#063
The Davis One-Arm Protractor is the most versatile
plotting tool available. Can be laid on any line of merid-
ian to draw course lines; turns any meridian into a
compass rose. This is especially handy on charts
where the compass roses are few and far between.
The base allows true course and reciprocal course to
be read at the same time. Includes east-west variation
scale, reads in degrees and compass points—32 point
to the scale. Made of durable, flexible and nearly
unbreakable clear plastic for use on uneven surfaces.
Overall length 15
1
/8” (38 cm). Base measures 4
5
/8”
(12 cm) square.
TO MEASURE COURSE ANGLE:
1) Place the one-arm protractor on the chart so that
the compass rose points true north. The grid on
the base plate should be parallel to a meridian of
longitude or a parallel of latitude.
2) Keeping the compass rose pointing north and the
grid parallel, rotate the arm and slide the protractor
along the chart until the “true index” edge of the
arm is along the course line.
3) Depending upon the direction traveled, read the
true bearing at the true index point or at the arrow
180° opposite to the true index point.
4) To read magnetic bearing, find the variation given
for your area in the center of the compass rose on
your chart. Count the number of degrees of varia-
tion to the right or left of the true index point on the
protractor. The line on the inner compass scale
that matches the degrees of variation on the outer
variation scale is your magnetic bearing.
TO DRAW A COURSE OR BEARING FROM A
POINT:
1) Place the protractor on the chart so that the com-
pass rose points true north and the center of the
base plate is over the given point. The grid on the
base plate should be parallel to a meridian of lon-
gitude or a parallel of latitude.
2) If the point is some distance from a meridian or a
parallel, check the alignment of the grid by rotating
the arm of the protractor until the “true index” edge
passes through the center of the nearest compass
rose. Read the true angle on the rose at the “true
index” edge of the arm. Holding the arm firmly in
place, rotate the base plate until the reading at the
true index point is the same as that of the com-
pass rose on the chart. The grid should now be
parallel.
3) Holding base plate firmly in place, rotate the arm
until the desired bearing is read at the true index
point or at the arrow 180° opposite to the true
index point (for true bearing) or at the variation
scale (for magnetic bearing).
4) Draw line along “true index” edge of arm. Slide
base plate as needed to extend line to the given
point.
OTHER USES OF ONE-ARM PROTRACTOR:
1) Measure distance using chart scales on arm. Be
sure to use the scale (1:80000, 1:40000 or
1:10000) indicated on your chart.
2) Handy centimeter scale on long edge of arm.
ONE-ARM PROTRACTOR
INSTRUCTIONS
Note: This product was discontinued as a separate
offering in 2005, but may still be purchased as part
of the Davis Charting Kit #083.
3465 Diablo Ave., Hayward, CA 94545 U.S.A.
Phone (510) 732-9229 • Fax (510) 732-9188
063_Inst.pdf October 2005
Highest quality printing: features sharp printing
process, high gloss finish on front and back, and over-
lamination for durability.
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