SURV LEC LESSON 6

• True Meridian

• Magnetic Meridian

• Grid Meridian

• Assumed Meridian

Four Types of Meridians

True Meridian

also knows as the astronomic meridian or geographic meridian.

True Meridian

the direction of a line passing through the astronomic north and south poles and the observer’s position.

Magnetic Meridian

is the direction taken by the magnetized needle of a compass at the observer’s position.

Magnetic Meridian

It is not parallel to the astronomic meridians since they converge at a magnetic pole which is located some distance away from the true geographic poles.

Grid Meridian

is a fixed line of reference parallel to the central meridian of a system of plane rectangular coordinates.

Assumed Meridian

is an arbitrary chosen fixed line of reference which is taken for convenience.

Assumed Meridian

this meridian is usually the direction from a survey station to an adjoining stations or some well defined and permanent point.

1. Establishing magnetic meridians by compass

2. Determining true north by the aid of Sun and a plumb line.

3. Determining true north by the rising and setting of the sun.

4. Determining true north by Polaris.

5. Determining true south by southern cross.

6. Determining true north (or south) by a wrist watch.

Expedient Methods of Establishing Meridians

1. Interior Angles

2. Deflections Angles

3. Angles to the right

4. Bearings

5. Azimuths

Kinds of Angles used in Surveying

COMPASS BOX

the _______ of the instrument has a horizontal circle which is graduated from 0-90° in each quadrant. (5-10 minutes reading)

LINE OF SIGHT

usually is fixed along the index mark on the north graduation of the circle

MAGNETIC NEEDLE

the compass needle is of magnetized tampered steel balanced at its center on a jeweled pivot so that it swings freely in the horizontal position.

• BRUNTON COMPASS

• LENSATIC COMPASS

• SURVEYOR’S COMPASS

• PLAIN POCKET COMPASS

• PRISMATIC COMPASS

• FORESTER’S COMPASS

• TRANSIT COMPASS

Types of Compass

BRUNTON COMPASS

is specialized instrument used widely by those meeting to make accurate deeper & angle measurements in the field (geologist archeologists, environmental engineers & surveyors.

LENSATIC COMPASS

is the typical military field compass, and is and principally to measure or layout magnetic bearings

SURVEYOR’S COMPASS

also known as CICUMFERENTOR is an instrument used in surveying to measure horizontal angles.

PLAIN POCKET COMPASS

is similar to the surveyor’s compass, except that it has no sight vanes.

PRISMATIC COMPASS

the graduation an found on a rotating card instead of being on the compass box

FORESTER’S COMPASS

is another type of pocket compass, which is usually made of aluminum or some type of metal which does not affect the free movement and positioning of the magnetic needle.

TRANSIT COMPASS

the engineer’s transit has a compass box which is similar in construction to the surveyor’s compass.

Magnetic Declination

Is the horizontal angle and direction by which the needle of a compass deflects from the meridian at any particular locality.