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Posted by Theodore Heise on July 31, 2010, 3:10 pm


On Wed, 28 Jul 2010 19:43:28 -0700 (PDT),
>> > Why would, for example, the scant detail base-map have an
>> > intersection between two major highways defined with less
>> > positional accuracy than a map set that has lots of details.
>> you may be right for intersections but a map doesn't only
>> consist of single points. Look at the street lines lacking much
>> curves.
> He's not right about intersections either. If you have a very
> detailed map where you are showing the map features down to an
> accuracy of 30' then you need to specify each point with lots of
> digits for the latitude and longitude. That level of detail and
> accuracy takes up more memory - not only because you have more
> points to specify, but also because each point needs more bits
> to hold the accurate position data.
> Retaining the same high level of accuracy of each point when
> storing a much coarser map (say where having the features known
> to within a few hundred feet is adequate) would be a waste of
> memory space.

This makes good sense, and is consistent with my observations.


> ...It's easy enough to see this by zooming way in on an
> area for which you have both the coarse basemap and a more
> detailed map (such as CityNav.). Features such as intersections,
> turns, etc. won't show up in *exactly* the same locations on the
> two maps.

Okay, I've seen that phenomenon before. It had puzzled me, and
it's nice to know why it happens.


> To answer the original question:
> The accuracy circle is supposed to be an indication that your
> actual position is likely to be somewhere within that circle *as
> portrayed on the displayed map*. So inaccuracies in the map
> data and uncertainty in the position measurement both enter into
> the size of the accuracy circle.

That was my original question, lost way back in the thread. :)

Thanks very much for the clear and satisfying answer! Appreciate
it very much.

--

Posted by Sunshine on July 31, 2010, 3:34 pm


On Wed, 28 Jul 2010 19:43:28 -0700 (PDT), Peter Rathmann

>To answer the original question:
>The accuracy circle is supposed to be an indication that your actual
>position is likely to be somewhere within that circle *as portrayed on
>the displayed map*. So inaccuracies in the map data and uncertainty
>in the position measurement both enter into the size of the accuracy
>circle.

It seems like uncertainty in the position measurements will affect the
size of the circle, while inaccuracies in the map data will only
affect the placement of that circle on the map.

How does inaccuracy of the map data affect the size of the circle?


Posted by Alan Whit on July 31, 2010, 5:16 pm


wrote:

>How does inaccuracy of the map data affect the size of the circle?

Probably only Garmin can answer that.
=20
--=20
Alan White
Mozilla Firefox and Forte Agent.
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in =
Argyll, Scotland.
Webcam and weather:- http://windycroft.co.uk/weather

Posted by Peter H. Coffin on July 26, 2010, 8:11 am


On Mon, 26 Jul 2010 08:42:25 +0200, Heinrich Pfeifer wrote:
> 25.07.2010 17:59, TheDuck wrote:
>> I don't agree with that. The only thing the program knows is where the
>> satellite signals think you are.
> and it knows the geometry of sat positions relative to each other
> affecting precision. And it knows the precision of the map shown. All
> this defines the size of the displayed ring. This is a fact.

EPE has nothing to do with the map, and can be figured without a map at
all. It entirely has to do with how different combinations of satellite
signals combine to report different positions and mathmatically
aaassigning an area that can be reported as "where the device is" with a
set level of confidence. The more closely those different combinations
agrees, the smaller the circle is.

--
73. I will not agree to let the heroes go free if they win a rigged
contest, even though my advisors assure me it is impossible for
them to win.
        --Peter Anspach's list of things to do as an Evil Overlord

Posted by Heinrich Pfeifer on July 26, 2010, 8:39 am


Am 26.07.2010 14:11, schrieb Peter H. Coffin:
> On Mon, 26 Jul 2010 08:42:25 +0200, Heinrich Pfeifer wrote:
>> 25.07.2010 17:59, TheDuck wrote:
>>> I don't agree with that. The only thing the program knows is where the
>>> satellite signals think you are.
>> and it knows the geometry of sat positions relative to each other
>> affecting precision. And it knows the precision of the map shown. All
>> this defines the size of the displayed ring. This is a fact.
> EPE has nothing to do with the map, and can be figured without a map at
> all.

this is correct, of course. But we did no talk about EPE. We talked
about the precision ring on the display of Garmins. This reflects both
EPE and map precision.

The GPS user looking at the map page is not interested in the precision
of the numerical coordinates found by the receiver. He wants to know how
far he is from a displayed map feature. And for this purpose both is
considered: EPE and map.


> It entirely has to do with how different combinations of satellite
> signals combine to report different positions

no. If it receives only four satellites then there is only one
combination but the EPE is calculated from their geometry anyway, as it
is all the time.


--

Heinrich
http://www.gartrip.de
mail: new<at>gartrip.de

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