Unit - 2

Compass Surveying

Q1) What is Compass Surveying: Introduction and Purpose?

A1)

Compass surveying is a form of surveying wherein the instructions of surveying strains are decided with a magnetic compass, and the duration of the surveying strains are measured with a tape or chain or laser variety finder. The compass is normally used to run a traverse line.

Compass surveying is an crucial department of surveying that's typically followed in figuring out the location of an item each through angular and linear measurements. Here angular measurements are taken the use of a compass and linear measurements are decided the use of chain or tape. Some priorities for compass surveying are as following- If there may be a huge location to be surveyed which include the coastal regions or the path line regions of the river. If the location is crowded with many information and has many limitations for carrying out chain surveying as triangulation turns into impossible.

If the surveyor has a particular time restriction for carrying out the surveying on a huge and specified location But there also are a few boundaries to compass surveying. It isn't always endorsed for the regions which might be fending off public interest and feature the presence of iron ore deposits, magnetic materials like steel or metal structures, electric powered cables conveying current, etc. Types of Compass Following are the forms of compass: Magnetic Compass Prismatic compass or Lensatic compass Surveyor’s compass Liquid compass Marine compass or Card compass Bas eplate compass or Orienteering compass Thumb compass Solid-country compasses Qibla compass GPS compass Gyro Compass Astor Compass Following are the compass sorts used for surveying. Prismatic compass Surveyor’s compass Level compass A short description of those sorts is given below.

Magnetic Compass

The magnetic compass is a compass having a magnetized needle consistent with the magnetic poles of the earth.

Magnetic Compass

The magnetic bearings of a line may be decided through a magnetic compass. The bearings may be measured both within side the Whole Bearing System (WBS) or the Quadrantal Bearing System (QBS)

Compass surveying is the department of surveying wherein the placement of an item is positioned the use of angular measurements decided via way of means of a compass and linear measurements the use of a series or tape. Compass surveying is utilized in following circumstances: If the surveying place is large, chain surveying isn't followed for surveying instead compass surveying is employed. If the plot for surveying has severa boundaries and undulations which prevents chaining.

Advantages

They are transportable and mild weight. They have fewer settings to repair it on a station The mistakes in course produced in a unmarried survey line does now no longer have an effect on different lines. It is appropriate to retrace vintage surveys.

Disadvantages

It is much less particular in comparison to different superior techniques of surveying. It is without problems subjected to diverse mistakes inclusive of mistakes adjacent to magnetic meridian, nearby appeal etc. Imperfect sighting of the ranging rods and erroneous leveling additionally reasons mistakes

Q2) What is True Meridian?

A2)

True meridian- the road on a aircraft passing via the geographical North Pole or geographical South Pole and any factor at the floor of the earth is referred to as proper meridian. It is likewise known as geographical meridian. The attitude among proper meridian and line is referred to as proper bearing of the road.

Q3) Define Magnetic Meridian?

A3)

Whilst magnetic needle is suspended freely and stability properly, unaffected via way of means of magnetic substance it suggest a course this course is called magnetic meridian. The perspective among magnetic meridian and line is called magnetic bearing of the line.

Q4) What is Geographical Meridian?

A4)

In geography and geodesy, a meridian (or line of longitude) is the 1/2 of of an imaginary polar extremely good circle or extremely good ellipse at the Earth's surface, a coordinate line terminated with the aid of using the North Pole and the South Pole. A meridian is the locus connecting factors of identical longitude, that's the angle (in stages or different units) east or west of a given high meridian (currently, the IERS Reference Meridian).

The role of a factor alongside the meridian is given with the aid of using that longitude and its latitude, measured in angular stages north or south of the Equator. Each meridian is perpendicular to all circles of latitude. Meridians are 1/2 of a extremely good circle at the Earth's surface. The period of a meridian on a current ellipsoid version of the earth (WGS 84) has been predicted at 20,003.ninety three km (12,429.nine miles)

Q5) Define True Bearing?

A5)

True Bearing:

Geographical north of earth is different from the magnetic north. Hence the angle which the survey line makes with the true geographical north is called as true bearing of the survey line.

Q6) Define Magnetic Bearing?

A6)

The magnetic needle of the compass always points towards the magnetic north-south (N-S) direction indicating earth’s magnetic axis. Since this direction is same at all the places on the earth’s surface, it is universally used as the reference direction. The angle made by survey line in a clockwise direction with reference to magnetic N-S line is called as magnetic bearing of the line. The value of magnetic bearing ranges from 0 to 360o.

Q7) What is Whole circle?

A7)

Whole circle Bearing (WCB):

The complete circle of angular measurement starts with north as 0 o and ends at north at 360o. The bearing of line directly obtained by magnetic needle ranging from 0 o to 360 o is called whole circle bearing.

• When the bearing of a line is measured with respect to magnetic north in clockwise direction, it is called as magnetic bearing or whole circle bearing.
• The value of W.C.B. Varies from 0° to 360°.
• Four quadrants I, II, III and IV are shown in Fig.
• The W.C.B. Of a line OA in the first quadrant is (between 0° to 90°).
• The W.C.B. Of a line OB in the second quadrant with respect to north is (between 90° and 180°) W.C.B. Of a line OC in the third quadrant with respect to north is (between 180° and 270°) and W.C.B. Of a line OD in the fourth quadrant with respect to north is , (between 270° and 360°).

Fig1: Whole Circle bearing

Q8) What is Quadrantal Bearing?

A8)

The quadrantal bearing is a not unusual place notation device of bearings. It's also called the decreased bearing. As the call indicates, quadrantal bearing relies upon the quadratic function of a line.

The horizontal perspective made with the aid of using a line with the magnetic north or south (whichever is nearer from the road) within side the eastward or westward course is the Quadrantal Bearing or Reduced Bearing of the road. In quadrantal bearing or decreased bearing, each north and south are taken into consideration as reference meridians. Depending upon the location of a survey line, the course of the reference meridian to the road may be both clockwise or anticlockwise. In the expression of the decreased bearing cost of a line, quadrant needs to be stated wherein the road lies.

• In reduced bearing, the bearing of survey lines is measured with respect to north and south line either in clockwise or anticlockwise direction towards east or west.
• Fig. Shows quadrantal bearing system.
• Any line in any of the four quadrant is expressed by an angle which is measured from either north or south.
• The directions are essentially written before and after the value of angle.
• This helps to identify in which quadrant the lines lie. The maximum bearing in such case is 90°.

Fig2: Quadrantal bearing

Q9) What is Prismatic Compass and Surveyors Compass?

A9)

Prismatic compass or Lensatic compass

Prismatic or lensatic compasses are used for lining up with an item whose bearing is to be determined. A prismatic compass has a tumbler prism or a lens and a lid with a hairline. This sort of compass frequently has a magnifying lens for map reading, a few form of mild for low mild conditions, and a ruler. It is frequently used for army functions or plotting. A prismatic compass is a surveyor’s hand compass held within side the hand while in use and prepared with peep attractions and a triangular glass prism so organized that the magnetic bearing or azimuth of a line may be examine thru the prism from a round graduated scale and on the equal time that the road is sighted over.

The compass is normally used to run a traverse line. The compass calculates bearings of traces with admire to the magnetic needle. The protected angles can then without problems be calculated the usage of appropriate formulation in case of clockwise and anti-clockwise traverse respectively. For every survey line within side the traverse, surveyors take bearings which can be fore bearing and again bearing which ought to precisely range via way of means of 180° if the nearby enchantment is in some way negligible.

Surveyor Compass

It is a device used often via way of means of surveyors for measuring horizontal angles and additionally for figuring out the magnetic bearing of a line of sight. The compass includes a pivoted magnetic needle, a graduated horizontal circle, and a sighting device. The magnetic needle swings freely over the horizontal circle that is divided into 360 degrees. The horizontal perspective is measured the usage of a couple of attractions positioned at the north-south axis of the compass. They are normally hooked up over a tripod and leveled the usage of a ball and socket mechanism

Major components of a Prismatic Compass are: Magnetic needle Graduated ring Adjustable mirror Sliding association for mirror Object vane Eye vane Metal field Glass cover Horse hair Adjustments of prismatic compass Two sorts of modifications: Temporary adjustment Permanent adjustment Temporary modifications Centering: it's far the technique of solving the compass precisely over the station. Centering is typically performed with the aid of using adjusting the tripod legs.

Also a plumb-bob is used to decide the correct centering of devices over the station. Leveling: the device needs to be leveled if it's far used as in hand or installed over a tripod. If it's far used as in hand, the graduated disc have to swing freely and looks to be absolutely degree in connection with the pinnacle fringe of the case. If the tripod is used, they typically have a ball and socket association for leveling purpose. Focusing the prism: Prism may be slide up or down for focusing to make the readings clean and readable. Permanent modifications are identical as within side the Surveyor’s compass. Surveyor’s compass Surveyor’s compass includes a round brass field containing a magnetic needle which swings freely over a brass circle that's divided into 360 degrees. The horizontal attitude is measured the usage of a couple of attractions placed on north – south axis of the compass. They are typically installed over a tripod and leveled the usage of a ball and socket mechanism.

Q10) What is Magnetic Declination?

A10)

Magnetic declination, or magnetic version, is the perspective at the horizontal aircraft among magnetic north (the course the north stop of a magnetized compass needle points, similar to the course of the Earth's magnetic subject traces) and actual north (the course alongside a meridian toward the geographic North Pole). This perspective varies relying on function at the Earth's floor and adjustments over time. Somewhat extra formally, Bowditch defines version as “the perspective among the magnetic and geographic meridians at any place, expressed in tiers and mins east or west to suggest the course of magnetic north from actual north. The perspective among magnetic and grid meridians is referred to as grid magnetic perspective, grid version, or gravitation By convention, declination is high-quality whilst magnetic north is east of actual north, and poor whilst it's miles to the west. Isogonic traces are traces at the Earth's floor alongside which the declination has the equal regular value, and contours alongside which the declination is 0 are referred to as agonic traces. The lowercase Greek letter δ (delta) is often used because the image for magnetic declination. The time period magnetic deviation is every so often used loosely to intend similar to magnetic declination, however extra effectively it refers to the mistake in a compass studying precipitated through close by metal objects, which include iron on board a deliver or aircraft. Magnetic declination ought to now no longer be stressed with magnetic inclination, additionally called magnetic dip, that is the perspective that the Earth's magnetic subject traces make with the downward facet of the horizontal aircraft.

Magnetic declination varies each from area to area and with the passage of time. As a traveler cruises the east coast of the United States, for example, the declination varies from sixteen stages west in Maine, to six in Florida, to zero stages in Louisiana, to four stages east in Texas. The declination at London, UK changed into one diploma west (2014), decreasing to 0 as of early 2020. Reports of measured magnetic declination for remote places have become not unusual within side the seventeenth century, and Edmund Halley made a map of declination for the Atlantic Ocean in 1700. In maximum areas, the spatial version displays the irregularities of the flows deep within side the Earth; in a few areas, deposits of iron ore or magnetite within side the Earth's crust can also additionally make a contribution strongly to the declination. Similarly, secular modifications to those flows bring about gradual modifications to the sphere power and route on the equal factor at the Earth.

Q11) What is Isogonic and Agonic Lines?

A11)

Isogonic lines

Isogonic traces are traces at the Earth's floor alongside which the declination has the equal consistent value, and contours alongside which the declination is 0 are referred to as agonic traces. The lowercase Greek letter δ (delta) is regularly used because the image for magnetic declination.

Agonic lines:

An imaginary line at the earth's floor connecting the north and south magnetic poles and passing via the ones factors wherein there's no magnetic declination and wherein a freely suspended magnetic needle shows real north.

Q12) What is Local Attraction and its adjustments?

A12)

The found bearings are corrected with the aid of using making use of correction to the stations laid low with the neighborhood attraction. For this, blunders at every of the station is observed after which beginning from an unaffected bearing, the bearings of the successive traces are adjusted with the aid of using making use of corrections to them

The magnetic needle is disturbed from its ordinary role if it's miles below the affect of outside appealing forces known as the re assets of nearby attraction. Such a demanding affect is called nearby attraction.

The time period is likewise used to indicate the quantity of deviation of the needle from its ordinary role. The nearby appealing re assets can be impartial together with the loads of magnetic rock or iron ore etc. and synthetic together with proximity of metallic structures, rails, iron-pipes, present day wearing wires etc. The iron made gadgets together with chains, arrows, ranging rods and different matters together with bunch of keys, knife, iron-buttons etc. must additionally be stored far from the compass at a secure distance apart.

Local appeal at an area may be de­tected with the aid of using staring at bearings, from each ends, of a line within side the location. If the fore and lower back bearings of the road vary precisely with the aid of using 180°, there's no neighborhood appeal at both station supplied instrumental and observational mistakes are eliminated. But if this distinction isn't same to 180°, then neighborhood appeal exists there both at one or at each ends of the road. Remedies for Local Attraction: There are the subsequent strategies of correcting the located bearings of the strains taken within side the location suffering from the neighborhood appeal: (i) The mistakes because of neighborhood appeal being equal for every of the bear­ings located at an affected station, the protected attitude calculated from the bearings taken on the equal station might be correct, even though, the station is suffering from the neighborhood appeal. Starting from the unaffected line and the usage of those protected angles, the proper bearings of the successive strains are calculated as already explained.

(ii) The determined bearings are corrected with the aid of using making use of correction to the stations suffering from the neighborhood appeal. For this, mistakes at every of the station is discovered after which beginning from an unaffected bearing, the bearings of the successive strains are adjusted with the aid of using making use of corrections to them.

Note: The significance and nature of mistakes at a station because of neighborhood appeal may be decided with the aid of using drawing a cartoon of the determined and accurate bearings of a line at that station. If the mistake is terrible then the correction is positive, and vice versa, the significance of the 2 closing same to every other. If the bearings are given within side the complete circle system, then keep in mind the subsequent rule for locating the character of mistakes

(ii) The determined bearings are corrected through making use of correction to the stations stricken by the neighborhood enchantment. For this, mistakes at every of the station is observed after which beginning from an unaffected bearing, the bearings of the successive traces are adjusted through making use of corrections to them.

Note: The significance and nature of mistakes at a station because of neighborhood enchantment may be decided through drawing a caricature of the determined and accurate bearings of a line at that station. If the mistake is bad then the correction is positive, and vice versa, the significance of the 2 closing same to every other. If the bearings are given within side the entire circle system, then don't forget the subsequent rule for locating the character of mistakes.