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It’s hard to cover all the details in a forum like this but
I’ll give you a quick primer.
I can send
you a powerpoint presentation that explains it in a little more detail.
To really learn how to read latitude and
longitude you should pick up a copy of “Chapman’s Piloting and Seamanship.”
The earth is divided into parallels of latitude and
meridians of longitude, also known as lines of position.
Latitude is measured north and south of the equator, with
the equator represented as 0 degrees, and the poles being represented as 90
degrees North or South.
Latitude lines
are paralel to the equator. For example, if
I was at the equator I would be at 0 degrees.
If I traveled exactly 60 nautical miles to
the north, I would be at 1 degree North, and if I traveled another 60 miles I
would be at 2 degrees north.
Your GPS
display will preface the Latitude measurement with an “N” for positions North
of the equator and an “S” for positions south of the equator.
Longitude measures your position east or west from the Prime
Meridian, which is a line represented as 0 degrees that bisects the earth from
north to south and passes through Greenwich England.
Halfway around the earth at the International
Dateline Longitude is 180 degrees.
Measuring
Longitude is a little more complicated because the lines are not parallel and
requires an accurate clock (your GPS) to compare time at your location relative
to the time in Greenwich England.
Your GPS display will preface the Longitude
measurement with a “W” for positions west of
Greenwich and
an “E” for positions east of Greenwich.
To make more accurate measurements each degree is divided
into 60 minutes.
Because the lines are
parallel, 1 minute of latitude is equal to 1 nautical mile.
Each minute can be further divided into 60
seconds.
Each second is roughly
equivalent to a distance of 100’
Instead
of seconds, the default setting on your Garmin breaks the minute down into
tenths, hundredths, and thousandths for meven more precise measurements.
Because they are not parallel, lines of Longitude
are measured the same way, but the distances vary depending on how far north or
south of the equator.
To find out where you are with a GPS, you need a map that
shows lines of latitude and longitude on it.
The lines will be labeled on the map or along the borders of the
map.
Most nautical charts show the
latitude measurements along the right
the left border of the map.
Longitude measuremnts will be shown along the
top and bottom edges.
×