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Latitude and Longitude
You can read the history of Latitude and Longitude
Here
The two things you need to know here to start is longitude
and latitude.
Latitude We can imagine the Earth as a sphere,
with an axis around which it spins. The ends of the axis are
the North and South Poles.
The Equator is a line around the earth,
an equal distance from both poles. The Equator is also the latitude
line given the value of 0 degrees. This means it is the starting point
for measuring latitude. Latitude values indicate the angular distance
between the Equator and points north or south of it on the surface of the Earth.
A line connecting all the points with the same latitude value is called a
line of latitude. This term is usually used to refer to the lines that
represent values in whole degrees. All lines of latitude are parallel to
the Equator, and they are sometimes also referred to as parallels.
Parallels are equally spaced. There are 90 degrees of latitude going
north from the Equator, and the North Pole is at 90 degrees N. There
are 90 degrees to the south of the Equator, and the South Pole is at
90 degrees S. When the directional designators are omitted, northern
latitudes are given positive values and southern latitudes are given
negative values.
Longitude
Lines of longitude, called meridians, run perpendicular to lines of
latitude, and all pass through both poles. Each longitude line is
part of a great circle. There is no obvious 0-degree point for
longitude, as there is for latitude. Throughout history many different
starting points have been used to measure longitude. By international
agreement, the meridian line through Greenwich, England, is currently
given the value of 0 degrees of longitude; this meridian is referred
to as the Prime Meridian. Longitude values are indicate the angular distance
between the Prime Meridian and points east or west of it on the surface of
the Earth
The Earth is divided equally into 360 degrees of longitude.
There are 180 degrees of longitude to the east of the Prime Meridian;
when the directional designator is omitted these longitudes are given
positive values. There are also 180 degrees of longitude to the west
of the Prime Meridian; when the directional designator is omitted these
longitudes are given negative values. The 180-degree longitude line is
opposite the Prime Meridian on the globe, and is the same going either
east or west.
In the US your location is always stated as so many degree's North and so many
degree's West. This is due to the United States location in reference to the Prime
Meridian.
Now that you understand Lat,Longs you will need to be able to chart. So, move
on to a page on charting.
All positions on a chart are
Always expressed as three numbers (030)
and on this side of the globe positions
are North and West. So, your position would be
xxx degrees North and xxx degrees West.
Lines of Longitude - or meridians are
numbered from the prime meridian which is
in Greenwich, England.
Prime Meridian ( or Greenwich, England)
The Prime Meridian is considered O Degrees.
There is 180 degrees East of Greenwich and 180 degrees
West of Greenwich.
Lines of Latitude -
Latitude is measured from the equator which is 0 degree's .
Latitude lines go 90 degree's North of the Equator and 90 degree's south of the
Equator.
For geometrical reasons, we use the minute
of latitude on charts to correspond to a nautical
mile rather than the minute of longitude.
Minutes of longitude shrink as they move away
from the equator and towards the poles; minutes of
latitude do not shrink. Take
Knots or Nautical Miles Per Hour - Statue miles per hour
is not the same. 7 knots is roughly 8 miles per hour.
The nautical mile was based on the circumference of the earth at the equator.
Since the earth is 360 degrees of
longitude around, and degrees are
broken into 60 so-called "minutes",
that means there are 360 * 60 = 21,600
"minutes" of longitude around the earth.
1 minute of longitude at the equator is equal to 1 nautical mile.
The device that sailors used to make their speed
measurement was called the "chip log". Chip as in
chip of wood, and log as in to record in a log.
The chip was a wedge of wood about 18" in size;
it was tied to one end of a rope on a large spool.
The rope had knots tied into it about every 47'3".
The number of knots in the rope that were
counted in 30 seconds, then, was equal to the speed of
the ship in nautical miles per hour.
because we need to establish international standards
and conversion factors, and because the earth is not
uniformly flat or round anywhere, and because even the
precise definition of the second has changed, the
official value for how many international feet in an
international nautical mile has changed. Likewise,
the measuring time of about "30 seconds" in the
sand glass is now calibrated at 28 seconds. Time
itself didn't change by 6.7%! But the cumulative effects
of new international standard definitions for time, feet,
statute miles, and nautical miles and more accurate
measurements of the actual size of the earth -- and the
fact that we don't use sand to measure time anymore --
have changed the modern calibration of the chip log.
Statue Mile - is used on inland waters such as
the Mississippi, Great Lakes, inner coastal waterway.
Statue Mile is shorter than a nautical mile
- 1 nm = 1,852 meters or 6,756 feet or 1.15 miles.
Fathom - depth of water is measured in feet,
in the open ocean it is measured in fathoms.
- One fathom = roughly six feet.
Chart depth is based on Mean Lower Low water, this is an average
taken over a period of 19 years.
Show Latitude lines on chart
Each degree of latitude is 60 minutes
(1 degree of Latitude = 60 minutes
1 minute of latitude = 1 nm
There are 360 degree's on a compass,
there are 32 points to a compass if you divide 360 by 32 you get 11.25
degree's to a point.
Dead Reckoning - OK, how about this one:
If you've driven for a half-hour and traveled 30 miles,
how fast are you going? Hardly rocket science, is it?
If you're nodding yes, and murmuring, "duh!" under your
breath, congratulations. You now understand the fundamentals
of dead reckoning.
Time, speed and distance. If you know two of them,
you can figure out the third. If you know all three
and what direction you're going in, you'll know where
you are. That's about all there is to it.
The value of dead reckoning is that is allows you to maintain
an good estimate of your position between fixes -- whether
you're sailing on the local bay or crossing the ocean. As such,
it is the backbone of all navigation.
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