LESSON 24

• MERIDIAN are fixed line of reference for determining direction of lines
• TRUE MERIDIAN is a North-South line passing through the geographical poles of the earth. It is sometimes known as the astronomic or geographic meridian
• MAGNETIC MERIDIAN are lines lies parallel to the magnetic lines of force of the earth and is indicated by the direction of the magnetized needle
• GRID MERIDIAN is a reference line parallel to the central true meridian of a system of plane rectangular coordinates, one central meridian which coincide with the true meridian is selected, and all other meridians are made parallel to this meridian.
• ASSUMED MERIDIAN is a line of reference arbitrarily chosen for convenience. This meridian is usually the direction from a survey station to an adjoining station or some well-defined permanent point.
• TRUE NORTH is the north point of the true meridian.

  -actual location of the earth's north geographic pole and is always shown along a vertical line
• MAGNETIC NORTH - a north point that is established by means of a magnetized compass needle when there are no local attractions affecting it.
• GRID NORTH a north point which is established by lines on a map which are parallel to a selected central meridian.
• ASSUMED NORTH is used to portray the location of any arbitrarily chosen north point.
• The two stars, Mersk and Dubhe, forming the side of the dipper which is the farthest from the handle is know as the pointer stars.
• POLARIS also known as north star, pole star, cynosure
• RADIANS is defined as the angle subtended at the center of the circle by an arc length exactly equal to the radius of the circle
• GRADS - centesimal system , circumference of the circle is divided into 400 parts
• DEGREE - sexagesimal system , circumference of the circle is divided by 360 parts or degrees
• MIL circumference is divided into 6400 parts
• Radian is sometimes referred to as NATURAL UNIT OF ANGLE because there is no arbitrary number in its definition.
• COMMONLY USED REFERENCE POINTS/DESIGNATION OF NORTH POINTS:

  TRUE NORTH

  MAGNETIC NORTH

  GRID NORTH

  ASSUMED NORTH