chart plotting and preparation 27
Parallel Rulers. Used for measuring direction. Th is instrument has two thin,
fl at sections of wood or plastic attached by hinges. You “walk” them across a chart.
Place pressure on one leg and hold it steady. Move the other leg. Th en, place pressure
on the leg you just moved, and move the trailing leg up. Continue the process of
alternating pressure from one leg to the other to move from one place to another.
Protractor. Used for measuring direction. Th is instrument has one fl at section,
scribed with a half circle that shows increments of degrees. To use the protractor,
place it along any vertical line, read the angle, and slide it to the place needed.
Some protractors, like the “Weems Plotter,” have wheels on one long edge to assist
in moving the instrument long distances.
Dividers. Used for plotting positions and measuring distance. Th is hand-
held instrument has two legs attached to a hinge. Sharp metal needles are
inserted into the ends of the legs opposite the hinge.
Plotting Compass. Used for plotting positions, measuring distance, and
marking the chart. Similar to dividers, but only one leg has a metal needle point.
The other leg contains a thin sliver of pencil lead with a pointed end.
Quick-and-Easy Review of Latitude and Longitude
Chartmakers created a system of vertical and horizontal intersecting lines to en-
able navigators to quickly and accurately plot a vessel’s position. Th ey named the
horizontal (i.e., east-west) lines latitude and the vertical (i.e., north-south) lines
longitude.
LATITUDE
Picture the earth as a perfect sphere balanced on a vertical axis. Th e North Pole lies
where the axis penetrates the top of the sphere, and the South Pole is located where
it penetrates the bottom. Now divide the sphere horizontally into two equal halves.
Th e resultant dividing line—a belt around the waist of the world—is the equator.
Th e equator is the starting point for latitude measurements, which is to say that
the latitude of the equator is 0 degrees. If we move north of the equator, the fi rst degree
of latitude is named 1 degree north, the second degree is 2 degrees north, and so on.
Latitude reaches a maximum of 90 degrees north at the North Pole. If we start from
the equator and move south, we’ll pass 1 degree south, then 2 degrees south, and so
forth to a maximum of 90 degrees^ south at the South Pole. All lines of latitude are
parallel with one another, which is why a line of latitude is also called a parallel.
Measure Latitude on the Chart’s Right or Left Margin Scale
On a nautical chart we measure latitude using the scales on the right- or left -side
margins of the chart. Th ese scales show latitude in degrees, minutes, and tenths