Magnetic to True North

CHANGING A MAGNETIC COMPASS HEADING

TO A HEADING RELATIVE TO TRUE NORTH

Sometimes it is necessary to convert from magnetic to ture North heading.   Magnetic readings are a problem for 2 reasons:   first, all FCC filings and authorized patterns are relative to true north (the geographic north pole); second, magnetic compass sightings are lucky to be within a one degree accuracy, unless they are taken with a surveyor's compass, similar to a transit.  

It is best to have the tower surveyed prior to Jampro doing the far field, full scale range work on an FM or TV antenna.   That way, there will be an accurate determination of the tower heading and have it reckoned relative to true north (usually).

Lacking that, there is an internet tool that can be used to find the magnetic declination at any site in the US.   (This is often times called the magnetic variation.)  

Step 1.   Go to the web site: http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp.

If you have the lat/long in degrees-minutes- seconds format (as appears on FCC documents), type that into the dialog boxes with space delimiters, such as "38 24 36".   Indicate North latitude and West longitude for the US   -- these are the defaults.   

Don't worry about the date.   The declination is slowly changing and the default is set to report the value on the current date.   The declination will be reported as westerly for regions in the eastern US and easterly in the western US.   There will be very small values in some portions of the midwest, as the magnetic north pole is nearly straight north of Chicago - many NFL players believe its somewhere near Green Bay, Wisconsin.

Step 2. Click on Compute Declination.   The result will appear on the refreshed screen just below this area.

Step 3. Use   the reported declination to correct the magnetic heading to a heading relative to true north:

If the declination is Westerly, Subtract it from the magnetic heading.

If the declination is Easterly, Add it to the magnetic heading.

Step 4. Pass this information to Jampro with your order.