Steel tapes are made of flat steel bands marked in various ways (see Fig. 5). The markings may be etched, stamped on clamps or soldered sleeves, or stamped on bosses. Steel tapes may be obtained in lengths up to 100 meters, although the most commonly used is 20 meters. Tapes are usually marked at 1 meter intervals, except the first and last meter, which are generally graduated in tenths.
Figure 5. Tapes
The Locke hand level, (see Fig. 6) is used for rough measurements of differences in elevation. It is used by standing erect and sighting through the eyepiece, holding the tube and moving the objective end up and down until the image of the spirit level bubble on the mirror is centered on the fixed cross wire. The point where the line of sight in this position strikes the rod or other object is then noted. The vertical distance from the ground to the surveyor's eye is used to determine the height of instrument and other ground elevations. A rough line of levels may be carried with the hand level for a distance of 10 to 15 meters, provided the length of each sight is not over 15 meters.
The Abney hand level, (see Fig. 7), is used in the same manner as the Locke Hand Level, except that it has a graduated arc for reading percent of slope. The spirit level is attached to the arc on the Abney level. The user sights through the tube and fixes the line of sight so that it will be parallel to the slope on which it is desired to measure the percent of slope. The indicator is then adjusted with the free hand until the image of the spirit level bubble is centered on the cross wire. The indicator is then clamped and the percent of slope is read. The Abney level may be used in the same manner as the Locke hand level for running a level line if the indicator is clamped at the zero reading.
Figure 6. Locke Hand Level
Figure 7. Abney Hand Level
The self-leveling level automatically levels its line of sight with great accuracy. There is no tubular spirit level and no tilting screw. It levels itself by means of a compensator after the circular spirit level is centered approximately. It is precise as well as simple and quick to operate and can be used for any type of level survey. (See Fig. 8)
This instrument is used primarily for measuring horizontal and vertical angles, prolonging or setting points in line, measuring approximate distances by the stadia principle, and for leveling operations. It can also be used as a compass when equipped with a compass needle. Horizontal and vertical plates graduated in degrees and fractions are provided for measurement of angles. They are mounted at right angles to the horizontal and vertical axis. Spirit levels are provided for leveling the horizontal plates. A telescope, equipped with a spirit level, is mounted at right angles on a horizontal axis supported by two uprights (standards) attached to the upper horizontal plate. In use, the instrument is mounted on a tripod and is equipped with a small chain and hook to which a plumb bob can be attached. (see Fig. 9) This provides a means of centering the instrument over a point. For details, refer to Annex 2.
Figure 8. Self-Leveling Level
Stadia rod and the range pole are two kinds of leveling rod which are commonly used in conducting site survey. (see Fig. 10)
The stadia rod is a four-piece 4 meters rod joined together with hinges and with a suitable locking device to insure stability. It has metal shoes on both ends. The face is approximately 3 ½ inches wide, which is divided into meters and tenths. This type of rod is designed primarily for use in making topographic surveys and is not equipped for use with a target.
The range pole, on the other hand, is a one or two-piece pole from 4 to 5 meters in length with alternate meters painted red and white. It is used generally to establish a “line of sight”.
The surveyor's compass, (see Fig. 11) consists essentially of a magnetic needle supported freely on a pivot at the center of a horizontal graduated circle. To this circle are attached a pair of sights. The needle and the graduated circle are enclosed in a brass case having a glass cover and the whole is attached to a tripod by a ball and socket joint. Leveling is aided by a circular spirit level. For details, refer to Annex 3.
Figure 9. Engineer's Transit
Figure 10. Stadia rod and range pole
Figure 11. The Surveyor's Compass
Measurement of Horizontal Distances are made either by direct and/ or indirect method. Direct measurements are made by pacing and chaining while indirect measurements are made by the use of stadia equipped instruments and graduated rods. The type of measurement used depends upon the required accuracy, access to the line and the time and cost involved.
a. Pacing. Measurement by pacing consists of counting the number of steps between two points and multiplying the number by a predetermined “pace factor”.
Pace factors for each individual is the average distance in meters per step. It can be best determined by pacing a measured distance (usually about 500 meters) several times. The pace factor is the distance in meters divided by the number of paces.
Measurement by pacing is permissible for preliminary and rough surveys, and for contouring by “gridding”.
b. Chaining. Chaining or taping is the method of measuring horizontal distances with the use of a steel tape. Accurate chaining requires skill in the use of the tape, plum bob, marking pins and range poles. In order to obtain sufficient accuracy in chaining, the following precautions should be observed:
Keep tape on line being measured, the rear chainman sights in the head chainman, the head chainman takes the zero of the tape;
Keep uniform tension on tape for each measurement;
Break chain on slopes as necessary to keep the chain level;
Accurately mark each station, the rear chainman calls out the number of his station and the head chainman marks his stake indicating one station has been measured; and
Keep accurate count of the stations.
c. Stadia. A rapid means of measuring distances is by the use of the stadia. The equipment required for measurement consists of a telescope with two extra horizontal hairs called stadia hairs and a graduated rod (stadia rod).
Most transits and levels have stadia hairs, one above and the other at an equal distance below the horizontal cross hair. The instrumentman observes through the telescope the interval in meters between the two stadia hairs when the rod is held vertically on some point. This interval, called stadia interval, is a function of the distance between the instrument and the rod. The ratio of this distance to the stadia interval is one hundred on most instruments, and to determine the distance to any given point, read the stadia intervals on the rod held at the point and multiply by 100, (see Fig. 12).
If the line of sight is on a grade (or slope) it will be necessary to apply a correction in order to obtain the true horizontal distance. This is done by the use of tables which give the corrections for various angles of slope. In fishfarm survey, however, corrections are seldom applied as slopes or grades encountered are rarely above or below 5 percent, and such percentage does not require correction.
Figure 12. Measuring Horizontal Distance with the Use of the Stadia
Methods used in computing areas are: (1) by the Double-meridian Method, (2) Simpson's Rule, (3) Triangulation Method, and (4) Trapezoidal Rule. The last two methods will be discussed as they are most easily understood.
a. Triangulation Method
Figure 13. Area Determination Using the Triangulation Method
In Figure 13, the field shown is an irregular closed figure that has been divided into small triangles. It is not necessary to measure any of the angles in order to determine the shape and size of the area. Where the length of the three sides of a triangle are known, the area may be computed by the following formula:
Since the above formula requires considerable calculations, it may be desirable to further subdivide each triangle into two to from right triangles. In the figure, perpendicular distances IG and DJ are drawn from the line AG. These distances are measured and the area of the right triangles are computed using the following formula:
Since the lines GD and FE are parallel, the area DEFG is a trapezoid. The area may be computed by using the following formula:
|h||=||height or distance|
The total area of the irregular figure is equal to the sum of A1, A2, A3, A4 and A5.
Example: Find the area of an irregular figure shown in Figure 13 using the triangulation method.
b. Trapezoidal Rule
Figure 14. Area Determination Using the Trapezoidal Rule
If a field is bounded on one side by a straight line and on the other by a curved boundary, the area may be computed by the use of the trapezoidal rule.
Along a straight line AB, Fig. 14, perpendicular offsets are drawn and measured at regular intervals. The area is then computed using the following formula:
|ho, hn||=||length of end offsets|
|Sh||=||sum of offsets (except end offsets)|
|d||=||distance between offsets|
Example: In Fig. 14, if the offsets from a straight line AB to the curved boundary DC are 35, 25, 30, 40, and 10, and are at equal distance of 30, what is the included area between the curved boundary and the straight line?
|=||117.5 × 30|
a. Laying out right angles. For instance it is required to lay out the center line of dike B (see Fig. 15) perpendicular to that of dike A using a tape. A simple corollary on the right triangle states that a triangle whose sides are in proportion of 3, 4, and 5 is a right triangle, the longest side being the hypotenuse. In the figure, point C is the intersection of the two dike centerlines. One man holds the zero end of the tape at C and 30 m is measured towards B. Again from C, measure 40 m distance towards A and then from A' measure a distance of 50 meters towards B'. Line CB' should intersect line A' B'. Therefore, line CB is formed perpendicular to line CA. It is always desirable to check the distances to be sure that no mistake has been made.
Figure 15. Laying Out Right Angles
b. Laying out parallel lines. In Figure 16, CD is to be run parallel to AB. From line AB erect perpendicular lines EF and GH in the same manner described in the previous discussion. Measure equal distances of EF and GH from line AB and the line formed through points C' and D' is the required parallel.
Figure 16. Laying Out Parallel Lines
a. Bench Mark (BM). A bench mark is a point of known elevation of a permanent nature. A bench mark may be established on wooden stakes set near a construction project or by nails driven on trees or stumps of trees. Nails set on trees should be near the ground line where they will remain on the stump if the tree will be cut and removed. Procedure on setting up a bench mark is attached as Annex 4. It is a good idea to mark the nail with paint and ring the tree above and below also in case a chain saw is used to cut down the tree.
The Philippines Bureau of Coast and Geodetic Survey has established bench marks in nearly all cities and at scattered points. They are generally bronze caps securely set on stones or in concrete with elevations referenced to mean sea level (MSL). The purpose of these bench marks is to provide control points for topographic mapping.
b. Turning Point (TP). A turning point is a point where the elevation is determined for the purpose of traverse, but which is no longer needed after necessary readings have been taken. A turning point should be located on a firm object whose elevation will not change during the process of moving the instrument set up. A small stone, fence post, temporary stake driven into the ground is good enough for this purpose.
c. Backsight (BS). Backsight is a rod reading taken on a point of known elevation. It is the first reading taken on a bench mark or turning point immediately after the initial or new set-up.
d. Foresight (FS). Foresight is a rod reading taken on any point on which an elevation is to be determined. Only one backsight is taken during each set-up; all other rod readings are foresights.
e. Height of Instrument (HI). Height of instrument is the elevation of the line of sight above the reference datum plane (MLLW). It is determined by adding the backsight rod reading to the known elevation of the point on which the backsight was taken.
The following describes the procedure of determining ground elevations using the engineer's level with a horizontal circle and stadia rod. A transit may be substituted for the level if care is exercised in leveling the telescope. It is assumed that a bench mark with known elevation has been established.
Establish your position from a point of known location on the map. In Figure 17, point B is “tied” to a point of known location on the map, such as corner monument C of the area. This is done by sighting the instrument at C and noting down the azimuth and distance of line BC. The distance of B from C is determined by the stadia-method discussed under area survey.
Figure 17. Establishing Position from a Point of Known Location on the Map
Take a rod reading on the nearest bench mark (BM), as shown in Figure 18, previously installed for such purpose. This reading is called the backsight (BS), the rod being on a point of known elevation. The height of the instrument (HI) is then found by adding the elevation of the bench mark (Elev.) and backsight (BS), thus:
H.I. = Elev. + B.S.
Figure 18. Transit-stadia Method of Topographic Survey
The telescope is sighted to point D, or any other points desired, and take the rod reading. The reading is called the foresight (F.S.), the rod being on a point of known elevation. Ground elevation of point D is then determined by subtracting the foresight (F.S.), from the height of the instrument (H.I.), thus:
Elevation = H.I. - F.S.
Similar procedure is used in determining the ground elevation of several points which are within sight from the instrument at point B. The azimuth and distance of all the points sighted from point B are read and recorded in the sample field notes such as shown in Figure 19.
Figure 19. Sample Field Notes on Transit-stadia Method of Topographic Survey
The following describes the procedure of conducting topographic survey by soundings using the tape, compass and stadia or sounding rod. Two bancas should be used.
The tide curve of the day the survey is conducted is drawn in advance. This provides the information on the heights of tide at different times of the day.
The instrument man at point A (see Fig. 20) locates his position on the map by sighting on known points and taking down the bearings and distances of his “tie lines”.
The stadia rod is held at some point M (under water) and the instrument man at A sights on M. Distance of M from A is measured. Man at M takes the rod reading of the water surface and notes the time.
The elevation of the ground surface at M is equal to the height of the tide at the time of rod reading minus the rod reading at M. Point M is located on the map by plotting its bearing and distance referred to from Point A.
Figure 20. Topographic (Hydrographic) Survey by Compass, Tape and Sounding Rod
The rod man at M then moves to another point and the instrument man at A again takes the rod reading and time. This process is repeated until such time the rod man has covered the area that the instrument man can see.
Instrument man at A moves to another point, B. Back bearing (BA) and the distance from A to B are taken. This locates point B on the map. Elevation of other ground points not yet covered at A are taken using the same procedure. A sample of a field survey notes is presented as Figure 21.
|hr. mi.||of Tide||Elev.||REMARKS|
|A||M||N 80 ° E||21||10.00||1.06 m|
|N||N 40 ° E||29||10.15||1.04 m|
|O||S 70 ° E||61||10.35||0.98 m|
|B||S 60 ° E||42||10.46||0.94 m|
|B||P||N 45 ° W||20||10.50||0.90 m|
|Q||N 45 ° E||26||10.55||0.85 m|
Figure 21. Sample Field Notes on Topographic Survey by Soundings