Surv1 Elementary Surveying_5

SURV1 Elementary Surveying

Unit V Measurement of Angles and Directions

Engr. Angela Davis

Lecture

Content

Unit I. Surveying Concepts

Unit II. Accuracy of Measurements

Unit III. Measurement of Distances

Unit IV. Levelling

Unit V. Measurement of Angles and Directions

Topics:

a. Definition of Terms

b. Types of Levelling Instruments

and Accessories

c. Care and Handling of Instrument

d. Differential Levelling

e. Profile Levelling

f. Reciprocal Levelling

g. Double-Rodded Levelling

h. Three-Wire Levelling

i. Curvature and Refraction

j. Common Errors and Mistakes in

Levelling

a. True and Magnetic Meridian

b. Magnetic Declination

c. Local Attraction

FINAL EXAM SCHEDULE:

MARCH 18, MONDAY “Study Day”

8:30-9:30 AM 172T

COVERAGE: Measurement of Angles and Directions

Measurement of Angles and Directions

True and Magnetic Meridian

MERIDIANS - a fixed reference line or direction which lies in a vertical plane passing through a fixed point of reference and through the observer’s position.

True Meridian- sometimes known as the astronomic or geographic meridian. It is the generally adapted reference line in surveying practice. This line passes through the geographic north and south poles of the earth and the observer’s position.


Magnetic Meridian- it is a fixed line of reference which line parallel with the magnetic lines of force of the earth. Its direction is defined by freely suspended magnetic needle of a compass held at the observer’s position.

Grid Meridian- a fixed line of reference which lies parallel to the central meridian of a system of plane rectangular coordinates. The use of grid meridians is applicable only to plane surveys of limited extent.


Assumed Meridian- an arbitrarily chosen fixed line of reference which is taken for convenience. This meridian is usually the direction from a survey station to an adjoining station or some well-defined and permanent point. It is used only on plane surveys of limited extent since they are difficult or may be impossible to re- establish if the original reference points are lost or obliterated.

Magnetic Declination The magnetic poles are not points but oval areas located

about 2,000 km away from the actual location of the geographic poles of the earth. These areas are not fixed and may move to a different location everyday, perhaps as far as 50 km.

The horizontal angle and direction by which the needle of a compass deflects from the true meridian at any particular locality is called the magnetic declination.

Local Attraction Local Attraction- is any deviation of the

magnetic needle of a compass from its normal pointing towards magnetic north. In the field, the compass user may not realize that magnetic bearings read with the compass have been affected by local attraction.

Content




Unit IV. Levelling




c. Local Attraction

d. Bearing and Azimuth

e. Types of Compass and Uses

f. Common Errors and Mistakes in

Compass Work

INTERIOR AND DEFLECTION ANGLES INTERIOR ANGLES

The angles between adjacent lines in a closed polygon are called interior angles.

INTERIOR AND DEFLECTION ANGLES DEFLECTION ANGLES

The angles between a line and the prolongation of the preceding line is called deflection angle.

Bearing and Azimuth BEARING

The direction of a line may be described by giving its bearing. The bearing of a line is the acute horizontal angle between the reference meridian and the line.

Bearing and Azimuth Quadrant System for defining bearings.

0 N

90 E 90 W

0 S

NORTH-WEST

(NW)

QUADRANT

NORTH-EAST

(NE)

QUADRANT

SOUTH-WEST

(SW)

QUADRANT

SOUTH-EAST

(SE)

QUADRANT

REFERENCE MERIDIAN

(N-S Line)

REFERENCE PARALLEL

(E-W Line)

Forward and Back Bearing When the bearing of line is observed in the direction in

which the survey progresses it is referred to as forward bearing.

If the bearing of the same line is observed in an opposite direction it is called back bearing.

Forward and Back Azimuth

Bearing and Azimuth AZIMUTH

The azimuth of the a line is its direction as given by the angle between the meridian and the line measured in a clockwise direction from either the north or south branch of the meridian.

Illustrative Problems

Compute the angles APB, CPD, and EPF from the following set of lines

whose azimuths are given.

a) AZIMN of line PA = 39˚48’ ; AZIMN of line PB = 115˚29’

b) AZIMS of line PC = 320˚22’ ; AZIMN of line PD = 62˚16’

c) AZIMN of line PE = 219˚02’ ; AZIMN of line PF = 154˚16’

Illustrative Problems

Convert the following bearings to equivalent azimuths.

a) AB, N 25˚25’ W

b) BC, due EAST

c) CD, S 50˚10’ E

Types of Compass and Uses

Uses

-this compass is suitable and sufficiently accurate for preliminary and exploratory surveys, engineering-type surveys, and topographic surveys requiring only low degrees of precision.

Types of Compass

Brunton Compass- is one of the most versatile and widely used. It

combines the features of a prismatic compass, sighting compass, hand level, and clinometer.


Uses

-it is designed for military use such as in reconnoitering, determining directions, orienting maps, artillery fire directions control, and other uses where magnetic azimuths are required.

Types of Compass

Lensatic Compass- it consists of an aluminum case containing a magnetic dial balanced on a pivot, a hinged cover with a sighting wire, a hinged eyepiece containing a magnifying glens for reading the dial graduations, and a sighting slot for viewing the distant object.


Uses

-was popularly used earlier for running limited plane surveys of reasonable accuracy. Now it is commonly used for forest surveys, in retracting old land surveys, and in geological explorations.

Types of Compass

Surveyor’s Compass- it has a compass box containing a graduated circle, two sight vanes, a magnetic needle, and two clamping screws. A glass cover protects the needle and the graduated circle.


Uses

-It is used in reconnaissance and exploratory surveys.

Types of Compass

Plain Pocket Compass- it is similar to the surveyor’s compass, except that it has no sight vanes. It is a small hand instrument used for obtaining roughly the bearing of a line.


Uses

-it is widely used by sailors for navigation at sea.

Types of Compass

Prismatic Compass- the graduations are found on a rotating card instead of being on the compass box. The card moves with the needle since both are fastened together. Two short sighting devices are provided and magnetic bearing are read by means of a prism at the same instant that the compass is sighted along the line.


Uses

-it is widely usedin forestry surveys as well as for geological and other similar exploratory surveys.

Types of Compass

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.


Uses

-it is used to check horizontal angles and directions measured or laid off during transit surveys. It is also used for orienting the transit with the magnetic meridian.

Types of Compass

Transit Compass- it is similar to the surveyor’s compass, only it is mounted on the upper plate of the transit.

Common Errors and Mistakes in Compass Work

Sources of Error in Compass Work

Bent Needle - when a compass with bent needle is used, all observed bearing will have a constant error.

Bent Pivot - the magnitude of the error will depend upon the direction in which the sight is made.


Sources of Error in Compass Work

Plane of Sight not vertical – the line of sight may be steeply inclined.

Local Attraction – the correct pointing of the magnetic needle toward magnetic north is usually affected by different forms of local attraction.

Errors in Reading the Needle - due to the inability of the observer to determine exactly the point on the graduated circle where the needle comes to rest.


Sources of Mistakes in Compass Work

Reading the wrong end of the magnetic needle.

Not releasing the needle completely and not allowing it to swing freely about the pivot.

Mixing or interchanging the recording of azimuths from north and south, magnetic and true bearings, clockwise and counterclockwise angles or forward and back bearings.

Content




Unit IV. Levelling




c. Local Attraction

d. Bearing and Azimuth

e. Types of Compass and Uses

f. Common Errors and Mistakes in

Compass Work

g. Compass Traverse

h. Adjustment of Data

Documents

Surv1 Elementary Surveying_5