Mexray
Super Nomad
Posts: 1016
Registered: 8-30-2002
Location: California Delta
Member Is Offline
Mood: Baja Time
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Some GPS Ref info...
...I took this from the July 2006 issue of 'Marine Reporter and Engineering News', a trade Mag I get...
Eye on the Navy
A Standard Point of Reference
By Robert Freeman, Office on the Navigator of the Navy.
Sine antiquity, mapmakers have struggled with the problem of representing a bumpy, ellipsoid (the earth) on a flat, rectangular surface (the map). To
do this requires some compromise in the relative position of individual points to each other. Historically, mapmakers have piked a point of interest
to them to be the center of their depiction, and placed various features in their relative position to that point.
For instance, an early English mapmaker might have chosen to place England at the center of his chart. More than just having personal appeal, this
makes sense in the terms of accuracy, since the farther one is from the reference point, generally, the less accurate the depiction. This was fine
for giving a general depiction of landmasses and water bodies. By laying a coordinate system (longitude and latitude, for example) over the
geographical features, you could provide a frame of reference that could be shared by other users.
But maps based on different central points could differ radically in the coordinate position of a location away from those centers.
In the modern world of interoperability and precision positioning, this problem is greatly amplified. The solution is to reference each chart and map
to a 'datum'.
A datum is a mathematical model of the shape of the earth used as a basic reference to calculate position coordinates, heights, and distances, as well
as to make maps and charts.
The datum defines the point from which all positions are referenced. For example, the WGS 84 point of origin is the center of the earth's mass, as
measured by the 1984 World Geodetic Survey (hence WGS 84). The North American Datum, NAD 27, is referenced to a ranch in Kansas, considered to be the
approximate center of the continental United States. For the Tokyo datum, all points are referenced to the center of Tokyo.
There are literally hundreds of datums in use. The DOD standard datum is WGS 84, and this is the one to which GPS defaults, but there is no 'World
standard datum'. GPS fixes positions based on WGS 84, so a position plotted on a chart using a different datum could differ by as much as a half mile
with respect to ports, and up to two miles with respect to isolated islands! Several ships have run aground due to a poor understanding of datums!
This problem becomes more critical when the subject is targeting. During the military action in Lebanon in 1983, Marine Corps spotters ashore were
directing the naval gunfire from a battleship. Unfortunately, the ship was using WGS 72 coordinates, while the Marines were using European Datum
coordinates. The target was missed and shells landed uncomfortably close to friendly forces. Fortunately, there were no casualties!
Until a world standard datum is adopted, the solution to this problem must reside in training. And while algorithms exist for translating from one
datum to another, this is no substitute for understanding. GPS is a wonderful innovation for navigation, providing instantaneous, real-time fixes at
an accuracy that fare exceeds conventional methods.
But navigators must be aware of its limitations and understand how to compensate when using charts based on other datums.
End of Article...
A friendly reminder that many of the charts we use for travel in Baja California use datum 'NAD 27 Mexico'. This entails changing the 'datum' in
your GPS when navigating in Baja to get the best accuracy...and remembering that most Mexican maps and charts are based on surveys taken long before
GPS came into being!
I'd be willing to bet our military has super-accurate maps and charts for most all our earth's land masses - maybe some day this kind of info will be
released to the public so more accurate maps can be made using one 'world standard datum' to make life easier for us GPS users! We can only dream.
As always, your GPS unit works with great accuracy when marking waypoints out in the field or on the ocean...so you can always find your way back to
that spot, no matter what the outdated map shows! Just make sure you're aware of the datum you've selected, and note it to others you might be
passing along your waypoints too.
I think' GPS' are the initials of an old g/friend I used to datum...
[Edited on 8-4-2006 by Mexray]
According to my clock...anytime is \'BAJA TIME\' & as Jimmy Buffett says,
\"It doesn\'t use numbers or moving hands It always just says now...\"
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Bruce R Leech
Elite Nomad
Posts: 6796
Registered: 9-20-2004
Location: Ensenada formerly Mulege
Member Is Offline
Mood: A lot cooler than Mulege
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Good stuff Mexray thanks.
Bruce R Leech
Ensenada
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GC
Junior Nomad
Posts: 70
Registered: 5-17-2006
Location: Punta Banda
Member Is Offline
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Some GPS ref info
Do you know the datum used for the Baja Almanac? I am looking at it now and I cannot see any reference to a map datum.
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David K
Honored Nomad
Posts: 64490
Registered: 8-30-2002
Location: San Diego County
Member Is Offline
Mood: Have Baja Fever
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| Quote: | Originally posted by GC
Do you know the datum used for the Baja Almanac? I am looking at it now and I cannot see any reference to a map datum. |
NAD27 Mexico
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