LESSON 27

• CATEGORIES OF MAGNETIC DECLINATION:

  DAILY VARIATION

  ANNUAL VARIATION

  SECULAR VARIATION

  IRREGULAR VARIATION
• DAILY VARIATION -also called Diurnal Variation -an oscillation of the compass needle through a cycle from its position over a 24- hour period
• ANNUAL VARIATION it usually amount to less than 1 minute of arc and thus considered negligible
• SECULAR VARIATION - slow, gradual, but unexplainable shift in the position of the earth’s magnetic meridian over a regular cycle
• IRREGULAR VARIATION - uncertain in character and cannot be predicted as to amount or occurrence
• ISOGONIC CHART A chart or map which shows lines connecting points where the magnetic declination of the compass needle is the same at a given time.
• LOCAL ATTRACTION -any deviation of the magnetic needle of a compass from its normal pointing towards magnetic north
• MAGNETIC DIP a characteristic phenomenon of the compass needle to be attracted downward from the horizontal plane due to the earth’s magnetic line of force
• TRAVERSE is a series of lines connecting successive points whose lengths and directions have been determined from field measurements
• TRAVERSING is the process of measuring the lengths and direction of the lines of a traverse for the purpose of locating the position of certain points.
• TRAVERSE STATION - any temporary or permanent point of reference over which the instrument is setup. Sometimes called angle points.
• TRAVERSE LINE are lines connecting traverse stations and whose lengths and direction are determined.
• ADVANTAGES OF A COMPASS SURVEY:

  BIOSED
• COMPASS SURVEY one of the most basic and widely practiced methods of determining the relative position of points where a high degree of precision is not required. This surveys which employ magnetic compass are performed by traversing.
• OPEN COMPASS TRAVERSE consists of a series of lines of known lengths and magnetic bearings which are continuous but do not return to its starting point or close upon a point of known position.
• CLOSED COMPASS TRAVERSE consists of series of lines of known lengths and magnetic bearings which forms closed loop or begin and end at points whose positions have been fixed by other surveys of higher precision.
• The lines drawn in Isogonic chart are ISOGONIC LINES
• AGONIC LINES Lines connecting a zero magnetic declination

  -defines true and magnetic north in the same direction
• FIELD WORK INVOLVED IN MOST COMPASS SURVEYS IS DIVIDED INTO TWP BASIC OPERATIONS:

  MEASUREMENT OF ANGLES AT TRAVERSE STATION

  MEASUREMENT OF LENGTHS OF TRAVERSE LINES
• TYPES OF COMPASS SURVEY:

  OPEN COMPASS TRAVERSE

  CLOSED COMPASS TRAVERSE