GPS digital navigation map data

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
Reply to
Al Balmer
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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:

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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."

Reply to
Mark Moulding

Yes. I use a handheld to double check position. I do not navigate with it [see below].

Aviation units compute location much faster. AFAIK there aren't any consumer oriented units that can reliably locate themselves while moving at 150 knots ... many turbocharged and/or multi engine aircraft can cruise faster than that.

Also aviation units with navigation plot great circle courses rather than straight lines.

I think it's more because GPS is controlled by the U.S.

George

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Reply to
George Neuner

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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Reply to
J. Clarke

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.

Reply to
David R Brooks

Would you do an approach without much assurance of the GPS satellite signal quality ? Assuming that the SA would be suddenly switched on during the approach (or a satellite would experience some sudden problem), this could have quite serious consequences. For any critical navigation, you really need some assurance about the signal quality.

A differential-GPS station can constantly verify the signal quality from all satellites and broadcast this signal quality information to nearby users (in addition to the differential correction). Without a DGPS transmitter at every airport, I don't see how you could make a safe approach with only a GPS receiver.

Paul

Reply to
Paul Keinanen

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

Reply to
Paul Keinanen

Dear Mark,

Thank you for your informative reply.

(I think your posting was the only relevant reply to the original question.)

Ray

Reply to
<Ray>

Does anyone know if there is an equivalent to "US Census Bureau's Tiger/Line database" in Australia?

Reply to
<Ray>

Google "WAAS".

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--John
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Reply to
J. Clarke

Not at all irrelevant to Anton's anecdote, which you snipped.

--
Al Balmer
Sun City, AZ
Reply to
Al Balmer

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

Reply to
Anton Erasmus

Hummm...are you talking about pilots trusting their lives to personal GPS?

Handheld aviation GPS units are remarkably accurate, and are great at facilitating navigation. But that is as far as the 'hand held' variety goes, I know of no pilots who actually trust their lives to the hand held units. think of it as a way to verify and cross check other nav aids.

My own flying, I use it to supplement the regular aviaonics...and the hand held is much more accurate than the standard VOR type units.

Reply to
jbeck

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