Advanced Navigation

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Advanced Navigation
Charts and Datums
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Objectives
1. Be able to define a datum. 2. Be able to
name the current datum used in producing
charts for US forces. 3. State causes for major
datum discrepancies. 4. Understand the
consequences of improper datums.
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Objectives
5. Understand the difference between accuracy
and precision. 6. Understand the
difference between relative and absolute
accuracy. 7. List at least four examples of
where datum discrepancies have affected US
forces. 8. Know where to look for datum info
and conversions on a chart.
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Objectives
9. Know the purpose for the NIMA product MADTRAN
and why it is useful. 10. Understand the
precautions when using charts with different
datums and when passing position info from them.
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THIS IS HOW A CHART IS CREATED
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THIS WORKS FINE FOR THIS EARTHS SHAPE
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BUT THIS IS WHAT THE EARTH IS LIKE
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THIS IS A CUT OF THE EARTH
AMERICA
EUROPE
ASIA
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SOMEONE TRIED TO FIND A MATHEMATICAL
EXPRESSION FOR THIS
SO HE SAID ITS AN ELLIPSOID
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BUT THE ELLIPSOID THAT FITS IN AMERICA DOESNT
FIT IN EUROPE
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SO SOMEONE INVENTED THE GEOID AND DESIGNED A
MATHEMATICAL EXPRESSION FOR IT
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BUT WHAT IS A GEOID???
???
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SUPPOSE THAT THE EARTH IS COMPLETELY COVERED BY
WATER
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LIKE IN THE MOVIE WATERWORLD?
!!!
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YES, BUT BECAUSE THE EARTH IS NOT HOMOGENEOUS,
THE WATER WILL FOLLOW THE GRAVITATIONAL
FIELD,CAUSING AN IRREGULARDISTRIBUTION
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SOMETHING LIKE THIS
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DIFFERENT GRAVITY DISTRIBUTION
SAME GRAVITY DISTRIBUTION
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SATELLITES WILL FOLLOW THAT IRREGULAR FIELD
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SURFACE
GEOID-ELLIPSOID RELATIONSHIP
GEOID
ELLIPSOID
THE GEOID IS THE MOST ACCURATE MODEL, BUT IN A
LOT OF CASES OLD CHARTS AND MANY COUNTRIES STILL
USE DIFFERENT ELLIPSOIDS
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NORTH AMERICA
EUROPE
O
O
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2
O
O
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The geoid and two ellipsoids. The ellipsoid which
fits well in North America will not fit well in
Europe, and must have a different origin.
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Ellipsoid of revolution with semi-major axis (a)
and semi-minor axis (b)
b
a
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Datum
Definition - The mathematical model of the
Earth we use to calculate the coordinates on
a map, chart or survey system.
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Many countries use their own datum to make their
maps charts called LOCAL DATUMS
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Datum
US Forces - Now use a datum called World
Geodetic System 1984, or WGS-84. - We
used to use WGS-72 and other datums to produce
charts that are still in use today.
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COMMON PROBLEMS
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THIS IS HOW THE GPS SYSTEM SEES THE EARTH
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THIS IS HOW GPS SEES THIS AREA BECAUSE
OF WGS-84
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THIS IS HOW ONE LOCAL ELLIPSOID SEES THE SAME
AREA
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THE PROBLEM STARTS WHEN YOU USE GPS COORDINATES
(IN RED) OVER OTHER CHARTED DATUMS.
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Accuracy vs. Precision

• 1. Position info can be precise without being
  accurate. Precision involves consistent
  positional info from many sources which provide
  similar results.
• 2. Accuracy requires position info be a set
  distance from some reference point.
• Absolute accuracy requires position to be correct
  WRT the earth (e.g. Lat and Long)
• Relative accuracy involves positioning info from
  a fixed reference point.

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Examples of Datum Discrepancies

• Persian Gulf War
• Navy cruiser anchoring off Italian coast
• USCG Bark Eagle on summer deployment
• Naval gunfire in Lebanon
• B-52 - local datum vs WGS-84
• Naval mine-clearing with Japanese
• American F-15s sold to Saudi Arabia

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Causes for Datum Discrepancies
- Multiple datums in use in one area - Datums
using same name and ellipsoid with different
initial points - Charts with missing or
incomplete datum information - Some foreign
products use datums with relationships to
WGS-84 not yet established
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Causes for Datum Discrepancies
- aged datums no longer accurate
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GPS / Chart Conversion

• All NIMA maps and charts will have the datum
  printed somewhere in the margin.
• MADTRAN - NIMA publishes this software which can
  convert coordinates between WGS-84 and over a
  hundred other datums. This program is on most
  ships.
• The bottom line ignoring the fine print in the
  margin of a chart could cause you to run aground
  or worse get you killed!

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Passing Position Information

• 1. Find out what datum is on your chart. Set
  your GPS receiver to read that datum.
• 2. Pass the datum along with the grid numbers
  when transmitting your position.
• 3. Use all available information, such as visual
  bearings and radar ranges FIRST, when navigating
  close to land.
• 4. Continue to compare GPS fixes with those
  obtained via alternative means.

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Questions?
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Compliments of Curtis Senior High School NJROTC