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# Working with Magnetic Declination by hcj

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Name:____________________ Date:______________ Bearings The compass is used primarily to take bearings. A bearing is a horizontal angle measured clockwise from north (either magnetic north or true north) to some point (either a point on a map or a point in the real world). Bearings are used to accurately travel to a destination or to locate your position. If you are working from your map, it is called a map bearing or true bearing and the angle you are measuring is the angle measured clockwise from true north on your map to this other point on the map. If you are taking a bearing off a real point on the landscape with a compass, you are using your compass to measure the angle clockwise from magnetic north to this point on the landscape. This is called a magnetic bearing or compass bearing. How to take a Map or True Bearing using a compass The following drawing will serve as a reminder of how to take a map bearing using a compass.

In the space provided, write instructions that will help you take a true bearing on a map using a compass 1. ________________________________________________________________ ___________________________________________________________________ 2. ________________________________________________________________ ___________________________________________________________________ 3. ________________________________________________________________ ___________________________________________________________________

Magnetic Declination – Which North to Use? So we have two types of north to contend with. When you look at your map, it is drawn in relation to true north; when you look at your compass, it points to magnetic north. We have seen that the difference between magnetic north and geographic north at a certain location is known as magnetic declination. Knowing the magnetic declination for a certain location is extremely important. Why? _________________________________ As you can see the following chart, failure to take declination into account can put you way off target. Declination or Degrees Off Course 1° 5° 10° Error Off Target after Walking 16 km (10 Miles)

280 meters (920 feet) 1402 meters (4,600 feet) 2795 meters (9,170 feet)

It is important to remember that we give declinations that are west a negative value and declinations that are east a positive value. There are two main ways to find out the magnetic declination for a certain location. 1. Determining the Magnetic Declination using a Map of Magnetic Declination Here is a Map of Magnetic Declinations for the U.S.

Using the given map, determine 1) The magnetic declination in New York, NY is ___________ or _____________ 2) The magnetic declination in Seattle, Washington is __________ or ____________ 3) The magnetic declination in Madison, Wisconsin is _________ or ___________. 2. Using an Online Magnetic Declination Calculator Here is an example of an online calculator found at http://gsc.nrcan.gc.ca/geomag/field/mdcalc_e.php http://www.ngdc.noaa.gov/seg/geomag/jsp/struts/declZip Magnetic declination calculator You can use the following form to calculate magnetic declination. Enter (longitude,latitude) or choose a city: Latitude: Longitude: Year: degrees, degrees,
2005 •Calgary

minutes ( minutes (

North West

South) East)

(1960-2005)
Calculate magnetic declination

What information is required to use the following magnetic declination calculator? ______________________________________________________________________________ And where can you find that information? ______________________________________________________________________________ In pairs, you will explore Lesson #4 of the program called Topographic Maps. This lesson gives you an introduction and overview of latitude and longitude. What are lines of latitude?________________________________________________________ _____________________________________________________________________________ What are lines of longitude? ______________________________________________________ ___________________________________________ Using this online magnetic declination calculator and a globe, find the magnetic declination for – 1) Baghdad, Iraq ______________ or ________________ 2) London, England ______________ or ________________

3) Beijing, China 4) Barrow, Alaska 5) Santiago, Chile

______________ or ________________ ______________ or ________________ ______________ or ________________

Working with Magnetic Declination So, how do we work with this thing called Magnetic Declination Remember, if the declination is West, it is __________________ and if the declination is East, it is _________________ The following will help you go from a True/Map Bearing to a Magnetic/Compass Bearing or vice versa. If the magnetic declination is west to go from a True/Map Bearing Magnetic/Compass Bearing (Map Bearing + Declination = Magnetic Bearing)

 

you

or to go from a Magnetic/Compass Bearing True/Map Bearing (Map Bearing - Declination = Magnetic Bearing)

you

and if the magnetic declination is east to go from a True/Map Bearing Magnetic/Compass Bearing (Map Bearing - Declination = Magnetic Bearing)

 

you

or to go from a Magnetic/Compass Bearing True/Map Bearing (Map Bearing + Declination = Magnetic Bearing)

you

Examples – (answers will be posted on board) 1. If you were in London, England, what compass bearing would you take if you wanted to travel a true bearing of 67? 2. If you were in Sydney, Australia, what true bearing would you be taking if you took a magnetic bearing of 210? 3. If you were in Tokyo, Japan, what compass bearing would you take if you wanted to travel a true bearing of 24? 4. If you were in Santiago, Chile, what true bearing would you be taking if you took a compass bearing of 75? Reference - http://www.princeton.edu/~oa/manual/mapcompass2.shtml

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