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>For example, GPS accuracy can vary widely over time for
>a given real-world location. If there's a problem with
>one of the satellites that's expected to provide part
>of the positioning info, or if weather or other conditions
>simply block or degrade its signal, then the GPS unit is
>suddenly providing location estimates that are much
>coarser than they were a few minutes ago (having fewer
>satellite data streams to correlate).
GPS signals are deliberatlly degraded for civilian use. Military GPS devices
can be accurate to within a handful of meters. Civilian GPS devices are
accurate to anywhere from 50 to 150 meters. This was done to prevent hostile
forces from using GPS to guide bombs to their targets.
>Say, for example,
>it's info regarding you and the nearby hazards a few
>hundred meters outside the harbour into which you are
>attempting to sail, in the fog... in the dark...
In a former life I use to navigate ships in the Coast Guard. We never used
GPS to navigate in a harbor, let alone in the fog, or in the dark. Actually,
navigating ships in a channel when fog has set in were some of the intense
experiences I have ever had.
>At the same time, the charts against which the GPS data
>might be presented are often drawn from survey information
>that is a hundred years out-of-date, or that is less
>refined than the GPS data which is being overlaid.
>So, as you glide into the foggy harbor, the GPS might be
>giving you your own position accurately to within a couple
>of meters, but location that it shows for the big rock
>on your map/chart was recorded by a near-sighted surveyor
>in 1886 and is wrong by 30 meters.
>Crunch. Ouch. Apparently it was 30 meters in the wrong
>direction. The cursor that represents you on the display
>is accurate, but the terrain features that were programmed
>to appear on that same display might not be.
Happens all the time. Navigational aids are moved. New piers, bridges,
towers, etc. are built. And my favorite; different nautical charts are
compiled using different datums. The datum is geographical reference system.
The current datum is World Geodetic System 1984 (WGS 84). It is expected to
be fully implement _this year_. I have seen two charts of the same area
using different datums and the latitute and longitude are differnent for the
same points of land. In fact, many Asian charts use the Tokyo datum which
differs from WSG 84 by more than 700 meters. That's about half a mile!
>GPS locates you with respect to an imaginary ball. Other
>people's data (geophysical survey, marine charts, dredging
>records...) that were not determined via GPS, are then
>overlaid onto the local section of that imaginary ball,
>so that you have some geographical features to relate to.
6 years ago the Coast Guard was just testing the positioning of navigational
aids using differential GPS. This is a form of GPS that compensates for its
inaccuracies by havnig fixed reference points. They have probably
implemented it now, but as you say, there are very few, if any, chart
features whose position's have been determined by GPS.
>Similarly, imagine a wheeled vehicle being navigated by
>GPS to enter an alley that is 20 meters up the street
>from a gasoline service station. While receiving signals
>from 4 satellites, the GPS is able to reliably show its
>own position to within 2 meters (just an example). Then,
>the vehicle enters the signal-shadow of a tall building,
>leaving only 3 satellite signals available. The accuracy
>decays to +/- 20 meters. Turn left at the alley, and
>you are home. Turn left a few meters on either side,
>and you've got a bit of property damage. Turn left
>into a gas-pump and you've got a five-alarm fire, not to
>mention a melted GPS. :-)
This is exactly why we did not navigate in a channel with GPS. When one of
my jobs was to make sure buoys were at their charted location, the threat of
a lawsuit was always on our minds. We would sometimes spend more than an
hour getting the bouy poistioned to within 10 yards of its charted location.
(It seems like a lot, but swim out into the middle of a lake and try to
judge distance.)
>The documents need to address the use to which the devices
>are to be put, or to warn the reader of the dangers in
>using it for non-documented purposes. The same device in
>different circumstances can be an asset or a liability.
Absolutely!
Mike Feimster
Information Design & Development
Computer Dimensions, Inc.
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