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[DPRG] Magnetic declination

Subject: [DPRG] Magnetic declination
From: David Anderson davida at smu.edu
Date: Tue Aug 19 13:39:19 CDT 2014


For those working on outdoor robots navigating to GPS coordinates using 
a magnetic compass or IMU-based magnetometers, the difference between 
magnetic north and true north needs to be incorporated into the heading 

In the before-time, the local magnetic variation was included in the GPS 
NMEA $GPRMC sentence in fields 10 and 11, but that no longer appears to 
be the case.  My GPS currently returns zero for both of those fields, 
and Steve Edwards confirmed this with his GPS at the last RBNO.


I think the GPS satellite guys just got tired of having to upload new 
magnetic maps all the time.  Too bad

So it looks like for jBot the magnetic variation will need to be entered 
by hand for any particular location.  That change has now been  made to 
jBot's navigation code.

There's probably an app for your GPS-equipped smartphone that can return 
the local variation, but I haven't found a good one.  In the meantime, 
the NOAA website at:


provides a calculator that allows you to enter a lat/lon (and date!) and 
returns the correct magnetic variation.  For the SMU campus in Dallas it 
currently returns 3.58 degrees E (i.e., add 3.58 to your compass 
heading) and says it changes by 7.5 minutes per year.

Anybody got a slicker way of automating this process?


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