32 seamanship secrets
10 ′ 50 ′ 40 ′A30 ′ 20 ′20 ′30 ′40 ′50 ′71 °W7 10 ′ 0 °W41 ° 10 ′N
10 ′ 50 ′ 40 ′B30 ′ 20 ′20 ′30 ′40 ′50 ′71 °W7 10 ′ 0 °W41 ° 10 ′N10 ′ 50 ′ 40 ′C30 ′ 20 ′20 ′30 ′40 ′50 ′71 °W7 10 ′ 0 °W41 ° 10 ′N
10 ′ 50 ′ 40 ′D30 ′ 20 ′20 ′30 ′40 ′50 ′71 °W7 10 ′ 0 °W(^103041) ° 10 ′N
At 1030 you obtain the following position from your GPS: Latitude 41o17.0'N; Longitude
70 o26.0'W. Below are the steps to box-plot your 1030 position:
Locate the grid box (A). First eyeball which latitude-longitude box surrounds your vessel.
In the accompanying illustration we’ve marked the four corners of this position box where the
41 o 10 'N and 41o 20 'N latitude lines intersect the 70o 20 'W and 70o 30 'W longitude lines. Th is
way, if you need to put aside the chart temporarily to attend to your boat, a quick glance
reorients you when you return.
Plot your latitude (B). Go to the right or left chart margin scale, whichever is closest to
your position box. Stick the point of your compass in the latitude mark that forms the
bottom of your box, in this case 41o 10 'N. Now open the other leg of the compass until
the pencil point is on your latitude position of 41o 17 'N. Maintaining that span, slide the
compass to your position box. Stick the point into each of the two bottom corners of the
box in turn and strike arcs through the corresponding meridians. Grab a straightedge and
draw a horizontal line between the two arcs.
Plot your longitude (C). Go to the top or bottom chart margin scale, whichever is closest. Stick
the point of your compass in the longitude that forms the right or left side of your position box (it
doesn’t matter which you choose as long as you’re consistent). In our case, we’ll stick our needle
point in 70o 20 'W. Th en we’ll open the other leg until the pencil point falls on our longitude reading
of 70o 26 'W. Slide the compass along the meridian until you reach the line of latitude you drew
between the arcs in the position box. Strike an arc over the line to mark your 1030 GPS position.
Neaten the plot (D). Circle your position, label it with the time of the position (1030)
and erase all extraneous lines from your chart. Always scrub your chart, which is the
nautical term for erasing, to make things as easy to read as possible. Extra lines and marks
frequently cause confusion and lead to navigational errors.