DAVIS Charting Kit - Manual (Page 2)

#063 ONE-ARM PROTRACTOR
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 variation 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 compass 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 longitude 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 compass
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.