4.2 Magnetic north map skills

Social sciences

Geography

Map work

Grade 9

Module 13

Magnetic north map skills

You became acquainted with basic map skills in Grade 8 and did exercises that involved the 1:50 000 topographic map of Alice. In doing that, you learned to:

determine location

gauge accurately and determine direction

calculate distance by making use of a scale

explain the naming of a map

recognise conventional map symbols

recognise representation of height on maps

identify contour patterns

This knowledge, together with what you will still be acquiring in using the 1:50 000 topographic map of Bloemfontein and a 1:10 000 orthophoto map of a portion of this will see you through this year’s work.

Before commencing the practical exercises, you have to learn a number of additional map skills.

Calculating distance on an orthophoto map.

Orthophoto maps are always drawn to the scale of 1:10 000. Follow the steps to determine distance:

Accurately measure the distance between two points in centimetres, using a ruler (or using a string if the line to be measured curves);

Convert this distance to kilometres or metres, depending on what is required.

Example

A________________________________B

The length between A and B is 10 cm.

Scale 1 1:10 000

What is the true distance in km?

10 cm x 10 000

100 000 cm ÷ 100 000

1 km

What is the true distance in m?

10 cm x 10 000

100 000 ÷ 100

1000 m

Why do we divide by 100 000 and 100?

The metrical units of measurement can be represented as follows:

We therefore have 100 000 cm in 1 km

and

100 cm in 1 meter

You may therefore follow the shorter method if you can remember that the scale of

1 : 10 000 is the same as:

1 cm = 10 000 cm

1 cm = 100 m (÷ 100)

1 cm = 0,1 km (÷ 100 000)

Activity 1: [lo 1.3]

1. Calculate the true distance in metres if the distance on a 1:10 000 orthophoto map is as follows;

1. 5,8 cm.
2. 10,1 cm.
3. Calculate the true distance in kilometres if the distance on a 1:10 000 orthophoto map is as follows;

1. 92,2 cm.
2. 15,6 cm. Magnetic North and Magnetic Declination

You have learnt how to gauge accurately in Grade 8. Remember the steps that have to be followed:

Connect the two points by means of a pencilled line.

• Draw a line to show True North through the point FROM which you have to measure.
• Position the protractor by placing 0° against the line indicating north and the centre at the point from which the measurement is to be taken.
• Measure the angle clockwise from the line for north to the connecting line (pencilled line of Step 1.)

Example:

On a map, North therefore indicates the North Pole, which we refer to as TRUE NORTH (geographic north.)

The earth also has a magnetic field that extends from north to south, for which we use a compass. The earth’s magnetic field does not correspond exactly with the true North and South Poles. North as indicated by the compass will always lie WEST of TRUE NORTH (TN) and is known as MAGNETIC NORTH (MN).

The difference between true north and magnetic north is known as the MAGNETIC DECLINATION (MD). The Magnetic Declination differs from place to place and continuously increases and decreases at any place, because magnetic north is not a fixed point.

To make it possible to determine the Magnetic Declination of any place, the following information is given in the margin of a 1:50 000 topographic map:

Example:

Calculate magnetic declination as at present:

Step 1:

Calculate the difference in years = 2003 – 2000

= 3 years

Step 2:

Multiply the years by the annual difference in minutes = 3 years x 4'

= 12'

Step 3:

Determine whether the angle increases (eastwards) or decreases (westwards).

Eastwards (smaller) -

Westwards (bigger) +

Step 4:

Add the minutes (westwards) or subtract them (eastwards).

In this instance it is eastwards, therefore:

21°30' - 12'

= 21°18' West of True North

REMEMBER that there are 60' in 1°.

If you should have to subtract and find that the degrees to be subtracted are too many, you “borrow” 1° and convert it to minutes.

Example:

21°30' – 34'

20°90' - 34'

= 20°56' West of True North.

If you should have to add minutes and get an answer that is more than 60’, you have to convert it 1°.

Example:

21°30' + 34'

21°64'

= 22°04' West of True North.

Step 5:

Remember to write West of True North alongside the new declination, because Magnetic North ALWAYS lies WEST of True NORTH.

Magnetic North

To gauge Magnetic North between two points, the degrees of TRUE NORTH must be added (+) to the Magnetic Declination and the answer must be written in degrees:

Example:

TN + MD = MN

50° + 21°18' = 71°18'

Assessment

Memorandum

ACTIVITY1:

• 580 m
• 1 010 m

• 9,22 km
• 1,56 km

ACTIVITY2:

1. True North / Bearing :

Magnetic Declination:

1990 – 2003 = 13 years

13 × 2’ = 26’ westwards

18 ⁰ 41’ + 26’ (westwards)

= 18 ⁰ 67’

= 19 ⁰ 07 west of true north

Magnetic Bearing: WN + MD = MN

…… + 19 ⁰ 07 = ………………………..

1. Actual Bearing :

Magnetic Declination: 1993 – 2003 = 10 years

10 × 3’ = 30’ eastwards (-)

20 ⁰ 15’ – 30’

19 ⁰ 75 – 30

= 19 ⁰ 45’ west of true north

Magnetic Bearing: WN + MO = MN

...................... + 19 ⁰ 45 =..................................