Types of Meridians

Types of Meridian

• True meridian
• Magnetic meridian
• Grid meridian
• Assumed meridian

True Meridian

• Also known as astronomic or geographic meridian
• Passes through the north and south geographic poles and the observer's position

Magnetic Meridian

• Fixed reference line lying parallel to the magnetic lines of force of the earth

Grid Meridian

• Fixed reference line parallel to the central meridian of a plane rectangular coordinate system

Assumed Meridian

• Arbitrarily chosen fixed reference line selected for convenience
• Typically the direction from a survey station to an adjoining station or permanent point

Methods of Establishing Meridians

• Establishing a magnetic meridian by compass
• Determining true north using the sun and a plumb bob
• Determining true north by observing the rising and setting of the sun
• Determining true north by using Polaris

Determining True North by Polaris

• Big Dipper is a useful reference constellation in the Northern Hemisphere
• Merak and Dubhe, which form the dipper side farthest from the handle, are known as pointer stars

Units of Angular Measurement

• Degree
• Grad
• Mil
• Radian

Designation of North Points

• True north is the north point of the true meridian with symbol asterisk or TN
• Magnetic north is the north point established by a magnetized compass needle with symbol half arrowhead or MN
• Grid north is the north point established by lines on a map parallel to a selected central meridian with symbol full arrowhead or GN
• Assumed north is used to portray the location of any arbitrarily chosen north point with symbol small black circle or AN

Converting Angles to Decimal Degrees

• Convert the angle 238° 25’ 50” into its equivalent in decimal degrees which is 238.431°

Converting Angles to Degrees, Minutes, and Seconds

• Convert to degrees, minutes, and seconds the decimal angle 325.7541667 which is 325°45'15"

Converting Degrees to Grads, Mils and Radians

• Convert 270° into its equivalent value in grads, mils and radians
• 270° x 400 grads/360° = 300 grads
• 270° x 6400 mils/360° = 4800 mils
• 270° x 2π rad/360° = 4.71 rad

Interior Angle

• Angle between adjacent lines in a closed polygon
• Re-entrant angle is an interior angle greater than 180°

Deflection Angle

• Angle between a line and the prolongation of the preceding line

Angle to the Right

• Angles measured clockwise from preceding to succeeding line

Bearing

• Acute horizontal angle between a reference meridian and a line
• Forward bearing is observed in the survey's direction of progress
• Back bearing is observed in the opposite direction

Azimuth

• Angle between the meridian and a line, measured clockwise from either the north or south branch of the meridian

Compass

• Hand-held instrument for determining the horizontal direction of a line, relative to the magnetic meridian

Essential Features of the Magnetic Compass

• Compass Box
• Line of Sight
• Magnetic Needle

Types of Compass

• Bruton
• Combines a prismatic compass, sighting compass, hand level, and inclinometer with a hinged brass case
• Lensatic
• Contains a magnetic dial balanced on a pivot in aluminum casing with sighting wire, hinged eyepiece, and magnifying lens; used by the military
• Plain Pocket
• Similar to a surveyor's compass, but lacks sight vanes
• Surveyor’s
• Includes a compass box with graduated circle, sight vanes, magnetic needle, and clamping screws
• Prismatic
• Compass graduations are on a rotating card instead of on the compass box
• Forester's
• Pocket compass usually made of aluminum or metal that doesn’t affect free movement and positioning of the magnetic needle
• Transit
• Compass box similar to a surveyor's compass, mounted on the transit's upper plate, used to check horizontal angles during transit surveys

Magnetic Declination

• Horizontal angle and direction by which a compass needle deflects from the true meridian at a given location

Variations in Magnetic Declination

• Daily variation (diurnal variation)- Compass needle oscillation through a cycle over 24 hours; greatest in higher latitudes
• Annual variation- Periodic swing of magnetic meridian with respect to true meridian, typically less than 1 minute of arc
• Secular variation- Variation across many years with not thoroughly understood cause or character
• Irregular variation- Variation type that has uncertain character and unpredictable amount of occurrence

Isogonic Chart

• Chart or map showing lines that connect points with the same magnetic declination of the compass needle at a given time
• Agonic lines are lines connecting parts of the chart with zero magnetic declination
• Needles have easterly declination in areas west of the agonic line
• Needles have westerly declination in areas east of the agonic line

Local Attraction

• Deviation of a compass needle from its normal pointing towards magnetic north

Magnetic Dip

• Characteristic of the compass needle to be attracted downward from the horizontal plane due to the earth's magnetic lines of force

Traverse

• Series of lines connecting successive points with lengths and directions determined from field measurements

Traversing

• Process of measuring the lengths and directions of traverse lines to locate the position of certain points

Traverse Station

• Temporary or permanent point of reference where an instrument is set up; sometimes called angle points because angles are often measured there

Traverse Lines

• Lines that connect traverse stations with determined lengths and directions

Types of Compass Surveys

• Open compass traverse
• Series of lines with known lengths and magnetic bearings, continuous, and not returning to the starting point
• Closed compass traverse
• Series of lines with known lengths and magnetic bearings forming a closed loop, or beginning and ending at points fixed by higher-order surveys

Sources of Error in Compass Work

• Bent needle
• Bent pivot
• Sluggish needle
• Plane of sight not vertical
• Electrically charged compass box
• Local attraction
• Magnetic variation
• Errors in reading the needle