# [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.

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

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
sidewalk.

the heading of the road, and my compass is now calibrated to True
North.

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,
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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
> >
> > _______________________________________________
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> >
>
>
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