# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Coastal Plotting Sheets
From: Bill B
Date: 2007 Mar 28, 16:26 -0400

```>> Bill B
>> I'm lost here.  If I draw a line segment between the two points, as I
>> perceive it the rest is about impossible.

> Bill Noyce
> Perhaps the confusion is the phrase "draw a line ... TO the base line."
> What Michael means is compute 'x'=90-(observed angle), then draw a
> line from object 1, extending out to sea, and forming angle 'x' with
> the baseline.  Draw a similar line from object 2.  You now have an
> isosceles triangle, with the two base angles = 'x', and the "point" is
> the center of your circle.  Use either of the two equal sides of that
> triangle as the radius when drawing the circle.

Yes indeed, I got tangled up in the words "...to the baseline."  It was
clear that the two points (objects) must form a chord of the circle, so the
rest makes sense now.

Thanks to all that helped set me straight, and to Michael for pointing out
the possible inaccuracy of the three object, two angle approach using a
three-arm protractor.

There is a general lack of objects to practice with on the southeastern
shore of Lake Michigan, so I have never invested in a 3-arm protractor.
Alex has a magnificent Soviet unit that is worth owning just to set on a
coffee table, but after being educated think I would leave it on the coffee
table and use a graphical method.

I have had excellent results (surprisingly good) using a combination of
hockey-puck compass and sextant 5-18 nautical miles off Chicago using the
skyscrapers as objects.  Variations on a theme: One compass bearing and
angular separation form sextant, two compass bearings, distance away from a
structure with the base below the horizon using the Bowditch formula with
modified constants, compass bearing to center object and angular separation
to objects on either side etc.  I also compared by using bearing and
distance from my GPS for fun.  At 17 miles from Chicago the
compass/distance-off plot was much closer to GPS lat lon than the plot using
GPS distance and bearing (my GPS reads out bearings in whole degrees).

I have also had one horrible result doing distance off from one object pre
GPS. Sailing down the eastern shore of Lake Michigan about 5 miles off (so
shoreline below the horizon) I picked a tower at the angle I wanted.  I kept
track of speed  and course adjusted for leeway until the tower was abaft of
abeam and took my second compass bearing; then proceeded to plot.  According
to the plot we were 2 miles offshore.  Not even close to a DR or EP
distance, nor eyeball distance.  A friend that knows the coast asked me what
object I had been using.  Then he broke out in laughter.  It was tower
delivering electricity from a nuclear power plant, and about 3 miles inland.
My bearings and plot were just fine, but I picked the wrong object.  Lesson
learned.

Bill B.

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