chart plotting and preparation 47
Save Time with Custom Distance Scales
Measuring distances on a 1:80,000 scale coastal chart is a snap, because the
latitude scale is broken down into degrees, minutes, and tenths of minutes.
Larger-scale charts (such as harbor charts) make distance plotting more
challenging, however, because their latitude scales are oft en shown in degrees,
minutes, and seconds. I find it convenient to draw my own scale line divided
into miles and tenths on such charts, making distance measurements faster
and more accurate.
- Selecting a spot for the scale. Locate an area on the chart that is not
needed for navigation. Th is keeps the scale from interfering with tracks
and annotations, or covering important navigation information. - Tape the scale. Pull off a few inches of dull-sided tape and lay it down
over the area where your scale will reside. If you use removable
tape, you can peel it off later if necessary. (You can remove more
permanent tape by rubbing a heated piece of metal over the tape,
peeling as you rub.) - Make the distance scale. Draw a line onto the tape equivalent to about 2 to
3 miles in distance. Mark one end of the line with a small tick mark and label
it 0. Next, fi nd that part of the latitude scale on the right or left margin that
breaks down a minute into seconds. Using your dividers or compass, pick off
6 seconds of latitude. Starting at the 0 point, strike arcs in 6-second (tenth-
of-a-mile) increments to the other end of the line. - Label the distance scale. Starting at the fi rst mark above the 0 baseline,
label each tenth of a mile 0.1, 0.2, and so forth, continuing to the 1-mile
mark. Th en continue above the 1.0-mile mark with 1.1, 1.2, and so forth, up
to the 2-mile mark. Make the whole- and half-mile tick marks longer than
those that indicate tenths of a mile. Tape over the scale to protect it from
smudging.
SEA-CRET TIP
X Stay away from using red markers when annotating charts.
Flashlights and cabin or bridge lights with red lenses, which are
favored because they do not degrade your night vision, make red
markings invisible. This is why magenta is used on charts.