[RFI] Compass Vectors

David Cole dave at nk7z.net
Tue Dec 1 09:01:25 EST 2015

If you are doing this by hand, as I do, then remember, to keep the map
and your bearings, in either True, or Magnetic North, I always keep my
maps in True North, and adjust my compass readings to True North, prior
to plotting them.  

If you take a Google map, it will be True North, while your
uncompensated compass, will be in Magnetic North.  I use Google Earth
images to draw my plots on.

You need to convert both your map, and your readings to the same units,
either True or Magnetic North, before you draw a bearing line on your
True North oriented Google map.  Here is how I get them to match the the
easy no math way.

I go here: http://www.acscdg.com/ and get a map of a long, straight,
road that runs dead N, S, E, or W in my area, the actual bearing does
not matter, it only matters that you know the bearing of the road
exactly, and can read your compass to that same accuracy, hence why I
use N, S, E, or W, they are easy to find on a compass, and almost every
city has N/S/E/W roads.  

I pick a road that is long, and has a sidewalk, that way I can stand on
the sidewalk, and sight down the sidewalk, not looking off to the side
and sighting down the road from the sidewalk, thus introducing parallax
issues.  I then take my compass to that road, and sight down the

I purchased an adjustable compass so I just adjust the compass to read
the heading of the road, and my compass is now calibrated to True

See: http://www.ngdc.noaa.gov/geomag/declination.shtml for why you need
to do this...

I then go home and take a bearing on something far away, from a known
spot on my property, and write that bearing down.  Then the next time I
go out and start looking for RFI, I just sight on the object again, from
the same spot, and adjust the compass if it has moved...  I'm now all
set to do True North bearing drawings...

Thanks and 73's,
For equipment, and software setups and reviews see:

For MixW support see;
For Dopplergram information see:
For MM-SSTV see:

On Tue, 2015-12-01 at 02:18 -0500, Roger (K8RI) wrote:
> Select your area with as much resolution as possible.
> Drive to a location, take a bearing.  using the Google distance ruler, 
> draw a line through your point.  Drive to another point and repeat.  Two 
> bearings are often sufficient, but three leave a small diamond for the 
> region of error.  Much easier to do than explain, although there is a 
> learning curve.
> It gets messy with RFI hunting when there are multiple sources, with 
> overlapping and similar patterns as in multiple networks or an area 
> where there are a mix og cable and outside antenna and the users just 
> wired the preamps and cable together.  This sometimes can lead to a much 
> larger region of error.
> 73 es good luck
> Roger  (K8RI)
> On 11/30/2015 8:16 PM, Karin Johnson wrote:
> > I'm planning on doing some RFI hunting and wanted to know if anyone
> > knows how to plot compass vectors on Google Maps?
> > I need to plot intersecting vectors from a direction finding loop to
> > find the source of an RFI generator.
> >
> > Karin   K3UU
> > Palm Harbor, FL
> >
> > _______________________________________________
> > RFI mailing list
> > RFI at contesting.com
> > http://lists.contesting.com/mailman/listinfo/rfi
> >
> ---
> This email has been checked for viruses by Avast antivirus software.
> https://www.avast.com/antivirus
> _______________________________________________
> RFI mailing list
> RFI at contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi

More information about the RFI mailing list