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On Wed, 2002-03-27 at 09:50, Hart, Geoff wrote:
>[snippo]
>
> This is only partially a documentation issue; the majority of the problem
> relates to an interface problem. Consider the difference in the usage
> contexts: a civilian GPS user can easily take time to read the manual, and
> can make usage decisions at a relaxed pace, with few consequences other than
> the need to clean up the data if they make a mistake. In contrast, a
> military user either won't have the documentation easily available, or will
> be too rushed and stressed out (perhaps even panicked) to have time to use
> it, and the consequences of a wrong decision may be fatal.
[...]
> civilian GPS (we study
> in-woods navigation of forestry machines using GPS in our research).
Geoff,
GPS is showing up in all kinds of applications, these days,
but in at least two that I know, the user had better learn/
discover everything before going operational. So, the
docs had better offer some incentive for learning BEFORE
a live, real-time situation.
Unlike the forestry equipment, that can simply come to a
stop if there's confusion or doubt, some situations proceed
too quickly for that kind of leisurely re-appraisal.
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). 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...
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that is a hundred years out-of-date, or that is less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.
Blub-blub-blub.
Hope your boat insurance was paid up.
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.
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. :-)
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.
Perhaps the GPS interface
could indicate the uncertainties by showing the usual
moving-map display, and then swapping it for "fuzzy zones"
every few seconds to show you how much uncertainty was
actually being hidden by the clear, crisp map lines.
The normal display might show a "safe" twenty-meter separation
between your position cursor (your boat) and the position of
that big rock, or of those gas-pumps, but if the fuzzy zone
of your own position were to overlap the fuzzy zone of
the "fixed" hazard, then you know it's time to stop
moving until you can locate both yourself and the hazards
by other means.
Hmm. Almost makes me want to stick with documenting
stuff that affects people's livelihoods, rather than
their lives. :-)
--
Kevin McLauchlan
Chrysalis-ITS, Inc.
"Ultimate Trust(TM)"
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