DAVIS One-Arm Protractor User Manual

#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 151/8” (38 cm). Base measures 45/8”

(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 [phone] • Fax [phone]

[e-mail] www.davisnet.com

063_Inst.pdf October 2005

Highest quality printing: features sharp printing
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