Variations In Magnetic Declination Isogonic Chart Use of the Compass Local Attraction Types of compass Surveys

1. VARIATIONS OF
MAGNETIC DECLINATION

2. Daily Variation
-also known as Diurnal variation
-extreme eastern position of the
needle usually occurs early in the
morning and the extreme western
position occurs just about after noon
time.
-usually neglected when observing
magnetic directions

3. Morning After noon

4. Annual Variation
• Also known as yearly
variation
• Amounts only less than 1
minute of arc and thus
considered negligible

5. Secular Variation
• Slow, gradual but unexplainable
• The meridian swings in one
direction for about 150 years and
comes to a stop then swings back
in the opposite direction
• It is important and not negligible
because of its magnitude

6. • No means of formula to
predict this variation
• Nature and behavior can
only be described from past
observations

8. Irregular variation
• Occurs unpredicted
• During magnetic storm and
disturbances associated with
sun spots and auroral display
occurrence

9. ISOGONIC CHART

11. Isogonic Chart
- tells you how far from true
north a compass will point
anywhere along an isogonic
line

12. Isogonic Lines Agonic Lines

13. Historical Magnetic
Declination
http://maps.ngdc.noaa.gov/viewers/
historical_declination/index.html
Find Magnetic Declination with
Google Maps
http://www.geosats.com/magdecli.html

15. USE OF THE
COMPASS

17. A
B

18. (Use the compass app)

19. LOCAL ATTRACTION

20. PROJECT

21. Local Attraction
The deflection of a magnetic
needle of a compass from
its true position due to the
presence of magnetic
influencing material

22. Local Attraction
may be constant or may vary
depending upon the surrounding
magnetic influences
draws the needle away from the
magnetic meridian

23. Local Attraction
present when:
two observed directions differ

24. 2

25. TYPES OF COMPASS
SURVEYS

26. TYPES OF COMPASS SURVEYS
1. OPEN COMPASS TRAVERSE
2. CLOSE COMPASS TRAVERSE

27. OPEN COMPASS TRAVERSE
• Are a series of lines connected to
each other with known bearings
and measurements.
• The line doesn’t connect back to
the original point and forms a
polyline.

28. Disadvantage :
• No arithmetic check available and
therefore needs an extra care on
measuring the bearings and the lines.
• Needs to measure the line twice and
take the mean as its length to make it
more accurate.

29. CLOSE COMPASS TRAVERSE
• Similar to an open compass traverse,
but the series loops back to the
original point.
• Are used extensively in construction
surveys, property and topographic
surveys due to its checking
availability.

30. Advantage :
• Ability to check the angular
measurements.
– Sum of interior angles =
(n-2)180degrees
• Where n = number of sides

31. ADJUSTING OPEN
COMPASS TRAVERSE

32. ADJUSTING OPEN COMPASS TRAVERSE
• 1st STEP: make a sketch of the
gathered data.
• 2nd STEP: determine which among the
traverse lines are free from local
attraction.
• 3rd STEP: performing the adjustment
by starting from the “best line” or the
unaffected line.

33. ADJUSTMENT OF A CLOSED
COMPASS TRAVERSE

34. Important steps performed during
adjustments
I. Plot or tabulate
II. Computing and adjusting the interior
angles
III.Selecting the best line
IV. Adjusting the observed bearings of
successive lines

35. The following forward and back bearings were observed
in traversing with a compass.
LINE
OBSERVED BEARINGS
FOREWARD BACKWARD
PQ S 37° 30’ E N 37° 30’ W
QR S 43° 15’ W N 44° 15’ E
RS N 73° 00’ W S 72° 15’ E
ST N 12° 45’ E S 13° 15’ W
TP N 60° OO’ E S 59° OO’ W
Calculate the interior angles and correct them for
observational errors. Assuming the observed bearing
of line PQ to be correct , adjust the remaining sides.

36. Sketch, sketch, sketch...

38. Solve for the interior angles

39. At Station P:
αPQ = S 37° 30’ E
(observed bearing for PQ)
αPT = S 59° 00’ W
(observed bearing for PT)
So, the interior angle at P is :
ϴP = αPQ + αPT
= 59° 00’ + 37° 30’
= 96° 30’ (COMPUTED interior angle at P)

40. At Station Q:
αQP = N 37° 30’ W
(observed bearing for QP)
αQR = S 43° 15’ W
(observed bearing for QR)
So, the interior angle at Q is :
ϴQ = αQP + αQR
= 180° - (37° 30’ + 43° 15’)
= 99° 15’ (COMPUTED interior angle at Q)

41. At Station R:
αRQ = N 44° 15’ E
(observed bearing for RQ)
αRS = N 73° 00’ W
(observed bearing for RS)
So, the interior angle at P is :
ϴR = αRQ + αRS
= 44° 15’ + 73° 00’
= 117° 15’ (COMPUTED interior angle at R)

42. At Station S:
αSR = S 72° 15’ E
(observed bearing for SR)
αST = S 12° 45’ W
(observed bearing for ST)
So, the interior angle at P is :
ϴS = αSR + αST
= 180° - (72° 15’ + 12° 45’)
= 95° 00’ (COMPUTED interior angle at S)

43. At Station T:
αTS = S 13° 15’ W
(observed bearing for TS)
αTP = N 60° 00’ E
(observed bearing for TP)
So, the interior angle at T is :
ϴT = 90° 00’ + αTS + (90° 00’ – αTP )
= 90° 00’ + 13° 15’ + (90° 00’ - 60° 00’)
= 133° 15’ (COMPUTED interior angle at T)

44. Adjust the interior angles...

45. • Sum of Interior Angles
SumA = θP + θQ + θR + θS + θT
= 96° 30’ + 99° 15’ + 117° 15’ +
95° 00’ + 133° 15’
= 541° 15’ (Sum of interior angles)
• Sum of Interior Angles of a Polygon
– Since the lot is a 5 sided polygon, its interior
angle is:
SumB = (n – 2) 180°
where: n = no. of sides
= (5 – 2) 180°
= 540° 00’ (THEORETICAL sum)

46. • Determine the error of closure
e = SumB - SumA
= 541° 15’ - 540° 00’
= 1° 15’ (error of closure)
• Obtain correction
Corr = e / n
= 1° 15’ / 5
= 15’. (correction per interior angle)

47. • If SumA is less than SumB, the correction is added
to each interior angle to determine the adjusted
interior angle. Else, subtract.
θ’P = θP - Corr. = 96° 30’ - 15’ = 96° 15’
θ’Q = θQ - Corr. = 99° 15’ - 15’ = 99° 00’
θ’R = θR - Corr. = 117° 15’ - 15’ = 117°00’
θ’S = θS - Corr. = 95° 00’ - 15’ = 94° 45’
θ’T = θT - Corr. = 133° 15’ - 15’ = 133°00’
Sum = 541° 15’ - 1° 15’ = 540° 00’

48. Solve for the adjusted bearings

49. • The adjustment is started from either end of the
best line.
- Adjustment for QR and RQ
θ’Q = 99° 00’
α’PQ = N 37° 30’ W
α’QR = 180° - (α’PQ + θ’Q )
=180° - (37° 30’ + 99° 00’ )
= 43° 30’ or
α’QR = S 43° 30’ E
(adjusted bearing of line QR)
-Also, α’RQ = N 43° 30’ W
(adjusted bearing of line RQ which
is opposite the adjusted bearing of QR)

50. -Adjustment for RS and SR
θ’R = 117°00’
α’RQ = N 43° 30’ E
α’RS = θ’R - α’RQ
= 117° 00’ - 43° 30’
= 73° 30’ or
α’RS = N 73° 30’ W
(adjusted bearing of line RS)
-Also, α’SR = S 73° 30’ E
(adjusted bearing of line SR
which is opposite the adjusted
bearing of RS)

51. -Adjustment for ST and TS
θ’S = 94°45’
α’SR = S 73° 30’ E
α’ST = 180° - (θ’S + α’SR )
= 180° 00’ –
(94° 45’ + 73° 30’)
= 11° 45’ or
α’ST = N 11° 45’ E
(adjusted bearing of line RS)
-Also, α’TS = S 11° 45’ W
(adjusted bearing of line TS
which is opposite the adjusted
bearing of ST)

52. - Adjustment for TP and PT
θ’T = 133°00’
α’TS = S 73° 30’ W
α’TP = 180° + α’TS - θ’T
= 180° 00’ + 11° 45’
- 133°00’
= 58° 45’ or
α’TP = N 58° 45’ E
(adjusted bearing of line RS)
-Also, α’PT = S 58° 45’ W
(adjusted bearing of line PT
which is opposite the adjusted
bearing of TP)

53. Tabulated of Adjusted
Traverse Area
STATION
COMPUTED
INTEROR
ANGLES
CORR.
ADJUSTED
INTERIOR
ANGLES
LINE
ADJUSTED BEARINGS
FORWARD BACK
P 96° 30’ 15’ 96° 15’ PQ S 37° 30’ E N 37° 30’ W
Q 99° 15’ 15’ 99° 00’ QR S 43° 30’ W N 43° 30’ E
R 117° 15’ 15’ 117° 00’ RS N 73° 30’ W S 73° 30’ E
S 95° 00’ 15’ 94° 45’ ST N 11° 45’ E S 11° 45’ W
T 133° 15’ 15’ 133° 00’ TP N 58° 45’ E S 58° 45’ W
SUM 541° 15’ 1° 15’ 540° 00’

54. • If local attraction exist at any
station, both backward and
forward bearing are equally
affected. Thus, interior angle
computed from forward and
backward bearing are
independent of local attraction.

55. PRECISION IN
COMPASS WORK

56. Sources of Error
In COMPASS Work

57. 1. Bent Needle
2. Bent Pivot
3. Sluggish Needle
4. Plane of Sight Not Vertical
5. Electrically Charge Compass Box
6. Local Attraction
7. Magnetic Variation
8. Errors in Reading The Needle

58. 1. Bent Needle
-All observed Bearings will have a constant
error, Either end of the needle will read
higher or lower than the correct value.
2.Bent Pivot
-A variable systematic error in magnetic
compass. The magnitude of the error will
depend upon direction in which the sight is
made.

59. 3. Sluggish Needle
—needle moves unusually slowly in
seeking magnetic north—it will
probably come to rest a little off the
magnetic meridian.
Most common cause:
- weakening of the magnetism of
the needle.

60. 4.Plane of sight Not Vertical.
When Observing the direction of a line,
the line of sight may be steeply inclined if the
sight vanes are bent.
5.Electrically charged Compass Box
happens when the glass cover of the
compass box is rubbed in the surface.
6.Local attraction.
The correct pointing of the magnetic
needle toward magnetic north is usually affected
by different forms of local attraction.

61. 7.Magnetic Variations

62. 8.Errors in Reading Needles
Most accidental errors in compass
work is due to the inability of the
observer to determine exactly the
point on the graduated circle where
the needle comes to rest.

63. MISTAKES IN COMPASS
WORK

64. MISTAKES OF COMPASS WORK
• Reading the wrong end of the
magnetic needle.
• Failing to observe the reverse
bearings or azimuths of lines in the
traverse.
• Misreading the quadrant letters
when taking a bearing near the
cardinal points of the compass.

65. • Bearing letters are not changed when
using the reversed bearing of a line.
• Setting off the magnetic
declination on the wrong side of
north.
• Failing to adjust the observed
traverse angles prior to
calculating bearings or azimuths
of traverse lines.

66. • Mixing or interchanging the
recording of azimuths from north
and south, magnetic and true
bearings, clockwise and
counterclockwise angles, or
forward and back bearings.
• Selecting a line for referencing
arbitrary directions which may be
difficult to locate later.

67. (END)