Equipment
1. 1 Prismatic Compass
2. 4 Ranging Poles.
3. 1 Engineer’s Chain (100ft or 30m)
4. 10 small Pegs or arrows for use with Chain.
5. 1 Survey field book
6. Pencil
7. Eraser
8. Tripod
9. Linen Tape (50m)
10.Plumb line
11. Optical Square
Mandatory Pre-Survey Checks
1. Check for compass error.
2. Check for true length of the Chain as this can introduce cumulative errors in measurements.
3. Ensure that you know the current magnetic declination of the area to be surveyed. This is particularly important when going to the field with a forest boundary schedule; as there will always be the need to add to each bearing, the Magnetic Variation or Declination of the area
4. Ensure that the survey is tied to a point or landmark, the co-ordinates of which can be obtained. (e.g. A forest Boundary Pillar or any other Gold Coast Survey Pillar (GCSP). These days, it is possible to read off Real World co-ordinates of points with the Global Positioning System (GPS), without reference to data from the Survey Department. Real World Co-ordinates are particularly important for placing the surveyed area accurately onto a map of Ghana
Before conducting a survey, it is often recommended that a reconnaissance survey be carried out. This is to ensure inter-visibility between adjacent stations. Although reconnaissance survey may not be obligatory, it is nevertheless a very helpful exercise.
If the survey is to be tied to a pre-existing pillar, a tripod is placed above the pillar and a plumb line made to fall vertically above the pin point of the pillar. In the absence of a plumb line; (a piece of lead with a tapering end hung to the base of the tripod with a thin piece of thread), a piece of stone is placed at the point of convergence of the legs of the tripod just below the compass, and made to drop freely onto the top of the pillar. If the stone falls right on top of the central pin of the pillar, the tripod may be considered as being vertically above the pillar. The Compass is then fitted to the tripod and balanced horizontally with the aid of the spirit level. A ranging pole is then placed as far from the starting point as is convenient and visible, for the bearing of this pole to be taken from the starting point. Never conduct a survey holding the compass in your hand. In the absence of a tripod, a wooden “peg” (monopod) may be cut for use. The length of the monopod is best at breast height of the compass reader. The top of this monopod should be about the size of the base of the compass and made flat for ease of placing the compass on it. Since the monopod cannot be placed conveniently on top the survey pillar, it is fixed directly behind the pillar, with the line of sight passing over the centre of the pillar, usually indicated by a pin. The bearing to the ranging pole is then taken after the compass, resting on the monopod, is levelled. In this case, measurement of the distance should be from the top of the pillar to the ranging pole and not from where the monopod (with compass on it) is positioned.
The tripod or monopod is then moved to the location of the ranging pole and the process repeated. The compass must always shifted to and be placed directly above the location of the ranging pole whose bearing has just been taken, for the next bearing to be taken.
IMPORTANT DEFINITIONS:
The Magnetic North (MN):
Our Earth has a magnetic axis inclined to the line of longitude, which divides the earth into two equal parts. This magnetic axis is the property that influences the needle of a compass. When a compass needle is allowed to swing freely and settle, it points to the northern pole of this axis, and the direction so indicated is referred to as the Magnetic North. The magnetic North therefore is the direction of the pole of the earth’s magnetic axis from any point on the earth’s surface as indicated by the freely suspended needle of a compass. It is important to note that the Magnetic North forms the basis for all angular measurements with surveying instruments. Without it, surveying with the theodolite and compass would not be possible. (See Fig. 2.1)
True North (TN)
The direction indicating the pole of the earth’s geographic axis in the Northern Hemisphere All other lines referenced to this are referred to as true north bearings. (See Fig.2.1) The figure below shows the Magnetic North (MN), the True North (TN), the True South, (TS) and the Magnetic South (MS).
The Azimuth:
The Azimuth is the smallest bearing to a point measured Eastward or Westward from a particular reference North. Azimuths may be measured either with reference to the Magnetic North or to the True North and referred to as Magnetic North and True North Azimuths respectively. The azimuth begins from °0 or 360° representing North and runs through 90° East, 180° South, 270° West and back to 360°North.
BEARINGS:
The mathematical analysis of survey data begins with the reduction of the bearings obtained from the analysis above, toquadrantal bearings.(See Fig. 3.1 below)
The next thing to do is to resolve the measured ground distances to their Horizontal and vertical components. We do this by finding the sine and cosine of each quadrantal bearing and multiplying by the measured distance on the ground. The sine of the bearing, multiplied by the distance gives the horizontal X axis value or Easting Component often referred to as the DEPARTURE while the cosine of the bearing multiplied by the measured distance gives the Vertical Y axis value or Northing Component often designated as the LATITUDE.
One very important characteristic to note about quadrantal bearings is that, they are measured relative to the N and S andNOT to the E and W cardinal points. Quadrantal bearings always have a preceding N or S followed by E or W depending on the quadrant in which the bearing falls. (N=north, S=South, E=East and W=West) (See Fig. 3.1) The primary objective of reducing whole circle bearings to quadrantal bearings is to reduce the bearings to values between 0 and 90 degrees. This facilitates the calculation of sines and cosines which, when multiplied by the distances measured will produce DEPARTURES and LATITUDES respectively.
Angles of the first Quadrant:
Angles equal to or less than 90 degrees retain their values and is placed in the first quadrant. 90-degree angles are referred to as Due East. For example a bearing of 75 degrees is referred to asN 75o E in quadrantal terms. A bearing of 90 o is referred to as DUE EAST.
Angles of the second Quadrant:
Bearings greater than 90o, but less than 180o, are usually subtracted from 180o. The resulting angle is then measured from the South cardinal point. South falls into the second quadrant. 180-degree bearings are referred to as Due South. For example, a bearing of 170o reduced to quadrantal bearings will be (180 o-170o) = S 10o E. A bearing of 180 o is referred to as DUE SOUTH.
Angles of the third Quadrant:
Bearings greater than 180o, have 180o subtracted from them to produce quadrantal bearings of the third quadrant. The resulting bearings are measured from the south cardinal point and written as S “bearing” W. For example, a whole compass bearing of 196o would be (196o -180o) = S 16o W in quadrantal terms. A bearing of 270 o is referred to as DUE WEST.
Angles of the fourth Quadrant:
Angles greater than 270 o fall into the fourth quadrant. To obtain quadrantal
bearings of the fourth quadrant, such bearings are subtracted from 360o. For example, a whole compass bearing of 288owould be (360 o -288o) = N 72o W. A bearing of 0 o or 360 ois referred to as DUE NORTH.
Local Attraction:
This is a term denoting any local influence that causes the magnetic needle to be deflected away from the magnetic meridian for that locality. This causes wrong measurements to be obtained. Measuring both the forward and the back bearing helps to detect local attraction. Some sources of local attraction are: permanently fixed objects of iron, steel and magnetite in the ground. Local attraction includes iron and steel articles about the person. High-tension lines are known to influence the needle of the compass and should be avoided where possible. Generally, the difference between the forward bearing (FB) and the Back Bearing (BB) is equal to 180o.
The Forward Bearing and Back Bearing:
When the bearing of a line is stated in a direction from an original point to a terminal point, it is known as a forward bearing. The back bearing is opposite in direction, to the forward bearing.
If the difference between the forward bearing and the back bearing is exactly 180°, then, the two stations are free fromlocal attraction. As an example, consider a survey line along stations A, B and C. If the forward bearing from A to B is 95°and the back bearing to A from B is 275, the difference between the two bearings is exactly 180° and there will be no reason to suspect any local attraction at stations B and A. If from stationB the bearing to station C is 240 (Forward Bearing) and the back-bearing from station C to B is 61, the difference between the two bearings will be 179°. Since it is already known that there is no local attraction at stations A and B, then there is good reason to suspect local attraction at station C. To confirm this suspicion a forward bearing is taken to station A from station C, and a back-bearing taken from A to C. Since A is known to have no local attraction, if the difference between the two bearings is not exactly 180°, then the presence of local attraction at station C is confirmed.
Magnetic Declination/Variation (MD or MV):
This is the angle the magnetic axis makes with the earth’s geographic axis.(angle q in Figure 12). It is often synonymously referred to as the Magnetic Variation because its value does vary from one point on the earth’s surface to the other. The Isogonic chart of Ghana Ghana
Where the bearings refer to the true North, the magnetic declination of the area in that particular year must be added to each bearing before using the schedule in the field. If however, the schedule is with reference to the Magnetic North, then the use of an ISOGONIC CHART becomes relevant.
A PROCEDURE FOR UNDERTAKING PRISMATIC COMPASS SURVEY
1. Collect a Prismatic Compass, a Sighting Pole and possibly a Chain for the Fieldwork. Try not to wear too many jewelleryor rings as the metals can interference with the compass readings.
2 Remember that Compass readings are made along straight segments of a boundary. Irregular paths (or boundaries) should therefore be first divided into straight segments before readings are taken. In this example, the straight segments of the boundary have been determined and marked for you.
3 To begin, pick the prismatic compass and locate the Starting Point (station 1). Let your partner move to station 2 with thesighting pole. Your partner must then hold the pole upright from the position marked station 2. Take a reading from your location (marked station 1) onto the sighting pole at station 2 and record the azimuth (angles) you get.
4 To verify whether the forward azimuth reading you made is correct, exchange positions with your partner (or preferably let your partner take a back azimuth onto the sighting pole now located at station 1). As a rule, if the forward azimuthis greater than 1800, you should subtract 180 from the forward azimuth to get the back azimuth but if the forward azimuth is less than 1800 you should add 180 to it to get theback azimuth. With the rule, make a quick check of the forward azimuth you made and record it if it is right. If it is wrong, redo the reading all over.
5 Record the forward azimuth you read earlier. See the next page for an example of how to record the readings on a page in a survey book.
6 Measure the segment of the boundary between station 1 and station 2 and record your answer beside the azimuth reading for this segment. You may use a chain or a tape and remember to take the measurement in feet. In tha absence of a chain or a tape, you may take the measurements by pacing along the boundary and counting the number of paces you make. Generally, a pace taken in a relaxed mood (not running) is about a yard (three feet) for many people. If you will use this method, you should first determine the length of your pace by marking three feet segments on the floor and walk along them for some time.
7 Walk along the boundary segment between station 1 and station 2 and make any other required readings such as resection or intersection then record such measurements on the page you have already opened. Make some sketches if necessary, to portray the features and positions you find in the field. See an example of the entry on the next page.
8 Now go to station 2 and let your partner move with the sighting pole to station 3. Take the forward and backward azimuths as explained above and record only the forward azimuth in your survey book. Check to make any required chain and compass readings along the segment between stations 2 & 3 and then move on to the next segment. Continue with the process in the same manner as described until all stations (or segments) are measured and the measurements recorded in your notebook.
9 Keep your note book entries for you shall use it to plot the shape of land you measure in the field. You will also hand in your note book entries for grading.
BOOKING OF THE FIELD DATA
Traversing involves taking bearings and distances from one station to the other until the last station is encountered. The convention for recording such data is shown in Fig.2.2. The survey book has two parallel lines running through the centre of each page. Booking is usually started from the last page of the book and from the bottom to the top of each page. The stations are represented as triangles enclosing serial numbers or letters specific to each station.
Fig.2.2 is an example of the booking of a closed traverse, which starts at A through B, C, D, E and back to A. The bearing from A to B is recorded at the top of the triangle enclosing A. The distance from A to B is recorded at the base of the Triangle enclosing B. Any feature encountered, such as the footpath shown in dotted line or the stream with an arrow head, is sketched at the point it crosses the survey line. It’s distance from the previous station is recorded just below the sketch as shown above.
The Magnetic Declination; (MD), of the area of the survey is recorded at the bottom right hand corner, together with the date of completion of the survey and the Name of the officer conducting the survey.
Interpreting the Note Book entries:
The azimuth from station A to station B was 199o and a distance of 120.37meters.
The azimuth from station B to C was 92o and a distance of 83.8 meters separates B and C.
A path crosses the traverse at 50.2 meters from B
The azimuth from station C to D was 65o and a distance of 40.75 meters separates C and D.
A river crosses the traverse at 30 meters from C.
The azimuth from station D to E was 353o and a distance of 59.83 meters separates D and E.
The azimuth from station E to A was 299o and a distance of 81.8 meters separates E and A.
Resection:
It is usually possible to locate your position on the map by means of terrain association but when one finds him/herself in an area where features are quite similar, one's position can be located by a procedure termed Resection. Resection is a procedure by which we sight on two known features in the distance from our known position.
A: To undertake resection without a Compass, follow the following steps:
1.Orient your map and locate yourself on it (Point A)
2.Select two outstanding features on the ground which you can identify on your map
3.Sight on the chosen features with a straight edge
4.On the map draw in the lines from the outstanding features back to your position so that where the two lines intersect will be your position
B: To undertake Resection with a Magnetic Compass:
1.Orient your map by compass and locate yourself on it (Point A)
2.Locate first position A on map and ground
3.Sight on the chosen position A with the compass (Magnetic Azimuth)
4.Convert to grid back azimuth (If the grid azimuth is greater than 180 convert by subtracting 180 from it)
5.Plot back azimuth on map using protractor (Line AC)
6.Repeat steps 2 through 4 for position B
7.Plot back azimuth on map using protractor (Line BC
8.The unknown location C is at the intersection of the two back azimuths
Intersection:
It is the process of locating unknown points on the map by successfully occupying two known positions and sighting on to the unknown point.
A: To undertake intersection without a Compass, follow the following steps:
1.Orient your map and locate yourself on it (Point A = your position)
2.Without moving the map, sight on the unknown point from your position with a straight edge (Pint C).
3.Draw in a line from your position along the edge of the ruler through the unknown position (C).
4.Move to a second position, whose location you know.
5.Repeat steps 2 - 4 above to locate B.
6.Where the two lines cross is the location of the unknown point.
B: To undertake Interception with a Magnetic Compass:
1.Orient your map and locate yourself on it (Point A)
2.Select two outstanding features on the ground which you can identify on your map
3.Sight on the chosen features with the compass (Magnetic Azimuth)
4.Convert to Grid Azimuth
5.Plot grid azimuth on map using protractor (Line AB
6.Move to position B and repeat steps 1 - 4
7.Plot grid azimuth on map using protractor (Line AB
8.The unknown location C is at the intersection of the two azimuths
why fore bearing is not written in the place of forward bearing ?
ReplyDeleteand which is suitable "forward bearing or fore bearing " ?