COMPASS SURVEY - Civil Engineering psc short notes

• The compass does not measure the angle directly, but measures magnetic bearings only 
• Relatively low accuracy 
• Used area to be surveyed large , fast surveying

SYSTEMS OF ANGULAR MEASUREMENTS

SEXAGESIMAL SYSTEM 
1 circumference : 360* 
1 degree : 60 min 
1 minute : 60 sec 

CENTESIONAL SYSTEM 
1 circumference : 400 grade 
1 grade : 100 centigrade 
1 centigrade : 100 cent centigrade 

HOURLY SYSTEM 
1 circumference : 24 hrs 
1 hour : 60 min 
1 minute : 60 sec 

CIRCULAR SYSTEM 
1 circumference : 2pi radian


SYSTEMS OF BEARING

WHOLE CIRCLE BEARING 
• The bearing of a line is measured from the reference meridian towards the line right round the circle 
• Value between 0 - 360* 
• The bearings observed in a prismatic compass or a theodolite is the whole circle bearings. 

REDUCED BEARING / QUADRANTAL BEARING
•Bearing is observed clockwise or anticlockwise from north or south whichever is the closest towards east or west 
• Value between 0 - 90* 
• Surveyors compass

TYPES OF COMPASS

PRISMATIC COMPASS
 Graduated circle is fixed to magnetic needle hence it does not move with line of sight 
 Broad form type needle 
 Viewing end has a prism 
 Sighting and reading done simultaneously 
 Graduations are in WCB 
 Graduations are inverted 
 Tripod may not be essential 
 0 marked at south 
 More accurate  

SURVEYORS COMPASS
Graduated circle fixed to the box hence it moves with the line of sight 
 Edge bar type needle 
 Viewing end has only a slit 
 Sighting and reading done separate 
 Graduations are in RB 
 Graduations are straight 
 Tripod is essential 
 0 marked at both north and south 
 Less accurate

DECLINATION
• The horizontal angle which the magnetic meridian makes with the true or geographical meridian. 
• Varies from place to place and time to time 
• TB = MB ± declination ( considering +ve and -ve decl. ) 
• Eg. 4° easts or 4° west

VARIATIONS IN DECLINATION 
1. SECULAR VARIATIONS : The magnetic meridian swings in one direction in 250 years gradually comes to rest and then swings in the opposite direction. 
2. ANNUAL VARIATION : 1 to 2 minutes deviation annually 
3. DIURNAL VARIATION : max 10 minutes variation daily. Maximum at poles, day time, summer 
4. IRREGULAR VARIATION : 1 - 2* variation due to natural disasters like earthquake volcano 

DIP
•Inclination of a freely suspended magnetic needle with the horizontal is known as the dip of the needle. • In northern hemisphere the north end of the needle is deflected downward ///. 
• The amount of dip varies in different parts of the earth 
• At equator - 0* , at poles - 90* 
• In order to counter dip, a sliding weight or rider in the form of fine brass is provided

TERMS 

 FOREBEARING : bearing taken in the direction of progression of survey 
 BACKBEARING : bearing taken in opp. Direction to that of progression of survey 
 DIRECT ANGLE : angle measured clockwise from preceding to following survey lines ( 0 - 360* )  DEFLECTION ANGLE : angle made by survey line with the projection of the preceding line ( 0 - 90* ) 

 ISOCLINIC LINE : line connecting points of equal dip 
 ISOGONIC LINE : line connecting points of equal declination 
 ACLINIC LINE : line connecting points of 0 dip 
 AGONIC LINE : line connecting points of 0 declination 
 LOCAL ATTRACTION 
• any external influence to the magnetic needle
• If local attraction exist BB ≠ F ± 180
• LA is equal for all reading taken from a location

 

BELOW ARE VIDEOS WITH PREVIOUS YEAR QUESTIONS BASED ON COMPASS SURVEY 

2 VIDEOS - 50 QUESTIONS 

 

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