Transcript Terrestrial Coordinate System/Chart Projections and Numbering

LESSON 02:
Terrestrial Coordinate System/
Chart Projections and
Numbering
• Learning Objectives:
– Comprehend the terrestrial coordinate
system
– Comprehend the location of positions on
the earth using latitude and longitude
– Comprehend the basic properties of the
most commonly used chart projections
Terrestrial Coordinate
System
• The earth is an oblate spheroid, but
for navigational purposes it is
considered a perfect sphere with a
circumference of 21,600 NM.
• On a perfect sphere at rest, all points
on the surface are similar; reference
points must be designated in order
to make any type of measurements.
Terrestrial Coordinate
System
• When rotation is introduced, the spin
axis introduces two reference points,
the north and south poles.
• The spin axis of the earth, together
with its poles, constitutes the basic
reference points on which the
terrestrial coordinate system is
based.
Terrestrial Coordinate
System
• Great Circle: the intersection of a
plane passing through two points on
the surface of the earth and the
center of the earth.
• Some key points:
– A great circle is the largest circle that
can be drawn on the face of the earth.
– A great circle represents the shortest
distance between two points on the
surface of the earth.
Terrestrial Coordinate
System
• Equator
• Meridian
Terrestrial Coordinate
System
• Small Circle- any circle not passing
through the center of the earth.
Terrestrial Coordinate
System
• Meridians of longitude are great
circles
• Parallels of latitude are small circles
(with the exception of the equator)
Latitude and Longitude
• The equator divides the earth into the
northern and southern hemispheres
and is the reference for parallels of
latitude.
• The prime meridian passes through
the original position of the Royal
Greenwich Observatory. It serves as
the reference for meridians of
longitude.
Latitude and Longitude
• Meridians are divided in half:
– upper branch
– lower branch
• Prime Meridian
– upper branch is known as the
Greenwich meridian
– lower branch is the 180th meridian
Longitude Defined
• The angular distance between
the Greenwich meridian and the
meridian passing through a
particular point on the earth’s
surface. Longitude is measured
in degrees of arc from 0 to 180
degrees, either east or west.
Latitude Defined
• The angular distance between
the Equator and the parallel
passing through a particular
point on the earth’s surface.
Latitude is measured in degrees
of arc from 0 to 90 degrees,
either north or south.
Longitude
Latitude
Measurement of Distance
• Since latitude lines are parallel, the
length of one degree of latitude is the
same everywhere on earth (60 NM).
• As the distance from the equator
increases, the length in miles of one
degree of longitude decreases, so
NEVER use the longitude scale to
determine distances on a chart.
Measurement of Distance
Chart Projections
• Desirable qualities of a chart projection:
– Correct angular relationships
– Representation of areas in their correct
proportions relative to one another
– True scale
– Rhumb lines represented as straight lines.
Note: Rhumb lines are lines on the surface
of the earth that cross all meridians at the
same angle. Ships on a constant course
follow rhumb lines.
– Great circles represented as straight lines
Mercator Projection
• Imagine a cylinder rolled around the
earth, tangent at the equator, and
parallel to the earth’s axis.
• Meridians appear as straight vertical
lines when projected outward onto
the cylinder.
Mercator Projection
Mercator Projection
Advantages
• Position, Distance,
and direction
easily determined.
• True shape of
features is
maintained for
small areas
(conformality)
• Rhumb lines plot
as straight lines.
Disadvantages
• Distortion of true
size of surface
features increases
with distance from
the equator.
• Great circles
appear as curved
lines.
Gnomonic Projection
• Surface features and reference lines
on the earth’s surface are projected
outward from the center of the earth
onto a tangent plane.
• Three basic types, depending on
point of tangency:
– equatorial gnomonic (tangent at
equator)
– polar gnomonic (tangent at either pole)
– oblique gnomonic (tangent somewhere
else)
Gnomonic Projection
Gnomonic Projection
Advantages
• Great circles
appear as straight
lines (shortest
distance between
two points)
• Tolerable
distortion within
1000 miles of the
point of tangency
Disadvantages
• Rhumb lines
appear as curved
lines
• Distance and
direction cannot
be measured
directly
• Not conformal
(true shapes are
not presented)
Gnomonic Projection
Gnomonic vs. Mercator
Gnomonic
• Since great circles
appear as straight
lines, used to
determine the
shortest route
between two
points. This
information is then
transferred to a
Mercator chart.
Mercator
• Used for everyday
navigation, due to
the ease of
measurement of
position, distance,
and direction.
Gnomonic vs. Mercator
Nautical Charts
• Two government activities are
responsible for producing charts:
• Defense Mapping Agency (DMA)
– ocean areas of the world outside U.S.
territorial waters
• National Ocean Service (NOS)
– inland and coastal waters of the U.S.
and its possessions
Chart Numbering System
• All charts are assigned a 1 to 5 digit
number, depending on the scale of the
chart
• No. of digits Scale
–1
no scale (supporting
publications)
–2
1:9,000,001 and smaller
–3
1:2,000,001 to 1:9,000,001
–4
misc and special non-nav
charts
–5
1:2,000,000 and larger
Chart Numbering System
• 1 Digit: supporting publications
• 2 or 3 Digits: depict large ocean
basins and subdivisions. First digit
is the ocean basin (there are nine
worldwide).
Ocean Basins of the World
Chart Numbering System
• 5 Digits: are of suitable scale to
depict coastal regions with the great
detail necessary for piloting.
– There are nine coastal regions in the
world
– Each is divided into several subregions
• The first number is the region
• The second number is the subregion
• The last three numbers represent the
geographic sequence of the chart
Coastal Regions of the
World
Chart Numbering System
• Thus, the chart numbering system
– indicates the scale of the chart (by the
number of digits in the chart number)
– indicates the area of the world depicted
(ocean basin, subdivision, coastal
region, and subregion)
– enables the navigator to organize the
charts into portfolios
Chart Scale
• An important point to remember:
SMALL SCALE = LARGE AREA
LARGE SCALE = SMALL AREA
Chart and Publication
Correction System
• Navigation is constantly changing!
• It is not practical to constantly
reprint navigational charts and
publications.
• DMA and NOS disseminate
corrections using two publications:
– Notice to Mariners (DMA, worldwide
coverage)
– Local Notice to Mariners (USCG,
changes pertaining to U.S. inland
waters)
Chart and Publication
Correction System
• Corrections must be made by hand
to the affected chart or publication.
• Fortunately, not all changes are
made. The corrections are kept on
file, using a correction card for each
chart.
• Changes are then entered on a chart
when a ship is scheduled to operate
in the area the chart covers.
Chart and Publication
Correction System
• Semiannually, DMA publishes a five
volume summary of all corrections
for the previous six months.
Other Correction Resources
• Broadcast Notice to Mariners
• Worldwide Navigation Warning
System (NAVAREAS)
• HYDROLANTS and HYDROPACS
• DMA Daily Memoranda
• Special Warnings (USN or USCG)
• Automated Notice to Mariners
System (ANMS)