Detailed engineering and ecological/environmental surveys of the fish farm site follows after site selection. Actual site surveys is done in connection with the full-scale planning and design layout and construction of the farm.
The engineering surveys may include measurements or verification of distances, directions, and areas, and topographic mapping. Ordinarily, existing topographic maps which include probable sites cover large areas such that the scale used is small and unsuitable for planning designing process. For purposes of fish farm project development, a more suitable or updated topographic map has to be drawn out. A topographic map shows the nature of the ground relief or its characteristics, such as differences in elevation, location and measurement of boundaries or fences, physical facilities (buildings, roads, rivers, canals, bridges, land use-tilled, swamps, woods) and other features. This map is of paramount importance because it gives the basic horizontal (areal) and vertical (elevation) controls in the planning/design of the farm. It provides the relationship of the site with the tidal fluctuation, determines direction of water movement, guides in locating water control structures and in estimating quantity of earthwork, and other factors which are closely tied-up with development costs.
The ecological/environmental survey may verify or provide more in-depth information about the physico-chemical and biological make-up of the environment, in addition to what has been known during the site selection process.
This topic, therefore, covers the survey procedures involved in the measurement of distances, areas, levelling contour mapping, including the ecological concerns and their application to fishpond design and construction.
There are a variety of equipment available for field survey work. The principal equipment are composed of the engineer's transit, levels, magnetic compass, surveying tape, levelling rod, and range poles. Added to these are minor tools such as hatchet, mallet, bolos, stakes, etc. Other equipment for actual field mapping work is place table with alidade.
(a) Engineer's transit. The cost depends on the model available which range from the simplest kind to the most sophisticated model. It is a versatile piece of equipment which is used for measuring vertical and horizontal distances; vertical and horizontal angles, for prolonging lines, for levelling operations, and others.
(b) Levels. Just like the transit, levels vary from simple or less accurate models of hand levels up to the sophisticated and precision models such as the self-levelling level. These are used mainly for measuring vertical and horizontal distances in levelling operations. Some models are equipped with horizontal circle to measure horizontal angle.
(c) Magnetic compass. The essential features of a surveyor's compass are: (i) a compass box with circle graduated from 0° to 90° in both directions from the N and S points and usually having the E and W points interchanged: (ii) a line of sight in the direction of the SN points of the compass box: and (iii) a magnetic needle supported freely on a pivot. The whole compass can be attached to a tripod by a ball and socket joint.
(d) Levelling rod. Also called target rod, this is usually made of wood graduated either in English or metric units for measuring vertical distances in conjunction with the transit or level. This comes in lengths of 2 to 4 m.
(e) Range poles. These are slender round poles usually made of metal or wood painted with alternate bands of red and white. These are stuck along the line of survey in order to establish a straight line of sight.
Distances in survey work are measured in either vertical or horizontal plane. Vertical distances or differences in elevation in fish farm planning are usually determined by the use of level instruments and level rods. Horizontal distances are determined in various ways depending on the accuracy desired. Among the available methods, the common and practical ones in use are pacing, taping, and the stadia method.
Distances may be roughly calculated by pacing when the desired accuracy is not greater than 0.6 m in 30 m. A pace is the normal length of a step or stride of an individual. The length of pace of an individual should be checked with an accurately measured distance in order to determine the so-called Pace Factor. Pace Factor (P.F.) is defined as the ratio of the measured distance in the number of paces made by an individual to cover the measured distance or:
In determining the P.F., the measured distance is at least 200 m or more. The 200 m distance is walked at normal pace, counting the number of paces to cover it. This is done at least three times. The average number of paces is used as the divisor in determining the P.F.
A person who has a P.F. of 0.70 means that the normal length of his step is 0.70 m. If the same person has walked 3 000 paces, the rough estimate of the distance covered is:
|Distance||= P.F. × Number of paces|
|= 0.70 × 3 000 = 2 100 m|
In pacing, one should be aware of the factors which vary the length of pace. Some of these are: (i) when walking through short or tall vegetation; (ii) when going up or down hill; (iii) when walking on wet or dry ground; on plowed or firm soil; and (iv) when crossing fences.
Tapes (Fig. 3.1) are used for direct measurements of horizontal distances. Commercially sold are made of steel, metallic cloth or fiberglass materials. These tapes are graduated in English, metric or combination of the two systems of units and come in various lengths of 50 ft or 15 m. 100 ft or 30 m. and as much as 100 m. The first and last foot or meter of the tapes are usually fully graduated, as small as tenth of a foot or in mm. The middle graduations are in full feet or meter graduation. There are, however, exceptions to this, Accurate taping requires skill in the use of the tape, marking stakes or pins, plum bob and range poles or flags. For accurate taping, the following should be observed:
(a) Pull tape tight enough avoiding too much sag especially when long lengths of the tape is suspended. Break the tape or use only a portion of a tape length when measuring horizontal distances on slopping ground; use also plumb bob.
(b) On the other hand, avoid too much stretching as in the case of fiberglass tape.
(c) Error due to expansion (in the case of steel pipe) during hot days.
(d) Alignment of tape during measurement. Use range poles or flags as guide in having straight line of sight. Rear tapeman should always align with the head of the tapeman during taping.
(e) Count number of tape lengths carefully. Taping pins should be used (Fig. 3.1b).
Fig. 3.1 Equipment for measuruing horizontal and vertical distances
(f) Inspect the full tape length always before using. When damaged or short tape is used, apply correction properly.
(g) Be proficient in measuring distance less than a tape length, and in reading graduation in tape.
A quick way of measuring distance is by the stadia method. The measurement of distance by stadia uses the transit or level instruments, having telescope provided with stadia hairs (Fig. 3.2) and levelling rod. The stadia hairs are equidistant from the horizontal cross-hair. The procedure of measuring distance on level ground is illustrated as follows (Fig. 3.2). Let us say that distance OA is to be measured.
From the position of the instrument, distance OA can be determined by getting the upper and lower rod readings which are intersected by the upper and lower stadia hairs of the telescope, respectively. The general formula is as follows:
Distance OA = (Upper rod reading - Lower rod reading) (100) + Stadia Constant
Old model transits and levels have stadia constants indicated in their box but the modern ones have this value as zero. With stadia constant equal to zero, the formula becomes:
Distance OA = (Upper rod reading - Lower rod reading)(100)
The difference between upper and lower rod readings is called stadia interval. The unit of the distance follows the unit of the rod used.
As an example, if upper and lower rod readings are 2.75 and 0.25 m, respectively, the distance is:
Distance OA = (2.75 - 0.25) (100) = 250
Fig. 3.2 Illustration of stadia method
The horizontal cross-hair is used as a check in the correctness of the distance. Since the horizontal cross-hair is located at the middle of the two upper and lower stadia hairs, the distance measure using either of the two stadia hairs and the horizontal cross-hair is one-half of the distance OA. Hence, the check is as follows:
Distance OA = (Upper or lower rod reading - Middle rod reading) (2) (100)
If the line of sight is on sloping ground, it is necessary to apply a correction in order to obtain the true horizontal distance. This is done by the use of tables which indicate corrections for various angles of slope. However, in the fish farm survey, corrections are usually ignored as slope encountered rarely exceeds 5 percent and such percentage does not require corrections.
The direction of any line is measured in terms of angle between the line and some reference line — usually the North-South line in the compass. The instruments used to measure angles are compass, transit, tapes, plane-table, alidade, and sextant.
In general, accomplishment of survey work revolves in the properly organized measurements of distances and angles. Angles just like distances, are also measured along the horizontal and vertical planes are called horizontal and vertical angles, respectively.
Angles and directions may be expressed in different ways, namely: (i) bearing; (ii) azimuth; (iii) interior angles; (iv) deflection angles; and (v) angles to the right. Among these, the first two are commonly used in fish farm survey. The method using interior angles is useful in checking or adjusting the plotted sides of an area (based on the field data gathered to have a closed survey.
(a) Bearing. It is the angle that is referred from the North and South, whichever is nearest with the added designation of east or west, whichever applies (Fig. 3.3). A bearing can never be greater than 90°. Examples of bearing are: N 37° E. N 45° 50' W, S 54° 15'30" W, S 89° 45' E, N 90° E or due East.
(b) Azimuth. The azimuth of a line is a clock wise angle measured from a reference direction usually North. The South end of the North-South line is also being used as reference direction for azimuth in geodetic surveys. Azimuths based from the North are called North azimuth; those referred from the South are South azimuth (Fig. 3.4).
Fig. 3.3 Sketch of example bearing of line
Fig. 3.4 Illustration of north and south azimuth of a line
Some examples of equivalent azimuth and bearings of a line are as follows:
|120°||300°||S 60° E|
|200° 30||20° 30'||S 20° 30'W|
|290°||110°||N 70° W|
|30° 45' 30''||210° 45' 30''||N 30° 45' 30''E|
Determining equivalent bearings and azimuths can best be done by figuring out in which quadrant the angle lies.
(c) Deflection angle. This refers to the angle between a line and the prolongation of the preceding line. Deflection angles are identified as right or left. Right deflection if the angle measured lies to the right (clockwise) of the extension of the preceding line. Left deflection if the angle lies to the left (counter clockwise) of the extension of the preceding line (Fig. 3.5).
Fig. 3.5 Definition sketch of deflection angles
In the case of deflection angles, the reference lines for lines (or sides of a field) BC and CD, is the prolongation or extension of the preceding lines AB and CD, respectively. However, those reference lines must be tied up with the reference line of AB which could be expressed either in bearing or azimuth.
Deflection angles may have values between 0° and 180° but they are not usually used for angles greater than 90°. In any closed polygon (or sides of a given fish farm site), the algebrate sum of the deflection angles (considering right deflection as plus (+) and left deflection as minus (-) is 360°.
(d) Angles to right. Angles may also be determined by clockwise measurements from the preceding line to the following line (Fig. 3.6). Such angles are referred to as angles to right.
Fig. 3.6 Angles to right
(e) Interior angles. In any closed polygon, the angles inside the figure between adjacent lines are called interior angles (Fig. 3.7). The sum of the interior angles in a closed polygon is equal to (N-2)(180°), where N is the number of sides. For a five sided field, the sum of the interior angles is 540°. Actual value of every interior angle is computed from the field data on directions, such as bearings, azimuths or the other methods. An error is incurred if the total interior angles obtained from survey data is more or less than the value determined by the formula.
Fig. 3.7 Illustration of interior angles
As an example, the interior angles in A and B (Fig. 3.7) can be computed as follows:
(i) For interior angle A, example bearings of line AB and EA of N 80°E, and N 11°E, respectively, will be needed in the computation including the cross-directional lines N-S and E-W. Hence:
Interior Angle A = 10° + 90° + 11° = 111°
(ii) For B, the bearings of lines AB and BC, N 80°E and S 95°E, respectively, will be needed. Therefore,
|Interior Angle B||=||(90° - 10°) + 85°|
|or||=||180° - 10° - 5° = 165°|
Only the common or simple methods are presented herein.
(a) Measurement of angles by tape. As an example, the angle between two sides of a field is to be determined as in Figure 3.8. The procedure is as follows:
From the figure, measure and mark with stakes a convenient distance, say 50-m each along line OA and OB. The points at O, A and B are marked by range poles in order to have a straight line of sight. Measure the distance between CD and locate the mid-point E. The angle is then determined by using the sine function and trigonometric table or calculator.
Let us say, distance CE is 25 m; then,
|Locate the angle with Sin = 0.5 from the table or by calculator, which is 30°|
|therefore, angle 0 = 30° × 2 = 60°|
Fig. 3.8 Angle measurement by taping
Another situation is how to establish a perpendicular line to either of the two lines OA and AB to make a right triangle (Fig. 3.9). The angle is determined by taking the necessary measurements in order to use the sine or tangent function. However, the above method is slightly quicker than this.
Fig. 3.9 Angle by taping, right triangle method
(b) Measurement of angles and directions by compass and transit
(i) Bearing of a line. To determine the bearing by compass, set the instrument over some point on the line. Level the instrument and lower the magnetic needle of the compass to make it swing freely — next, sight along the line the bearing of which is sought. The bearing is then read on the graduated circle at the point of the needle which will be less than 90° and either in the West or East of the North or South. Example, if the needle stands 45° east of north, then the bearing is N 45°E (Fig. 3.10). Take note that the magnetic needle of the compass always aligns itself towards the magnetic North. During reading the North and South end of the needle is distinguished from each other by the counterweight, it being located in the South end.
Fig. 3.10 Measurement of line bearing
The engineer's transit is also provided with compass which is mounted on its upper or vernier plate. In taking bearing, the instrument is set-up on the line and properly levelled. Then the telescope line of sight, the 0° mark of the horizontal circle, as well as its vernier, are all locked and all in one line directed towards the magnetic North to coincide with the released free-swinging needle of its compass. After attaining a steady position, the horizontal circle is unlocked to make it rotate with the telescope. The telescope is then rotated about its vertical axis and directed along the line until the vertical cross hair bisects the marker stake or range pole. The angle read on the horizontal circle and the vernier corresponds to the bearing of the line. The directional N-S and E-W are taken from the compass.
(ii) Azimuth of a line by transit. Let us say that the azimuth of the line BC is desired (Fig. 3.11). Set the instrument at B. Backsight point A with the vernier set to read the azimuth of the line BA. When the telescope or line of sight is rotated to C, the vernier reading will be the azimuth of line BC.
Fig. 3.11 Measurement of azimuth
(iii) Deflection angle. Referring to Fig. 3.12, the deflection angle at B can be determined as follows: A backsight is taken at A with the vernier set at 0°. Then the telescope or line of sight is rotated vertically or plunged in altitude to point in the direction BD. Then the line of sight is rotated until it sights C. The vernier reads the deflection angle DBC.
Fig. 3.12 Measurement of deflection angle
This is usually encountered in the actual layout of pond dike. The job is easily done with transit but in its absence the use of tape is also convenient.
For example, it is desired to layout the centreline of dike CD (Fig. 3.13) perpendicular to dike AB at point D.
Fig. 3.13 The 3-4-5 method in laying out perpendicular lines
(a) The 3-4-5 method. It is a common knowledge that a right triangle is one whose sides are in the proportion of triangle, with shorter sides 3 and 4 perpendicular to each other while the longest side 5 is the hypotenuse. To lay out the perpendicular lines AB and CD using the same principle, the procedure is as follows:
(i) One tape length of 100 m is convenient to use such that the 0, 15, 35, and 60-m graduation marks can be held as a loop in one set-up.
(ii) Three men have to do the work. First man holds the zero and 60-m graduation of the tape, the second man, the 15-m and the third, the 35-m mark.
(iii) The tape is held tight enough, and the first and second man are aligned along AB while the third man adjusts himself as necessary to keep the tape stretched.
(iv) The points D and C are then marked and extended.
(v) This can be checked by using larger proportions of distance such as 30, 40 and 50 m.
(b) Intersection method
Fig. 3.14 Intersection method
(i) From Fig. 3.14 measure equal distance of 30 m from both sides of point D.
(ii) While one man holds the tape at 0, another man describes an arc using, say a full tape length of 50 m.
(iii) The procedure in (ii) is repeated at point 0'.
(iv) Point C is located by the two intersecting lines. Line CD is then perpendicular to AB.
The intersection method applies on relatively clear ground where the described are can be marked or seen. An alternative quicker procedure is to use one tape length of 50 m and an equivalent length of rope or two ropes of equal length. Two men hold one end of the tape/rope and each of them stays at point 0 and 0'. Another man holds the other end of the two ropes pulled tight and point C is located.
Suppose in Fig. 3.15, dike CD is to be laid parallel to and at 65 m distance from dike AB. From AB erect perpendicular lines EF and GH in the same way described in the previous topic. Extend the line from points E and G until it exceeds 65 m from AB. Measure equal distances of 65 m along EF and GH from AB. The line CD formed passing through points F and H is the desired parallel.
Fig. 3.15 Laying out parallel lines
In ordinary land surveying, the area of a tract of land is taken as its projection upon a horizontal plane and not the actual area of the surface of the land.
Available methods used in computing areas are the: (i) planimeter method — where boundaries of the farm are plotted to scale and area is determined by the use of planimeter; (ii) double-meridian-distance (DMD) method — where area is calculated from the coordinates of the farm; (iii) trapezoidal rule and Simpson's ⅓ rule for calculating areas of land bounded by irregular curves; and (iv) by plotting the boundaries to scale and dividing the tract into regular geometric figures (such as triangles, rectangles, or trapezoids), scaling the dimensions of these figures and computing their areas mathematically. Likewise, the tract of land may also be actually divided into regular figures and all necessary measurements of sides are taken.
Among these methods, the trapezoidal rule and the last method of subdividing into regular geometric figures are easily understood. Moreover, with the advent of precision pocket calculators, direct computation of areas is now convenient to do. The principles and procedures for two methods are illustrated in Appendix D.
This kind of survey requires technical know-how and skill in levelling operations. The ultimate objective in doing this survey is to reflect on map the relief or changes in elevation of the fish farm site including other relevant ground features.
This is a basic operation in engineering survey that leads to the production of a topographic map. Direct levelling is commonly used among the methods in levelling available and it is the process by which differences in elevation in the site is determined with the use of a level or transit instruments (Fig. 3.16) together with a level or stadia rod. When it is necessary to locate or fix the ground points as in the case of full-scale topographic survey, additional information on directions (angles) and distances are obtained.
There are two kinds of direct levelling — differential and profile levelling. Differential levelling is the operation which determines the difference in elevation of two points which are distance apart. Profile levelling is the operation that determines the differences in elevation of points along a prescribed line and at measured intervals.
The following terms and their definitions are useful in understanding the principle of levelling.
(a) Elevation — refers to the vertical distance of a ground point from the reference datum plane (MLLW).
(b) Bench mark (BM) — it is a station or point on the ground of known elevation and of a permanent nature. BM provides the reference elevation from which relative elevations for other stations are calculated. A BM may be established on permanent objects/structure on wooden or bamboo stakes driven firmly near a construction project.
Fig. 3.16 Level instruments
(c) Station (Sta) — any point where a rod reading is taken and is generally along the line being run.
(d) Backsight or plus sight (BS) — a rod reading taken on point of known elevation. This is used for obtaining the level line of sight or HI. Also known as plus sight since it is always added.
(e) Foresight or minus sight (FS) — a rod reading taken on any point of unknown elevation. Also known as minus sight since it is always subtracted.
(f) Turning point (TP) — it is generally impossible to take all the readings along the direction of survey without moving the instrument. The TP is an intermediate station or reference point whenever the instrument is moved from one set-up to another. A point which is no longer needed after the necessary readings have been taken.
(g) Height of the instrument (HI) — is the relative elevation of the line of sight of the instrument as referred to the elevation of the datum plane, bench mark or turning point.
(h) Ground profile — a graph of the ground surface which shows change in elevation (along vertical y-axis) with distance (along horizontal x-axis).
The principle involved, keeping of differential level note, the arithmetic check, and acceptable degree of accuracy are given below.
(a) Principle of differential levelling
Case 1: Two points visible. From Fig. 3.17, the difference in elevation, H = Ha - Hb; where Ha and Hb, are rod readings at points A and B, respectively.
Fig. 3.17 Case of two points visible from the instrument
To determine the elevation of point B, the elevation of point A must be known. Assuming that the elevation at A is ELA, the elevation of point B, ELB is:
|ELB||= ELA + Ha - Hb||Note that ELA + Ha = HI; and since the elevation at A is known, then Ha is a BS; Hb is a FS since the elevation of B is unknown.|
|= ELA + BS - FS|
|ELB||= HI - FS|
Case 2: The objective points are not visible from each other or far apart. To determine the difference in elevation between points A and B (Fig. 3.18), a series of differential levelling is done. This situation occurs when another bench mark is to be established in the fish farm project. Instrument is set approximately midway of turning points. The corresponding differential level note as obtained in the field for Fig. 3.18 is presented in Table 3.1.
(b) Arithmetic check. Completion of the level note for HI's and elevations follow after the field work. To check for the accuracy of addition and subtraction, the difference in the sums of the backsights and foresights must be equal to the numerical difference in elevation between BM2 and BM1, or:
Sum BS - Sum FS = Elev. BM2 - Elev. BM1
From the given example of a complete differential level note (Table 3.1), the corresponding arithmetic check is as follows:
|Sum BS =||9.53||Elev. BM2 =||4.32|
|Sum FS =||6.71||Elev. BM1 =||1.50|
|2.82 m||Check||2.82 m|
(c) Allowable error of closure. The arithmetic check determines only the correctness of the addition and subtraction done in completing the table. It does not tell the degree of error incurred in the conduct of the field survey. Depending on the nature of the job, the accuracy of work must be within the allowable limit prescribed. In fish farm site survey, the allowable error is largely dictated by the range of tidal fluctuations. The allowable error of closure for rough and ordinary levelling works are as follows (Davis, et al., 1966):
|Type of work||Allowable error, in feet (0.305 m)|
|(For reconnaisance or preliminary survey)||+ (0.4) ( / M ): Distance of sights is about 300 meters|
(Most engineering work)
|+ (0.1) ( / M ): Distance of sights is about 150 meters|
Where M = length of traverse or level circuit in mile (1.609 km).
Fig. 3.18 Levelling procedure when objective points are not visible in single instrument set-up
Differential level note for Figure 3.18
|BM1||1.50||3.00||-||1.50||BM, located on an undisturbed or permanent structure|
The actual error incurred during the field survey must not exceed the allowable error. Actual error is determined by completing the level survey loop or circuit. In other words, the levelling operation from point A to B (Fig. 3.18) is continued back (B to A), thus completing the loop and taking into consideration its length. An example of a level circuit is shown in Fig. 3.19.
Fig. 3.19 An example of levelling circuit
The actual error is the difference in values of elevation at A at the beginning of survey and computed elevation (also at A) at the end of the survey.
An important aspect in pond design and construction is devoted to control of water movement. Water must be conveyed in desired direction and at controlled velocities. To accomplish this, it is necessary to measure accurately differences in elevation along a definite line. Such line may be the centreline for a water supply canal or drainage ditch. The procedure in conducting profile levelling is illustrated in Fig. 3.20. The figure is the centreline of a freshwater supply canal for regulating salinity or other purposes in a brackishwater fish farm.
Profile levelling begins by setting up the instrument at a convenient location where several stations are visible. As BS is taken from the BM, then FS readings are taken as many as the instrument man can clearly read. When no more FS can be read, the level instrument has to be transferred. Before transferring, an FS is taken at the TP. At the new location of the level, a BS is taken from the TP, then additional foresights (FS) are again taken. The whole process is repeared until the work is finished. The column for elevations in the level note is completed by computations (Table 3.2).
There are only two HI's under the illustration. To compute for the elevation of the stations, the foresights from Station 0 + 00 up to 1 + 00 shall be subtracted from HI1. At TP1, a new HI must be determined. Elevations of remaining stations (1 + 25 to 2 + 00) are determined by subtracting the foresights from the HI2 of TP1.
Fig. 3.20 Illustration of profile levelling procedure
The profile level note for Figure 3.20
Beginning of canal on top of dike
End of canal
0 + 00
1 + 00
1 + 25
1 + 50
1 + 75
2 + 00
2 + 30
From the completed profile level notes, the ground profile is drawn using a cross-section paper (Fig. 3.21). The elevation is plotted on the vertical axis, while the stations or distances represent the horizontal axis. The scale for elevations are usually made larger than the horizontal scale to emphasize differences in elevation. Example of information obtained from the profile are available ground slope for determining possible velocity in canal, and location and depth of cuts or fills.
Fig. 3.21 Profile of centerline of supply canal
Contour lines show the configuration or changes in elevation of the ground in a topographic map. Each contour line represents points of same elevation and are spaced according to the difference in elevation between two adjacent lines. Hence, contour line (C.L.) 0.50 means that every point on that line has elevation of 0.50 m above the reference datum. If the next contour lines lower and above C.L.0.50 are 0.25 and 0.75, respectively, the contour interval is 0.25.
Topographic or contour survey is commonly done with three methods — the laying — out-square, random shot, and by sounding methods. These methods use combined knowledge of differential and profile levelling.
Accurate measurements of lengths and directions of the farm boundaries are done prior to taking rod readings during contour survey.
(a) Laying-out-square method. This method is done by literally setting up squares in the area with the use of either tape or transit. Each intersection or corner in the square is marked with stake and represents ground point where rod reading is to be taken. Proper identification of ground points must be done for proper note keeping and plotting during mapping. Identification is usually done using numbers in one direction and letters in the other direction (refer to Fig. 3.22). The format for data recording is given in Table 3.3
Fig. 3.22 Laying-out square method
Format for the level note for laying-out-square method
Note: “n” refers to the last number of observation
The above menthod is appropriate for small areas with less vegetation. With this method plotting of points on the scaled map is also easy. For large areas, however, laying out squares in the field becomes laborious and impractical.
(b) Random shot method. This method has more advantage than the preceding method when large areas are involved. Field survey is done faster, but plotting of ground points on the scaled map may take longer time since it involves measurement of angles and distances. A transit equipped with stadia hairs is best to use in this method. However, frequent check of the transit telescope level need to be done to minimize error. A plane table with telescopic alidade can also be employed in this method.
The sequence of field survey activities in the random shot method is as follows:
(i) The first step consists in measuring the boundaries of the field to be surveyed, determining the length, directions and angles of all sides.
(ii) The instrument stations have to be established in the area. The locations of the instrument position on the ground must be fixed so that it can be accurately plotted on the map. The instrument stations can be established either separately and ahead of the actual taking of rod readings or simultaneously as the work progresses. However, when the work is quite large, it is better to establish the instrument stations separately together with additional turning points or bench marks as necessary.
(iii) The instrument is set at Instrument Station 1 (or simply Station 1). Before random shots are taken, the location of Station 1 should be fixed or tied-up with at least two concrete corner points or monument of the land boundary. This is necessary in locating the positions of the instruments during map construction.
The location of point on the ground is fixed if measurements are made of: (a) its direction and distance from a known point: (b) its direction from two known points: (c) its distance from two known points; and (d) its direction from one and distance from another known point.
The position of the instrument at Station 1 in Fig. 3.23 is fixed by the two lines connecting Station to boundary monuments at points A and B.
(iv) With the instrument set at Station 1, a reference line for the radiating random shots is selected. This reference line can either be the North-South line of the instruments' compass or the line connecting two adjacent instrument stations. The illustration in Fig. 3.23 uses the line between Stations 1 and 6 as the reference line or the zero-degree (0°) line. To take random shots (rod readings) the transit telescope deflects incremental angles in clockwise direction from the 0° — line. Incremental angles of 3°, 5°, 10°, etc. may be used depending on the situation and convenience. Along the line of sight of these incremental angles, random rod readings are taken as many as necessary. Each rod reading, however, must be accompanied with corresponding angular reading and distance of the ground point from the instrument measured by the stadia method. Once the 360° - turn is completed, the instrument is transferred to Station 2. Before going to Station 2, a FS is taken from the selected TP1. Once the instrument is set at Station 2, a BS is taken from the TP1. At Station 2, the ground points not covered in Station 1 are now taken using the same procedure. The survey proceeds until the whole area is covered with radiating shots. Fig. 3.23 illustrates the general procedure in the random shot method.
Fig. 3.23 Illustration of instrument stations within an area and the random shots for each angle
(v) Once the field survey is completed, the elevation of all ground points are computed to complete the suggested format for level notes (Table 3.4).
(vi) Construction of the topographic map follows. A suitable scale to be used is selected based on the size of the paper and purpose of the map. Then the boundaries of the land is drawn to scale, followed by the loop of the instrument stations. Radial lines corresponding to the incremental angles at each station is drawn by using a protractor; distances along each line corresponding to the ground points are also plotted to scale.
The computed elevations are then noted on their respective ground points. Each building, road, creek, river, etc. should also be located with the proper symbols. Contour lines are then drawn out in order to show the topography of the land.
(c) Contouring. The contour interval (C.I.) to be used must be decided on before drawing the contour lines. The C.I. depends on the desired accuracy and purpose of the contour map. Generally, however, wider C.I. is used in area with rugged topography and closer such as 0.15, 0.25, 0.5 m for flat or gently sloping area as in the case of most fish farm sites. Examination of Fig. 3.24 provides better understanding on how to construct a contour map. The first contour line to be drawn may be based either from the lowest or highest ground elevation. Take note that a contour line always passes between ground points with values lower and higher than the value or number of the given contour line. Elevations denoting each contour line must be indicated, otherwise it does not mean anything.
Fig. 3.24 A contour map
Format for the level notes for random-shot method
The location of each contour line as it passes between points on the map can be determined by inspection or eyeballing and more accurately by interpolation method.
(d) Characteristics of contour lines. Certain knowledge on characteristics, of contour lines will facilitate contouring as well as its interpretation. Some of these characteristics are as follows:
All points on a contour line are of the same elevation.
Contour lines always close to itself within or outside the confines of the map. Where they close outside the boundaries of the map, they are ended at the boundaries.
Contour lines that close within the confines of the map represent either a rise or a depression.
Contour lines regularly spaced on the map represents uniform slope.
Contour lines never intersect nor cross each other except in the case of an overhanging cliff.
Contour lines never branch nor split.
Contour in crossing a valley run up the valley at one side, cross the stream, and run back on the other side.
The steepest slope is always at right angles to the contours.
It is a common situation in coastal fish farm site to conduct topographic survey on an area that is under shallow tidal water. The usual method used is by sounding. The following describes the procedure on how the survey is done by using magnetic (surveyor's) compass, stadia/levelling (sounding) rod, tape and two small boats. One boat is for the instrument (compass) man and the other for the rod man.
(a) The predicted tide curve for the day the survey is conducted is prepared in advance. This curve provides tide level information at any time of the day.
(b) The surveying party establishes the boundaries of area under survey as well as fixing location of some landmarks or ground points with reference to corner points of boundary lines.
(c) The instrument man at point A (refer to Fig. 3.25) locates his position on the map. Whenever appropriate, the instrument man chooses to tie-up directly from the corner points of the boundary or from fixed landmarks such as trees, in order to locate his position on the map. His position is located by taking down the bearings of at least two of his “tie lines” (i.e., the corner points or landmarks). The bearings obtained when plotted will intersect at one point which defines the position of the instrument man. The actual distance of the boat from the “tie lines” may be measured by tape to check. It is assumed that the water is shallow enough to permit actual measurements by tape. Otherwise, distance is measured approximately while on boat.
(d) Then the instrument man signals to the rodman that he is ready to sight him (the rod). The rodman holds the rod straight down to the ground surface (under water) at ground point (GP) 1 and the instrument man gets the bearing of GP 1. Rodman records the water surface level or depth of water at GP1 and the corresponding time. The process is complete after taking the distance by tape between points A and GP1.
Fig.3.25 Illustration of topographic survey by soundings
At this point, the ground surface elevation at GP, can already be computed as follows:
|Elevation of GP1=||Height of tide (from predicted tide curve) at the time rod reading is taken minus the rod reading at GP1.|
The location of GP1 is located on the map by plotting its bearing and distance with reference to point A.
(e) The rod man at GP1 then moves to another ground point, GP2 and the instrument man at A reads the bearing, the rodman records the depth and time; and the tape men measure the distance from A to GP2. This process is repeared until such time the rodman has covered the area that is visible from the instrument man.
(f) The instrument man moves to another position B. The instrument man now at point B backsights points A to get the back bearing of BA. Similarly, the tapeman measure the distance between AB. Hence, this locates point B on the map. Elevations of other ground points not yet covered at point A are subsequently taken using the same procedure. A sample of field survey data is presented in Table 3.5.
After a site has been selected, there are more information (in addition to those already gathered during the site evaluation) needed which are useful in the project development. It should be remembered that the site may have some strong and weak points during the evaluation. During the detailed planning and design, it is therefore necessary to identify the positive and especially the negative factors that would affect productivity of the cultured species, in order to make the necessary modifications in the engineering design and cultural management practices.
Among others, the following need more in-depth attention and evaluation:
More information on the physico-chemical and microbiological characteristics of the water should be obtained. The water temperature, pH, dissolved oxygen, biological oxygen demand (BOD), nutrients (NO3, P2O5, etc.), salinity, pollution and other information such as COD and measures of productivity must be looked into in detail. These values, aside from being used in design process, would also help in evaluating the environmental changes once the site has been cleared and the project established.
There should be at least two samplings done. Once at minimum and once at maximum tidal range. If possible, this sampling should also be done at different seasons. This allows calculation of data for intermediate ranges. Water samples or measurements should be obtained at least from the surface, mid-length and near the bottom of the sampling site. On each sampling occasion, observation may be done at each depth at hourly interval over a tidal cycle.
The normal salinity during high and low tide at different seasons of the year should also be known. The following are of particular importance and should be known: (i) the rate of flow and velocity in the rivers or canals during each season in relation to saltwater intrusion and formation of saltwater-freshwater wedges; (ii) the length of wedge from the river mouth, the depth of saltwater and freshwater layers within the wedge, and salinity values along the depth with wedge region; (iii) along with these, the frequency and duration of freshwater flooding. This is important to know if there is a need to induce mixing or destroy the wedge to alter the salinity; to minimize siltation along the river or canal, or in selecting the site for gate construction and others.
Sample field data on survey by soundings
|Height of tide |
|Instrument ground point||Water depth |
Verification of tidal fluctuations and currents is desirable. Particular attention should be given to changing winds and current patterns at different times (wet and dry seasons) of the year.
The added information on prevailing currents are useful in the planning and designing of erosion control measures to protect dikes or gates; finding out possibility of siltation or sedimentation in water control structures and how to solve the problem such as building of sediment trap within the water supply system and so on.
(a) Natural food. Abundance of food organisms in aquatic environment are indicators of productivity and considered the most valuable resource for extensive fish farming. The composition of plankton as well as some macrophytes are the major groups of food organisms that are of interest.
Plankton tows and netting should be done at some selected survey stations. Three net sizes, with mesh openings of 0.063 mm, 0.21 mm and 1.0 mm and towing speeds of 1.5 to 3, 3 to 5, and 6 to 8 km per hour will retain the majority of phytoplankton and the smaller and larger mesoplankton. All samples taken during plankton tows and netting should be preserved for subsequent identification and analysis.
(b) Bio-fouling. Any structure constantly immersed in water are susceptible to marine fouling and boring organisms. The rate and severity of attack depends on the kind of materials, location of the structure in relation to depth and hydrographic conditions and biological conditions in the area.
The foulers are grouped as microfoulers and macrofoulers. The latter should be given more importance than the former because macrofoulers include most of the “borers” such as shipworms (Teredo), bivalves (Martesia), gribble (Limnoria), and such encrustations as tubeworms, barnacles, bryozoans, algae, hydroids, mussels and oysters.
Among the borers, wood boring organisms is a factor in deciding the type of material to use in sluice gates. Examination of the kind of organisms should be done and evaluated if it is a problem. Old pieces of wood stuck in the ground, or wooden boats of residents in the area should be examined to determine which group of borers cause the damage.
(c) Seed resources. Continuous supply of fish seed (fry) is imperative in fish farming operation. In addition to ascertaining the supply from hatcheries or dealers, and assessment of local resource, evaluation of the availability and seasonality of cultivable species other than the identified species for culture should be surveyed. Assessment should also be done on the impact of clearing the mangrove on the local seed resource.
(d) Predators and competitors. Predators prey upon other animals while competitors may compete for food and space in the pond. Hence, these two are important factors affecting water and cultural management practices. The kind of screenings and pre-stocking procedures must be effective to control predators and competitors.
Examples of some predators that must be checked and evaluated are given below. Aside from the finfishes, benthic community in the site must also be examined. The quantified data on the benthos is desirable.
Some known predators and competitors of milkfish in the region are: Mozambique tilapia (Tilapia mossambica); tarpon (Megalops cyprinoides); ten pounder (Elops hawaiiensis); apahap (Lates calcarifer); erong-erong (Therapon jarbua; Therapon theraps); Indian white shrimp (Penaeus indicus); green tiger shrimp (Penaeus semisulcatus): yellow shrimp (Metapenaeus brevicornis).
(e) Vegetation. This is well discussed in Chapter 2.