chart plotting and preparation 31
- Locate the grid box. Find the pair of latitude lines (parallels) and the pair
of meridians (longitude lines) that form a box around your boat and mark
the corners for easy reference. - Plot your latitude. Measure latitude on the right or left chart margin
scales. Transfer the measurement to each vertical side of the box by
sticking the point of the compass (or dividers) in one corner of the box and
sweeping an arc with the compass lead across the corresponding meridian.
If using dividers, plant the other divider point on the meridian and mark
that spot with a sharp #2 lead pencil. Duplicate this on the other meridian.
Th ese are your latitude marks. Use any straightedge to draw a line joining
the two marks. - Plot your longitude. Measure longitude on the top or bottom chart margin
scales. Follow one of the meridian lines bounding your box until you come
to the horizontal latitude line you just drew. Place the compass (or divider)
point on that intersection, sweep an arc through the horizontal line (or plant
the other divider point there and mark with a pencil), and you’re done. - Neaten the plot. Place a small circle around the intersection of the latitude
and longitude. Erase all lines and arcs to neaten up the chart.
How to Convert Minutes, Seconds, or Degrees to Miles
Most nautical charts are based on the projection of a round globe onto a cylinder.
Th is projection, called a Mercator projection, thus contains some distortion. For
instance, because meridians of longitude appear as straight lines on your nautical
chart (on a globe, they converge at the poles), you must take care in measuring
distance. Always use the latitude scale (i.e., the right or left chart margin) and
get into the habit of taking your distance measurement at a place along the
latitude scale, close to your position. For instance, if your position is near the
top of the chart, take distance measurements from the top of the chart.
Because each line of latitude parallels all the others, the distance between
two such lines is always consistent. Therefore each minute of latitude equals
one nautical mile, no matter where on the globe you may be.
1 ' (minute) = 1 nautical mile
1 ° (degree) (60 minutes) = 60 nautical milesTh is relationship does not hold true for lines of longitude, since these converge
at the north and south poles. Th ey look parallel on a Mercator chart, but only
because the chartmakers have distorted things for convenient navigation. A
minute of longitude equals one nautical mile only at the equator and diminishes