VARIATIONS OF MAGNETIC DECLINATION

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

Morning After noon

Annual Variation• Also known as yearly

variation • Amounts only less than 1

minute of arc and thus considered negligible

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

• No means of formula to predict this variation

• Nature and behavior can only be described from past observations

Irregular variation• Occurs unpredicted

• During magnetic storm and disturbances associated with sun spots and auroral display occurrence

ISOGONIC CHART

Isogonic Chart

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

Isogonic Lines Agonic Lines

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

USE OF THE COMPASS

A

B

(Use the compass app)

LOCAL ATTRACTION

PROJECT

Local Attraction

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

Local Attraction

may be constant or may vary depending upon the surrounding magnetic influences

draws the needle away from the magnetic meridian

Local Attraction

present when:

two observed directions differ

2

TYPES OF COMPASS SURVEYS

TYPES OF COMPASS SURVEYS

1. OPEN COMPASS TRAVERSE

2. CLOSE COMPASS TRAVERSE

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.

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.

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.

Advantage :

• Ability to check the angular measurements.

– Sum of interior angles = (n-2)180degrees • Where n = number of sides

ADJUSTING OPEN COMPASS TRAVERSE

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.

ADJUSTMENT OF A CLOSED COMPASS TRAVERSE

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

The following forward and back bearings were observed in traversing with a compass.

Calculate the interior angles and correct them for observational errors. Assuming the observed bearing of line PQ to be correct , adjust the remaining sides.

LINE

OBSERVED BEARINGSFOREWARD 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

Sketch, sketch, sketch...

Solve for the interior angles

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)

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)

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)

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)

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)

Adjust the interior angles...

• 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)

• 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)

• 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’

Solve for the adjusted bearings

• 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)

-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)

-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)

- 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)

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’

Tabulated of Adjusted Traverse Area

• 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.

PRECISION IN COMPASS WORK

Sources of Error In COMPASS Work

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

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.

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.

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.

7.Magnetic Variations

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.

MISTAKES IN COMPASS WORK

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.

• 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.

• 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.

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