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From: (no name) (no email)
Date: Tue Feb 01 2005 - 14:52:58 EST
In a message dated 2/1/05 1:21:31 AM, writes:
> The calibration technique in GPSy, Fugawi
> and several other programs requires selecting three points of
> exactly known position, preferably close to opposing edges of
> the chart.
Therein lies the problem. You've perfectly described it.
Three points allows for a linear fit in 2 dimensions and a single rotational
shift. Unfortunately, the scanning process (and the charts for that matter)
don't have linear error. Their error is much more complex.
When Maptech scans a chart (with a $50,000 scanner made for professional
scanning) they correlate the data with 50 to 300 data points. They keep
adding points until an analysis of each pixel results in a less than one
pixel error across the image. The transformation from lat/lon to pixel
position isn't a linear one. It's a 12 term polynomial equation providing a
non-linear fit. By the way, Maptech isn't alone in doing this. Other
commercial "for-navigation" data producers do the same thing. Individuals
are the only ones who do the "three point shuffle." See a pattern there?>>
REPLY:
This is spurious precision. If the geographical contours to be represented
are imprecise, it doesn't matter how accurately the charted information is to be
displayed. It is as if your car speedometer showed speeds to the 1/1000 of a
mph.
Despite the fact that satellite imagery is collected in the data base, the
results still have to be interpreted by a cartographer utilizing Mark 1
eyeballs, then transferred to master sheets. It may be years before the data finds it
way to released NOAA charts. That Pacific seamount was in the data base but
the latest issue charts used by the submarine did not show it. In my own neck of
the woods, the Hudson River, a waterway that has been navigated heavily for
300 years, was not accurately scanned for depth until a Columbia University
research vessel used a side scan sonar to plot bottom contours seven years ago.
Geographical features relevant to navigation that changed 20 years ago still
haven't shown up on NOAA charts. My own annotated charts of the Hudson River
area are probably more accurate than any you can buy.
The use of $50,000 drum scanners to scan mylar master sheets is irrelevant if
the data depicted is imprecise. The release of the scanned material at 266
(or as you say, 254 lpi) adds additional ambiguity as the scans are merged and
compressed.
> The actual chart data is in the public domain. Anyone can
> reproduce published government charts and many companies
> do so with their own variations (Bellingham, etc).
First, the US is about the only country that does this. Almost every other
country retains their copyright rights and requires royalties for
reproduction, even in an electronic form. Worse than that, they have
draconian rules for the storage and use, especially of electronic data.
I've found that most people in the US don't know this.>>
<<Now to your point, yes, everyone has access to the map images that NOAA
produces at no cost. Does this mean that Bellingham gives away their
charts? Are all of those ICW chartbooks now free at Amazon? Of course not.
Each publisher charges a fee based on their production costs and what the
market will bear. The same is true for electronic chart data. This gets a
little more complicated with NOAA's CRADA agreement with Maptech (which
expires this year), but still, there are competitors with similar electronic
chart packages at similar prices (SoftChart). Transforming paper media to
electronic media which is licensed for the purpose of navigation isn't cheap
to do.>>
REPLY:
That's the rub isn't it.
I do most of my boating in US waters. My taxes paid for the charts. I want
them. What do I care about the draconian copyright rules of other countries? I
guess I will have to contact some of my colleagues in China to see about
publishing low cost CDs of US navigation charts. It may be as profitable as bootleg
Microsoft software. But if I can scan my own charts almost for free, the price
will have to be pretty low for the Chinese to compete.
<<Hey, if you enjoyed scanning the charts and using them, go for it. I'm
certainly not suggesting that you change. For most people, and I mean
almost everyone, it's a poor option producing less than expected results.
Even worse, as many people rely on their computer screen to know where they
are, scanning your own charts creates a dangerous environment in my opinion.
>>
REPLY:
Jeffrey,
I want to thank you for making so many of my points. And I do agree that
looking at the computer screen instead of out the windscreen is a dangerous habit.
But I take umbrage at the idea that scanning my own charts puts other boaters
at risk. You seemed to ignore how I used self scanned charts. My charts are
intended for recreational boating only, not for navigating the Exxon Valdez. My
boat draws 4 ft. and is quite maneuverable. It cruises at the speed of a slow
jog. I rarely travel at night or in fog and when I do, I use radar or night
vision glasses. Any chart depths greater than 4' are irrelevant except when
looking for an anchorage. I need charts to tell me where I am and I will be
satisfied to know that within a few hundred yards.
I suspect my needs are similar to those of the vast majority of TWL members,
give or take a few feet in draft and a few knots in cruising speed. For us,
many of the features in the commercial navigation programs (Cap'n, Nobletec,
MaxSea, ChartView, etc.) are overkill.
But then I don't write navigational software or sell charts.
Larry Z
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