harbors. This is also suitable for knowing the amount of material removed by dredging.
Location by Range and Time Interval
In this method, the boat is positioned in range with two signals provided on the shore. Then, the boat is rowed at constant speed and time required to reach the instant of sounding is measured which gives the distance of total point along the range. This method is more suitable for less width channels or rivers. It is not so much accurate.
Sounding by Observations with Theodolite or Sextant
Theodolite or sextant is used to measure angles in surveying. In this method, the sounding is located by measuring angles. Here also, there are a lot of subdivided methods are there to locate sounding. They are
By range and one angle from the shore
By range and one angle from the boat
By two angles form the shore
By two angles from the boat
By one angle from the shore and one angle from boat
By intersecting angles
By Range and One Angle from the Shore
In this method, boat is kept in range line with the help of two signals on the shore. The boat is moved and the point where sounding is measuring is observed by the theodolite or sextant and angle is noted. Using this angle, we can fix the point in the range.
Likewise, all other soundings are observed from different stations. The angle should be more than 30 degrees otherwise fix should be poor.so, whenever the angle is less than 30o, new instrument station is selected. This method is so accurate and easy for plotting the sounding details.
By Range and One Angle from the Boat
This method is similar to the above method, but in this case, the angular measurements are taken from the boat to different stations positioned on the shore. This is also having similar accuracy to the above method.
But, there are some advantages in this method as compared with above method. Angle measured from the shore from different stations is difficult when compared to angle observed from the boat to all stations.
So, the surveyor in this case has better control over the operations. Check can be made by measuring second angle towards some other signal on the shore for important fixes.
By Two Angles form the Shore
In this method, two instrument stations are fixed on the shore with proper distance. Two instruments and two instrument men are required to do this job. From the two instrument stations, angular observations are made and a point is located where sounding is measured.
If the angle made by instrument is less than 300 then new instrument station is selected. In this case, primary setting out and erecting range signals are eliminated. This method is useful when water currents are strong and difficult to row the boat along range line.
By Two Angles from the Boat
In this method, three constant points on the shore are selected. Using three-point problem, boat is positioned in range line and angles are observed from the boat to two of the three known positions.
The known positions may be light house, church spire, etc. like objects on the shore. If fixed positions are not available, then go for shore signals or ranging rods.
By One Angle from the Shore and One Angle from Boat
This method also requires two instruments and two men to operate. This is the combination of above two methods. In this method, two instrument points are located on the shore and instrument is placed only at one point. Other instrument is placed in the boat.
The first angle is measured from the first point on the shore to boat and from the boat second angle is measured from boat to second point. At that instant sounding is measured.
By Intersecting Angles
In this method, sounding is determined periodically at same points. This method is essentially used for harbors, reservoirs etc. to know the amount of silting or scouring happened at that points.
Number of signals are erected on the shore and a boat is rowed perpendicular to the shore and measure the sounding at a point where inclined line of signal intersect the line of signal next to it as shown in fig.
Flag rods are erected at sounding points to avoid confusion for the next round of measuring.
In this method, tachometer is placed on the shore and staff is placed on a boat. The staff intercept “s” is known by tachometer from this the distance between boat and instrument is known. This method is suitable when water is stable and sounding location is nearer to the shore.
Sounding by Theodolite Angles and EDM Distances from Shore
In this method, EDM and Theodolite are placed on the shore in fixed positions. From this set up, the reflector placed on the boat is targeted and point of sounding is located.
This method is more accurate when the water is still. This is one of the modern methods of fixing sounding vessel.