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STEM Challenge ONE

Magnetic Declination

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Subject Areas

• Declination
• Earth Science
• Geolocation
• Geomagnetism
• Navigation

References

• About Declination
• About the AC5JW Geolocation Table and Derivative Tables
• How To Use a Compass with a USGS Topographic Map
• Journey Along a Field Line (PDF)
• Pentazed - Expanded True Compass Rose
• The Earth
• True Compass Rose
• 1 |  2.   USGS FAQs.  What is Declination?

Activity

Oh, no!  Your GPS goes out and you must resort to a portable magnetic compass to show you the way.  

You just put in a new directional antenna as your primary aerial, and now you want to set its aim properly and complete the installation.   You are keen to take this aerial out for a spin and see what it can do on a radio net today.   You decide to take your compass to the aerial and see firsthand that it aims properly.   As you know where your aerial is positioned, and you also know where the net control station aerial is sited, you determined earlier that the great circle path to the net control station aerial is 90° due east from your own.  

To complete this task, you must first compute a true direction, or "beam heading," that you can read on a magnetic compass.   Then you must transfer the true direction from your compass to the aerial itself.   Next, you must lock in this direction as 90° on your mast rotator controls so that you will not have to go back up there until next year.   With only an hour to go before the net control station opens the radio net, can you do it?  

Find Out

1. What is the aerial location?   Given.   Use AC5JW at 36.08335, -79.783138


2. What must the compass read so that you get the proper direction to aim the aerial at 90° true?   Unknown.   Compute Magnetic True East (MTE) for this direction.  

Solution

1. Visit Magnetic Field Calculators at the NOAA National Geophysical Data Center (NGDC) website.  


2. Enter location coordinates and collect Estimated Value of Magnetic Declination from the International Geomagnetic Reference Field (IGRF) model at the proper date.  


• NOTE.  Defaults are today's date and World Magnetic Model (WMM).   Make sure you use the proper date and model.  
• NOTE.   Magnetic Field Calculators do not accept minus signs input with coordinates.  
• NOTE.   Include proper hemisphere for northing and easting by translating any minus signs (-) to South latitude or West longitude.  


3. Apply declination and compute Magnetic True East (MTE) for this point.  


• NOTE.   For Magnetic True North (MTN), start from 0.000° or 360.000°. as appropriate.  
• NOTE.   For Magnetic True East (MTE), start from 90.000°.  
• NOTE.   For Magnetic True South (MTS), start from 180.000°.  
• NOTE.   For Magnetic True West (MTW), start from 270.000°.  
• NOTE.   Declination can be applied by starting from any necessary direction on the True Compass Rose.   The most common direction to compute is MTN.  


• NOTE.   Refer to About Declination for more specific guidance and information about:
• applying declination,  
• calibration of aerials and other directional equipment,  
• computing magnetic true values,  
• International Geomagnetic Reference Field (IGRF).  

  

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