GPS digital navigation map data

David R Brooks wrote:


Doesn't anyone sailing near the Horn without real Admiralty charts deserve what he/she gets? What little sailing experience I've had tells me yes.
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Jim Stewart wrote:

When I was learning coastal navigation, about 15 years ago, my instructor was over the moon about GPS. In fairness, he didn't specifically say to dump my paper charts, but he did say to dump my sextant, & use the saved cash to buy a back-up GPS unit. I still take leave to differ with that view.
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On Tue, 27 Dec 2005 22:28:19 -0800, David R Brooks

I read an article In one of the Electronic trade magazine some time ago. This article was written by an engineer who had been evaluating the GPS technology, chipsets and devices available at the time. One of his hobbies was sailing, and he took the several GPS devices along on a weekend sailing trip around some islands in the west indies. One thing he picked up was that very few of the devices gave a confidence level (Or a tolerance) on the highly precise value displayed on the device. He also said that if he had relied on the GPS devices on a particilar trip where he had a 100m wide channel to navigate, he would have ended up on the rocks. I personaly think that devices should display a precision related to the acuracy of the data. People tend to believe the numbers even if they should be ignored. If one displays a range for instance with an accuracy of +- 1m, then display say 143 m, not 143.000 m. Better would be to display 143 +-1m or (142 to 144m). On a GPS device with a map display, a circle could be displayed that indicated that one is somewhere in the circle in stead of a dot. If the minimum height intersects the ground anywhere in this region, an alarm should sound.
Regards Anton Erasmus
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On Wed, 28 Dec 2005 09:44:03 +0200, Anton Erasmus

Was "some time ago" prior to the removal of Selective Availability?

At least some GPS units do exactly that, though it's usually a separate screen. It wouldn't surprise me if all of them do.

Seems useful only to pilots, and would require topographic maps, which are available but not needed by most lay GPS users.
--
Al Balmer
Sun City, AZ
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Al Balmer wrote:

Also, take a look on comp.risks: there's an item in the current Risks bulletin on in-car GPS issues: as noted on this thread, they all derive from a single database, which is *not* error-free. So using two different makes of GPS won't help either.
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wrote:

That is irrelevant as long as the system contains the option of SA. How can you be sure that the SA is not activated tomorrow ?
Paul
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wrote:

Not at all irrelevant to Anton's anecdote, which you snipped.
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Al Balmer
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wrote:

I cannot remember but believe it does not make much of a difference. As long as SA can be activated, then for safety critical applications one has to be able to operate as if it is switched on. The biggest problem was that during a storm, when the navigational data accuracy is at it most critical, one can have problems aquireing one or more of the GPS satelites. With a lower signal to noise ratio, it takes longer to get an acurate position. If one is moving, then one can up with position data of which the accuracy is severly down graded. The author of the aricle I was refering to was greatly concerned that this degrading of accuracy was not made clear in the documentation of the GPS devices, never mind actually having a clear indication on the unit that the current data has degraded accuracy. Naturally the GPS device manufacturers has a dentancy to indicate the accuracy one gets under ideal weather conditions. For any safety critical application one must never rely on best or even typical performace specs, but on worst case performace specs.
That is also why I mentioned the accuracy versus precision thing. Most people have a tendancy to equate precision with accuracy. This can have disastrous consequences, hence my opinion that GPS devices should display the data in a format which clearly indicates the CURRENT accuracy when displaying the data. Even if the map database data was 100% accurate, one should still be careful to rely on the data, since the accuracy of any GPS device can vary greatly depending on what the current local conditions are. From "No data" up to the resolution limit of the GPS system as a whole. Because of filtering, the user might not even know that the current displayed position is based on "No Data" values for the last number of readings.
[Rest Snipped]
Regards Anton Erasmus
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Anton Erasmus wrote:

The question is whether he would have ended up on the rocks due to inaccuracy or to his failure to set the correct reference point (the "datum"). If the GPS is using one datum and the charts another then even if the GPS is giving accuracy to the nanometer its reading will differ from that on the chart. This is something that most GPS users don't even seem to be aware of.

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--John
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<SNIP>

That is too verbose. May I propose a notation from my physics experiments 143K2m accurate to 0k05m (Conventional, half of last dec.) A bar under the 2 accurate to 0k1m A twiggle under the 2 accurate to 0k2m
This should do the job and is unobtrusive. Come to think about it, it should be a standard feature on digital volt meters. At last those display only significant digits, even if that is a cost issue more than anything else.

That sounds sound.

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Albert van der Horst, UTRECHT,THE NETHERLANDS
Economic growth -- like all pyramid schemes -- ultimately falters.
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Albert van der Horst wrote:

You're a bunch of naive optimists ;) I remember a sophomore EE lab ~1962. Goal - design 1 transistor amplifier Classmates regularly requested bias resistors spec'd to less than 1% Value depended on beta which had < order of magnitude precision ;/
I'll odds that typical user would assume you were trying to identify center of circle as *precise* location.
Then again, a circular blob might work where a circle outline might not. Then again, I've written test procedures which intelligent persons have followed "literally" getting some strange results ;{
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Richard Owlett wrote:

Hey, at least they weren't looking for ten farad 5000 volt capacitors to filter the power supply.

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saving up my nickles and dimes to get one, been doing the research.
NO, there is no common format. Far from it.
There is the GPX format which is meant to be a common ground.
There is a program called GPSBabel which is meant to translate most manufactures to/from GPX and to each other.
AFAIK, most if not all can stream NEMA GPS data to their serial port which can be read and translated. I had an old magellen that was meant to do this, and did, however it was exceeding faulty, registering my Los Angeles home as a moving target at some 700 mph...
I have been cosnidering Trails.com as a source of hiking trails, and had hoped to join them and generate nice maps to follow. As it turns out, you can trace a map, and export a GPX format route. I D/L'd Babel and converted my route to CSV format, so it does function in that respect...
<Ray> wrote in message> GPS navigators such as TomTom, NavMan, Garmin, Magellan etc. become

type
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You might take a look at the US Census Bureau's Tiger/Line database. These files are freely available, and contain the basic source data that all of the (US, anyway) GPS mapping companies use for their products. Look at:
http://www.census.gov/geo/www/tiger /
These files contain information on all streets, geographic boundaries (lakes, rivers, etc.) zip codes, railways, governmental boundaries (city, count, state, military, parks, etc.) address information (including left/right side and one-way) for all the streets. In short, it's all you'd need to write your own mapping software (along with a bit of work, of course).
The GPS and mapping companies (deLorme, Microsoft, AAA, MapQuest, etc.) then supplement this information with phone-book databases to add points of interest, freeway exit information, etc. They also invest quite a bit of effort in producing routing algorithms to make use of this raw data, and user interfaces to (hopefully) make it easy to use. Invariably, the raw data is massaged into their own proprietary format for speed of access.
-- Mark "I prefer heaven for climate, hell for company."
<Ray> wrote in message > GPS navigators such as TomTom, NavMan, Garmin, Magellan etc. become

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Dear Mark,
Thank you for your informative reply.
(I think your posting was the only relevant reply to the original question.)
Ray
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Does anyone know if there is an equivalent to "US Census Bureau's Tiger/Line database" in Australia?
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