* DEFINITIONS

  • Earth's surface 

                         The earth is not a true sphere and is slightly flattened at the poles. Its polar axis is somewhat smaller in length (about 43.45 km) than that of its equatorial axis. Any section of the earth parallel to the equator is a circle and any of its section parallel through the poles is an ellipse. Such a figure may be generated by revolving about its minor axis and is called an oblate spheroid. Precisely, the equatorial section is also slightly elliptical and therefore such a figure should be called an ellipsoid. Precise observations indicate that the southern hemisphere is a trifle larger than the northern. Therefore, all the polar sections are oval and can be called ovaloid.

                               In fact, no geometrical solid represents the true shape of the earth. However, for all measurement purposes in surveying, the irregularities of the earth's surface, as discussed above, may be assumed to be absent and the resultant surface be considered a spheroid. The earth is also recognised by a new name, geoid,

  • 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

  • Great circle

                         Imagine a plane passing through the centre of the earth (Fig. 1.1). The intersection of such a plane with the mean level surface of the earth is termed as the great circle of the earth.

  • Meridian

                             It is the line defined by the intersection of an imaginary plane, passing through the poles and any point on the earth's level surface (Fig. 1.2).

  • Plumb line
                            The plumb line is normal to the meridian. Considering the mean level surface of the earth as spherical, these lines converge at the centre of the earth (Fig. 1.2).

Note Earth being an oblate spheroid, the perpendiculars to the surface do not converge at any point. The irregular distribution of the earth's mass also causes some deviations. But for plane surveying, all such deviations are ignored and plumb lines are assumed to converge at the centre of the earth.
   
  • Latitude
                       The latitude of a place (say point A) is the angle subtended at the centre O from the equatorial plane along the meridian through the point (Fig. 1.3). Thus, it is the angle between the



direction of a plumb line at the place and the equator, the reference plane. If the point A is above the reference plane, the latitude is positive (or northerly) and in case it below the reference plane, the latitude is negative (or southerly)Latitude range up to 90 deg (or 90 deg north), and down to - 90 deg (or 90 deg south). Latitudes of 90 deg and - 90 deg correspond to the north and south geographical poles, respectively.

  • Longitude

                     7. The longitude of a place is the angle between a fixed meridian (prime meridian) and the meridian of the place of the observer (Fig. 1.3). The prime meridian adopted for surveying work is that at Greenwich and will therefore be of zero longitude. The longitude of a line is measured with respect to this longitude. The longitude of the place may be east or west of the Greenwich meridian and are termed accordingly the east longitude (positive) or the west longitude (negative). The longitude can range up to 180 deg (east), and down to - 180 deg (west). 

  • Note -The location of a point on earth's surface is fixed by the latitude and longitude of that point.
  • Level line

                         . Any portion of the line lying on the great circle of the earth is called a level line. It may also be defined as a line lying on the level surface and normal to the plumb line at all the points:

  • Horizontal plane and line

                                     . A plane through any point on the earth's mean level surface and tangent to the surface at that point is known as horizontal plane. A line lying in the horizontal plane is termed as horizontal line. Through any point on the earth's surface, there can be only one horizontal plane but infinite horizontal lines.

  • Vertical plane and line

                                     A line through a point perpendicular to the horizontal plane is called a vertical line. A plane passing through that point and containing the vertical line is termed as vertical plane. Through any point on the earth's surface, there can be only one vertical line but infinite vertical planes.

  • Spherical triangle

                                            Imagine three points A, B and C (Fig. 1.4) on the mean level surface of the earth. The three points when joined form a triangle having a curved surface ABC, and AB, BC and CA being the arcs. The triangle ABC is known as a spherical triangle and the angles A' B and C are spherical angles. The amount by which the sum of the angles of a spherical triangle exceeds by 180 deg is called spherical excess.


  • Grade- It is defined as the slope of a line. It is also called gradient.


  • Elevation

                           It is the vertical distance of a point above or below the reference surface (datum). When elevations are with respect to the earth's surface, the datum is the mean sea level. The datum is a curved surface and, therefore, its curvature should be given due consideration even while determining elevations in plane surveying. An imaginary line joining points of equal elevations is known as contour.




 

Comments

Post a Comment

Popular posts from this blog

Image
      FUNDAMENTAL CONCEPTS      *INTRODUCTION Surveying is one of the oldest arts practised by man. History reveals that the principles and practises of surveying were used, consciously or unconsciously, even in the primitive ages, albeit in a crude manner. In the past few decades, however, these have become more rational and channelised. The introduction and practise of surveying is indispensable to all branches of engineering. The training that a student receives, irrespective of his branch of engineering, in the art of observing, recording and computing data, as well as in the study of errors, their causes and effects, directly contribute to his success in other professional courses. He develops inter alia qualities such as self-reliance, initiative and the ability to get along with others. This also helps an engineer get acquainted with the reasonable limits of accuracy and the value of significant figures. A knowledge of the limits of accuracy ca...
Read more
Image
 CLASSIFICATION OF SURVEY                                                             An attempt has been made here to group the types of survey. However, it is not that significant or satisfactory as there are differences in objectives and dissimilarities in the procedures employed to distinguish between them.   Based on Accuracy Desired Plane survey- Survey in which the mean surface of earth is regarded as plane surface and not curved as it really is, is known as plane surveying. The following assumptions are made: (a) A level line is considered a straight line and thus the plumb line at a point is parallel to the plumb line at any other point. (b) The angle between two such lines that intersect is a plane angle and not a spherical angle. (c) The meridians through any two points are parallel.       When we dea...
Read more