Fce 552 part2

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  • AARSEC 09/30/12 University of Nairobi
  • AARSEC 09/30/12 University of Nairobi
  • AARSEC 09/30/12 University of Nairobi
  • AARSEC 09/30/12 University of Nairobi
  • AARSEC 09/30/12 University of Nairobi
  • AARSEC 09/30/12 University of Nairobi
  • AARSEC 09/30/12 University of Nairobi

Transcript

  • 1. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology DIRECTION OFA LINE:Astronomical Meridian A plane passing through a point on the surface ofthe Earth and containing the Earth’s axis of rotationdefines the astronomical meridian at the point. The direction of this plane may be established byobserving the position of the sun, or a star, or byobserving a planet. By popular usage the intersection of this meridianplane with the surface of the Earth is known as thetrue meridian. B.Sc. (Civil Engineering) University of Nairobi
  • 2. FCE 552: Engineering Survey IV Dept. of Geospatial & Space TechnologyMagnetic Meridian The Earth acts very much like a bar magnet with a northmagnetic pole located considerably south of the north pole definedby the Earth’s rotational axis. The magnetic pole is not fixed in position, but ratherchanges its position continually. A magnetised needle freely suspended on a pivot will cometo rest in a position parallel to the magnetic line of force acting inthe vicinity of the needle. The direction of the magnetised needle defines themagnetic meridian at the specific time. Unlike the true meridian, whose direction is fixed, themagnetic meridian varies in direction. B.Sc. (Civil Engineering) University of Nairobi
  • 3. FCE 552: Engineering Survey IV Dept. of Geospatial & Space TechnologyAssumed Meridian For convenience in a survey of limited extent, any line ofthe survey may be assumed to be a meridian or a line ofreference. An assumed meridian is usually taken to be in thegeneral direction of the true meridian.Convergence of Meridians True meridians on the surface of the Earth are lines ofgeographic longitude, and they converge toward each other as thedistance from the equator towards either of the poles increases. The amount of convergence between two meridians in a givenvicinity depends on i) its distance north or south of the equator andii) the difference between the longitudes of the two meridians. B.Sc. (Civil Engineering) University of Nairobi
  • 4. FCE 552: Engineering Survey IV Dept. of Geospatial & Space TechnologyAzimuth of a line The azimuth of a line on the ground is the horizontal anglemeasured from the plane of the meridian to the vertical planecontaining the line. Azimuth gives the direction of the line w.r.t. the meridian. It is usually measured in a clockwise direction w.r.t. eitherthe north meridian or the south meridian. In astronomical and geodetic work azimuths are measuredfrom the south meridian. In plane surveying azimuths are generallymeasured from north. A line may have an azimuth of between 0 – 360. B.Sc. (Civil Engineering) University of Nairobi
  • 5. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology Azimuths are called true azimuths when measured from thetrue meridian, magnetic azimuths when measured from the magneticmeridian, assumed azimuths when referred to an arbitrary north-south line, and grid azimuths when referred to the central meridianin a grid system. The magnetic poles do not coincide with the poles definedby the Earth’s rotational axis, and certain irregularities in theEarth’s magnetic field cause local and regional variations in theposition of the magnetic needle. The amount and direction by whichthe magnetic needle is off the true meridian is called the magneticdeclination. Two azimuths differ from magnetic azimuths by themagnitude of the magnetic declination at the time. B.Sc. (Civil Engineering) University of Nairobi
  • 6. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology Traverses, triangulation and trilateration particularly, require azimuth control. The direction of a survey line w.r.t. true north – the azimuth of the line – is determined from theodolite observations to the stars or the sun, which itself a star, presents special problems when observed from the Earth. The quantities observed are:1. The horizontal angle between the star (sun) and a reference object (RO) placed at some suitable point of the survey, and either2. The vertical angle to the star (sun) at the same instant as the horizontal angle; or3. The accurate time of the horizontal observation. B.Sc. (Civil Engineering) University of Nairobi
  • 7. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology The position of the star (sun) at the instant of horizontal/vertical angle observation is extracted from the star almanac. The accuracy with which these quantities are observed depends on the accuracy required of the final azimuth, the particular method used, and the position of the observer on the Earth.B.Sc. (Civil Engineering) University of Nairobi