File Name: fore bearing and back bearing in surveying .zip
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Local attraction is the phenomenon by which the magnetic needle is constantly prevented to point towards the magnetic north at a place. This is because that these magnetic compass is influenced by other magnetic objects at that locality such as wires carrying electric current, rails, steel and iron structures, steel tapes etc. The occurrence of local attraction can be detected by observing the difference between the fore and back bearings. If there is no influence of local attraction and other error, this difference will be
In navigation , bearing is the horizontal angle between the direction of an object and another object, or between it and that of true north. In nautical navigation the absolute bearing is the clockwise angle between north and an object observed from the vessel. If the north used as reference is the true geographical north then the bearing is a true bearing whereas if the reference used is magnetic north then the bearing is a magnetic bearing.
An absolute bearing is measured with a bearing compass. The measurement of absolute bearings of fixed landmarks and other navigation aids is useful for the navigator because this information can be used on the nautical chart together with simple geometrical techniques to aid in determining the position of the vessel.
A grid bearing also known as grid azimuth is measured in relation to the fixed horizontal reference plane of grid north , that is, using the direction northwards along the grid lines of the map projection as a reference point.
A compass bearing , as in vehicle or marine navigation, is measured in relation to the magnetic compass of the navigator's vehicle or vessel if aboard ship. It should be very close to the magnetic bearing.
In nautical navigation the relative bearing of an object is the clockwise angle from the heading of the vessel to a straight line drawn from the observation station on the vessel to the object.
The relative bearing is measured with a pelorus or other optical and electronic aids to navigation such as a periscope , sonar system , and radar systems. Since World War II, relative bearings of such diverse point sources have been and are calibrated carefully to one another.
The United States Navy operates a special range off Puerto Rico and another on the west coast to perform such systems integration. Relative bearings then serve as the baseline data for converting relative directional data into true bearings N-S-E-W, relative to the Earth's true geography. By contrast, Compass bearings have a varying error factor at differing locations about the globe, and are less reliable than the compensated or true bearings.
The measurement of relative bearings of other vessels and objects in movement is useful to the navigator in avoiding the danger of collision. A bearing can be taken on another vessel to aid piloting. If the two vessels are travelling towards each other and the relative bearing remains the same over time, there is likelihood of collision and action needs to be taken by one or both vessels to prevent this from happening.
A bearing can be taken to a fixed or moving object in order to target it with gunfire or missiles. This is mainly used by ground troops when planning on using an air-strike on the target. Moving from A to B along a great circle can be considered as always going in the same direction the direction of B , but not in the sense of keeping the same bearing, which applies when following a rhumb line.
Accordingly, the direction at A of B, expressed as a bearing, is not in general the opposite of the direction at B of A when traveling on the great circle formed by A and B.
For example, assume A and B in the northern hemisphere have the same latitude, and at A the direction to B is east-northeast. Then going from A to B, one arrives at B with the direction east-southeast, and conversely, the direction at B of A is west-northwest.
The bearing consists of 2 characters and 1 number: first, the character is either N or S. Next is the angle value. Third, the character representing the direction of the angle away from the reference ray - thus, either E, or W. The angle value will always be less than 90 degrees. An azimuth is specified in the same angle units. From Wikipedia, the free encyclopedia. In navigation, horizontal angle between the direction of an object and another object.
See also: Azimuth. This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.
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In navigation , bearing is the horizontal angle between the direction of an object and another object, or between it and that of true north. In nautical navigation the absolute bearing is the clockwise angle between north and an object observed from the vessel. If the north used as reference is the true geographical north then the bearing is a true bearing whereas if the reference used is magnetic north then the bearing is a magnetic bearing. An absolute bearing is measured with a bearing compass. The measurement of absolute bearings of fixed landmarks and other navigation aids is useful for the navigator because this information can be used on the nautical chart together with simple geometrical techniques to aid in determining the position of the vessel.
The bearing we end up with represents the angle between a line from magnetic north to our location, and a line from our. Surveying is a very important part of civil engineering. Welcome to this video lecture series on basic surveying and today we are going to. The subject matter is canvassed along with theory, variety of examples, various types. The name prismatic compass is given to it because it essentially consists of a prism which is used for taking observations more accurately. Surveying is a science as well as an art, by which studies are made of the objects present on the ground and below the ground at different points. In this section, we will examine the back and fore bearing.
bearings, Conversion of bearings, Local attraction, Correction of compass In this section, we will examine the back and fore bearing; and the steps to be taken.
Bearings of lines may be calculated if bearing of one of the lines and the included angles measured clockwise between the various lines are given. In a closed traverse, where local attraction is not suspected i. The ordinary compass cannot read directly the angle between two lines. The angles can be determined by observing the bearings of the two lines from their point of convergence.
Taking a forward bearing. The first compass bearing we need to take is from our known location to our cabin on the far shore. The bearing we end up with represents the angle between a line from Magnetic North to our location, and a line from our location to the cabin. These bearings in the opposite direction are called back bearings or reciprocal bearings. How to determine a back bearing.
Bearings measured in the direction of progress of the survey are known as fore bearing and bearings measured opposite to the direction of the survey are known as back bearing. The bearing of a line is the direction with respect to a given meridian; Meridian is a fixed reference line. While setting out a survey line, the bearing readings are necessary.
Taking a forward bearing. The first compass bearing we need to take at the lake is from our unknown location to our cabin on the far shore. The bearing we end up with represents the angle between a line from Magnetic North to our location, and a line from our location to the cabin. How to determine a back bearing One technique is to do the math. Forward and back bearings as seen through a sighting compass. There are also lots of tricks that avoid doing the arithmetic entirely….
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