Differentiate between the following (2x10)
Fore bearing and Back bearing
Fore Bearing (F.B.)
The bearing of a line in the direction of progress of the survey is called the fore or forward bearing.
Back Bearing (B.B.)
The bearing of a line in the opposite direction of progress of the survey is known as back or reverse bearing.
The bearing of a line is indicated in the order in which the line is lettered. Thus, the bearing from A to B is α the fore bearing a of the line AB. whereas the bearing of line AB in the direction B to A is its back bearing β.
Triangulation and Trilateration
When the area to be surveyed is of a considerable extent, triangulation is adopted. The entire area is divided into a network of triangles. Anyone side of any of the triangles so formed, is selected and is measured precisely. Such a line is called a baseline. All the angles in the network are measured with a transit. The lengths of the sides of all the triangles are then computed, from the measured length of the baseline and the observed corrected angles, using the sine formula.
Global Positioning System (GPS) is a satellite navigation system based on the principle of trilateration. Trilateration is a method used to find the location of a point if its distances from at least three other stations of known coordinates are predetermined.
Mean sea level and Level surface
A level surface is a curved surface, every point on which is equidistant from the centre of the Earth and every surface element is normal to the plumb line. It is parallel to the mean spheroidal surface of the earth. However, for plane or ordinary surveying, a level surface at any point is assumed to be a plane surface perpendicular to the plumb line at that point. The particular surface at the average sea level is known as mean sea level.
Rise and Fall and Height of Instrument method of levelling
It consists of determining the difference of levels between the consecutive points by comparing their staff readings. The rise or fall is obtained by calculating the difference between the consecutive staff readings. A rise is indicated if the backsight is more than the foresight, and a fall if the backsight is less than the foresight. Then the reduced level of each point is obtained by adding the rise to, or by subtracting the fall from the reduced level of the preceding point.
Check: ⅀B.S. – ⅀F.S. = ⅀rise – ⅀fall = last R.L. – first R.L.
Differentiate levelling and Reciprocal levelling
Determining the difference in elevation between two or more points without any regard to the alignment of the points is called differential levelling. It is used when: (a) two points are a large distance apart, (b) the difference in elevation between the two points is large, and (c) some obstacle intervenes between the points.
It is the operation of levelling in which the difference in elevation between two points is accurately determined by two sets of reciprocal observations. This method is very useful when the instrument cannot be set up between the two points due to an obstruction such as a valley, river, etc., and if the sights are much longer than are ordinarily permissible.
The three-point problem and two-point problem in plane table surveying
Two Point Method
The two-point problem consists of locating the position of a plane table station on the drawing sheet by observation of two well-defined points, whose positions have already been plotted on plan.
Three Point Method
The three-point problem consists in locating the position of a plane table station on the drawing sheet by means of observation of three well-defined points, whose positions have already been plotted on plan. Let A, B, and C, be the three well-defined points and let their plotted positions be a, b and c. It is required to fix ground station T on plan as t.
Vertical photographs and Oblique photographs
In a particular case, the perspective and the orthographic, produce similar images. This is when the ground is flat and the film is horizontal, i.e., the optical axis of the camera is vertical. Such a photograph is known as a vertical photograph and it may be considered to represent the plane of the ground.
In case the vertical axis of the camera is unintentionally inclined to the vertical by not more than 3°, the photograph is called tilted. However, sometimes the vertical axis of the camera is intentionally inclined to the vertical and then the resultant photograph is known as oblique.
True error and most probable error
- True Error. A true error may be defined as the difference between a measurement and its true value.
- Most Probable Error. The most probable error is defined as that quantity which added to, and subtracted from, the most probable value fixes the limits within which it is an even chance the true value of the measured quantity must lie.
Principal point and Isocentre in an aerial photograph
It is the point of intersection of the optical axis of the aerial camera with the plane of the aerial photograph and is shown as k in following figure. It is considered to coincide with the intersection of x and y axes of the photograph. The point K is known as ground principal point where the line Ok when produced meets the ground.
It is the point on an aerial photograph in which the bisector of the angle of tilt meets the photograph. In given figure, i is isocentre. It lies on the principal line at a distance of f tan (t/2) from the principal point.
Sensors and Platforms
Platforms are commonly called the vehicles or carriers for remote sensing devices. A platform is a synonym for any orbiting spacecraft, be it a satellite or a manned station, from which observations are made. In most instances, the platforms are in motion and by movement they automatically proceed to new positions from where, they target new objects. A satellite orbiting the Earth is a typical platform however, the platforms range from balloons, kites (low altitude remote sensing) to aircrafts and satellites (aerial and space remote sensing). As we go higher in the sky, larger area is viewed by the sensor.
The sensor systems are simply the eyes of the satellites that view and record the scene. Sensors are the special instruments mounted on the platforms (aeroplane or satellite) usually having sophisticated lenses with filter coatings, to focus the area to be observed at a specific region of EMS. Solar radiation is the main source of EMR and is a combination of several wavelengths such as gamma ray, x-ray, visible, infrared, thermal and microwaves. Sensor systems mainly operate in the visible, infrared, thermal and microwave regions of EMR.
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