Disadvantage of chain surveying is that, in it only distances are measured and hence area is to be covered with a network of triangles. If the length as well as angle of a line can be measured with respect to a known direction then it is possible to plot a line, independent of length of other lines. Hence, in such cases there is no compulsion of going for a network of triangles only.
Compass is an instrument which can be used to measure the direction of a survey line with respect to magnetic north-south. The magnetic north-south direction which is the reference direction is called meridian (reference direction) and the angle between the line and the meridian is called bearing. Use of compass for measuring direction of a line simplifies the surveying to a great extent.
Types of Compass
The types of compass that are used commonly are:
(i) prismatic compass; and (ii) surveyor compass.
The essential parts of both type are:
(i) a magnetic needle, (ii) a graduated circle, (iii) a line of sight, and (iv) a box to house them.
There are some differences in the essential parts of the two type of compass. The construction of the two types of compass is explained and the difference in them is pointed out in this article.
1. Prismatic Compass: Figure 1(a) shows the cross-section of a typical prismatic compass [see Fig. 1(b) also].
A magnetic needle of broad form (1) is balanced on a hard and pointed steel pivot (2). The top of the pointed pivot is protected with agate cap (3).
An aluminium graduated disk (4) is fixed to the top of the needle. The graduations are from zero to 360° in clockwise direction when read from top. The direction of north is treated as zero degrees, east as 90°, south as 180° and west as 270°. However, while taking the readings observations are at the other end of line of sight.
Hence, the readings are shifted by 180° and graduations are marked as shown in Fig. 2. The graduations are marked inverted because they are read through a prism.
The line of sight consists of object unit and the reading unit. Object unit consists of a slit metal frame (5) hinged to the box. In the centre the slit is provided with a horse hair or a fine wire or thread (6).
The metal frame is provided with a hinged mirror (7), which can be placed upward or downward on the frame. It can be slided along the frame. The mirror can be adjusted to view objects too high or too low from the position of compass.
Reading unit is provided at diametrically opposite edge. It consists of a prism (8) with a sighting eye vane (9). The prism magnifies the readings on the graduation disk just below it.
For focussing, the prism is lowered or raised on the frame carrying it and then fixed with the stud (10). Dark sunglasses (11) provided near the line of sight can be interposed if the object to be sighted is bright (e.g., sun).
The bottom of the box (12) which is about 85 mm to 110 mm supports the pivot of needle firmly at its centre. The object vane and the prism are supported on the sides of the box. The box is provided with a glass (13) lid which protects the graduation disc at the same time permit the direct reading from the top.
When the object vane is folded on the glass top it presses a lifting pin (14) which activates lifting lever (15) lifts the needle off the pivot. Thus, it prevents undue wear of pivot point. While taking reading, if graduation disc vibrates, it can be dampened with a spring (16).
For pressing spring a knob or brake pin (17) is provided on the box. When not in use prism can be folded over the edge of the box. The box is provided with a lid to close it when the compass is not in use. The box is provided with a socket to fit it on the top of a tripod.
2. Surveyors Compass: In this type of compass graduation disc is fixed to the box and magnetic needle is free to rotate above it. There is no prism provided at viewing end, but has a narrow slit. After fixing the line of sight, the reading is directly taken from the top of the glass cover. Hence, graduations are written directly (not inverted).
In this compass graduations are from zero to 90°, zero being to north or south and 90° being to east and west. An angle of 20° to north direction to the east is written as N 20° E, and an angle of 40° to east from south is written as S 40° E. Always first direction indicated is north or south and the last letter indicates east or west direction.
In this system graduated circle rotates with line of sight and magnetic needle is always towards north. The reading is taken at the tip of needle. Hence, on the compass east and west are marked interchanged and marked [Ref. Fig. 3]. Fig. 3(c) shows the photograph of a surveyors compass.
The difference between prismatic and surveyor’s compass are listed in Table 13.1.
|Sr. No.||Prismatic Compass||Surveyors Compass|
|1||Graduation circle is fixed to broad type needle. Hence, it will not rotate with the line of sight.||Graduation circle is fixed to the box. Hence, it rotates with the line of sight.|
|2||There is a prism at viewing end.||At viewing end there is no prism. There is only a slit.|
|3||Sighting and reading can be done simultaneously.||Sighting and viewing cannot be done simultaneously|
|4||The magnetic needle do not act as an index.||Magnetic needle acts as index while reading.|
|5||The graduations are in whole circle bearing.||The graduations are in quadrantal system.|
|6||Graduations are marked inverted since its reflection is read through prism.||Graduations are marked directly. They are not inverted.|
|7||The reading is taken through a prism.||The reading is taken by directly viewing from top glass.|
|8||Tripod may or may not be used. It can be held on a stretched hand also.||Tripod is essential for using it.|
Method of Using a Compass
To take a reading from a compass, the following temporary adjustments are required:
(i) Centring: The compass should be fixed to the stand and set over the station. To centre the compass legs of the tripod stand should be moved inward-outward or in a circumferential direction. To check centring plumb may be used or a pebble dropped from the centre of the compass.
(ii) Levelling: In compass survey perfect levelling is not necessary, but it should be sufficient to permit free suspension of magnetic needle. For checking levelling a bubble level is provided in many compasses. After centring bubble should be ensured in the middle of the circle provided for it in the level. If it is not within that circle, circumferential movements may be provided to the legs of tripod so that without disturbing centring the levelling is achieved.
(iii) Focussing the prism: In prismatic compass, to focus the prism on graduated circle, its attachment is slided up or down till the readings are clearly visible. There is no such requirement in surveyors compass.
The following steps are required for observing bearing of a line, say, AB:
- Centre the compass over A.
- Level the compass.
- Focus the prism, if prismatic compass is used.
- Rotate the box till ranging rod at B is sighted through the line of sight.
- Bring the needle to rest using knob.
- Take the reading and note it in the field book.
Care should be taken to see that the line of sight is not disturbed between the line of sighting the object and the time of reading the bearing.
As stated earlier a bearing of a line is the angle made by the line with respect to a reference direction, the reference direction being known as meridian.
The direction shown by a freely suspended and properly balanced magnetic needle is called magnetic meridian and the horizontal angle made by a line with this meridian is known as magnetic bearing. The points of intersection of earth’s axis with surface of the earth are known as geographic north and south-pole.
The line passing through geographic north, south and the point on earth is called true meridian at that point and the angle made by a line passing through that point is called true bearing.
While traversing along lines A, B, C, D …, the bearing of lime AB is called fore bearing of AB and the bearing of BA is called back bearing. Fore bearing and back bearing differ by 180°.
Whole Circle Bearing and Reduced Bearing
In whole circle bearing (WCB) the bearing of a line at any point is measured with respect to a meridian. Its value varies from zero to 360°, increasing in clockwise direction. Zero is north direction, 90° is east, 180° is south and 270° is west (Ref. Fig. 2). This type of bearing is used in prismatic compass.
In reduced bearing (RB) system, bearings are measured from north or south direction towards east or west. Hence, angles are from 0 to 90° as shown in Fig. 3.
This system of measuring bearings is used in Surveyor’s compass and it is also known as Quadrantal Bearing (QB). The bearing measured is designated with letter N or S in the beginning to indicate whether it is from north or south. The letter E or W written after the angle indicates whether the bearing read is towards east or west, respectively.
The conversion of the bearing from one system to the other system can be easily carried out by drawing a sketch to indicate WCB or RB as shown in Fig. 4. It may be observed that conversion table is as given below:
Computation of Angles
At any point, if bearings of any two lines are known, the angle between these two lines can be easily found by drawing a neat sketch, and then noting the difference.
Declination and Dip
The magnetic meridian and the true meridian may not coincide with each other in a place. The horizontal angle between these two meridians is known as magnetic declination. The magnetic north at a place may be towards east or west of true north (Fig. 5).
If it is towards east, it is known as eastern or +ve declination. Western declination is known as –ve declination. Eastern declination is to be added to observed magnetic bearings to get true meridian.
To find magnetic declination at a point true meridian should be established from astronomical observations and magnetic meridian by a compass. Maps are made with respect to true meridian.
Magnetic declination varies from time to time and also from place to place. In the noon sun is exactly on the geographical meridian.
In India, ‘Survey of India’ department conducts astronomical survey and publishes Isogonic Charts from which magnetic declinations at any point can be found. The lines joining the points at which declination is the same at the given time are called ‘Isogonic Lines’. Lines joining points of zero declinations are called ‘Agonic Lines’.
The isogonic lines are quite irregular near geographic poles. The isogonic charts show lines of equal annual change in declination. The following type of variations are observed in declination:
(i) Secular variation, (ii) Annual variation, (iii) Daily variations, and (iv) Irregular variations.
(i) Secular Variation: The magnetic meridian swings like a pendulum to the left and to the right of true meridian. Its period of variation is approximately 250 years.
(ii) Annual Variation: It is observed that in a year declination varies from 1′ to 2′.
(iii) Daily Variation: The daily variation of magnetic declination is as much as 10′. This variation is also known as ‘Dirunal Variation’. The following factors influence its magnitude:
- It is more in day and less in night.
- It is more in summer and less in winter.
- The amount of variation changes from year to year.
- It is more near magnetic poles and less near equator.
(iv) Irregular Variation: Due to earthquakes and volcanic eruptions, magnetic storms occur, resulting into changes in magnetic meridian. Such changes are from 1° to 2°.
Magnetic Dip: A perfectly balanced, freely suspended magnetic needle dips towards its northern end in northern hemisphere and towards its southern end in southern hemisphere. If it is at north-pole, the needle takes vertical position.
The vertical angle between the horizontal and the direction shown by a perfectly balanced and freely suspended needle is known as the magnetic dip at that place. Its value is 0° at equator and 90° at magnetic poles. To counteract the dip, a sliding rider (weight) is provided on the needle.
A freely suspended and properly balanced magnetic needle is expected to show magnetic meridian. However, local objects like electric wires and objects of steel attract magnetic needle towards themselves.
Thus, needle is forced to show slightly different direction. This disturbance is called local attraction. The list of materials which cause local attraction are:
- magnetic rock or iron ore,
- steel structures, iron poles, rails, electric poles and wires,
- key bunch, knife, iron buttons, steel rimmed spectacles, and
- chain, arrows, hammer, clearing axe etc.
Surveyor is expected to take care to avoid local attractions listed in (3) and (4) above.
Detecting Local Attraction: For detecting local attraction it is necessary to take both fore bearing and back bearing for each line. If the difference is exactly 180°, the two stations may be considered as not affected by local attraction.
If difference is not 180°, better to go back to the previous station and check the fore bearing. If that reading is same as earlier, it may be concluded that there is local attraction at one or both stations.
Correcting Observed Bearings: If local attraction is detected in a compass survey observed bearings may be corrected by any one of the following two methods:
Method I: It may be noted that the included angle is not influenced by local attraction as both readings are equally affected. Hence, first calculate included angles at each station, commencing from the unaffected line and using included angles, the corrected bearings of all lines may be calculated.
Method II: In this method, errors due to local attraction at each of the affected station is found starting from the bearing of an unaffected local attraction, the bearing of the successive lines are adjusted.
Chain and Compass Surveying Field Work
In compass survey chain or tape is used for linear measurement. If the surveying starts from a station, goes round an area and ends at the starting station it is called closed traverse.
If survey starts from a point, goes along a number of interconnected lines and ends at some other point it is called as open traverse.
Closed traverse is used for preparing plan of an area while open traverse is useful in the road, rail or canal projects. The following are required for chain and compass survey:
(i) Compass and stand (ii) Chain and tape (iii) 10 arrows (iv) 5 to 6 ranging rods (v) Ranging poles (vi) Pegs and hammer (vii) Plumb bobs (viii) Line ranger, cross staff etc.
Field Work: Field work involves:
(i) reconnaissance survey (ii) preparation of location sketches of stations (iii) measurement of directions (iv) measurement of lengths and offsets, and (v) recording measurements.
(i) Reconnaissance Survey: The entire area to be surveyed is inspected to select survey stations. Important points to be considered in selecting stations are:
- Adjacent stations should be intervisible.
- Lines to be chained should be free of obstacles.
- Number of survey lines should be minimum.
- Survey lines should run close to the important objects, so that offset lengths are small.
An index map is prepared with pencil and stations are marked. If necessary, changes may be made in survey lines and corresponding changes in the index plan.
Location Sketches: Before commencing surveying a line, the location sketches of the stations of that line should be prepared. At the beginning of the field book few pages should be reserved for drawing location sketches.
Direction Measurement: The following precautions should be taken in measuring the direction of a survey line with compass.
- Centre the compass on the station correctly.
- Level the compass and ensure needle is free to move.
- Take the reading only after vibration of graduation circle/needle is stopped. Use the knob, if necessary.
- Gently tap the top of the glass of compass to remove sluggishness of the needle and take the reading after vibration stops.
- While taking reading parallax should be avoided.
- Care should be taken to keep away steel and iron objects like key bunch, metal framed spectacles, iron buttons, chain, arrows etc.
- If handkerchief is used to clean top of glass of campass, the glass is charged with electricity. As a result of it local attraction is induced. To avoid this problem apply moist fingers to clean the glass.
- If the compass is not in use, fold the prism and object vane on the top of glass plate, so that needle is lifted from the pivot to avoid unnecessary wear of the pivot.
- For all survey lines fore bearings and back bearings should be taken. If any other survey station is visible, bearing should be taken to that station also, which helps in checking survey work.
Measurement of Lengths and Offsets: This is similar to the one used in chain survey. However, it may be noted that for the objects of less importance one can measure the bearing of the offset and its length.
Recording Chain and Compass Measurements: The type of field book used in chain survey is used in this survey also. Apart from recording linear measurements in this survey the bearings taken also should be recorded. Figure 6 shows a page of typical field record.