GPSmap 76 CSx accuracy
Thibaud Taudin Chabot
by the manual as far as I could find out.
1. When I have a accuracy of 7 meters (in the satelite screen) and the D
is being displayed in teh bars after some times indicating that
WAAS/EGNOS is being used I would expect that the accuracy of 7 meters
jumps to a much lower level like 2 or 3 meters. But the accuracy remains
the same. How can?
2. Again having a accuracy of 7 meters and switch to the map display I
see a blue circle. I understood that this circle is the area of
uncertaincy. However the radius of that circle can be 30 meters when the
accuracy indicates 7 meters. How can?
Has anybody an explanation of those 2 items?
Thibaud
Tom
> I used my 76CSx last week a lot. Two items I saw couldn't be explained
> by the manual as far as I could find out.
>
> 1. When I have a accuracy of 7 meters (in the satelite screen) and the D
> is being displayed in teh bars after some times indicating that
> WAAS/EGNOS is being used I would expect that the accuracy of 7 meters
> jumps to a much lower level like 2 or 3 meters. But the accuracy remains
> the same. How can?
WAAS/EGNOS does not always make a big difference, and with my Map60CSx
(same electronics/firmware as 76 CSx) I usually see little difference,
so I leave WAAS off to reduce computation time and maybe save a little
battery. Satellite geometry can play a big role in accuracy.
>
> 2. Again having a accuracy of 7 meters and switch to the map display I
> see a blue circle. I understood that this circle is the area of
> uncertaincy. However the radius of that circle can be 30 meters when the
> accuracy indicates 7 meters. How can?
>
The error circle includes both the gps positional fix error (7 meters
in your example) and the error in the maps themselves. The map error
is probably a general add-on of xx% even though some roads might be
off by 20 meters and others within 2 meters.
Thibaud Taudin Chabot
I wonder about the mapaccuracy. My experience is that the map is pretty
good, certainly no large errors that can explain an error circle of 30
meters when the satelite page indicates 4 meter. The POI's on the maps
are indeed not always correctly positioned.
Using a topo map of the Netherlands of which I know the source is
extremely good gives the same indications when I am in an open field
with 11 satelites with full bars!
If there would be a map error then that error should be in the mapinfo
in order to get an overall error like you say, I doubt if that kind of
info is prsent in a map.
Thibaud
Bert Hyman
Tom <tbha...@gmail.com> wrote:
> The error circle includes both the gps positional fix error (7 meters
> in your example) and the error in the maps themselves. The map error
> is probably a general add-on of xx% even though some roads might be
> off by 20 meters and others within 2 meters.
How can the receiver know about errors in the maps?
--
Bert Hyman St. Paul, MN be...@iphouse.com
Tom
> Innews:4f0b5c1c-cab2-4106...@e4g2000prn.googlegroups.com
>
>
> How can the receiver know about errors in the maps?
>
> --
> Bert Hyman St. Paul, MN b...@iphouse.com
The receiver "knows" what it is programmed to know in addition to the
positions it calculates. The Garmin engineers probably have some data
or estimates of what the map error is *likely* to be. We are also
talking about the maps that are resident in the gps, not say a topo
map that one might download data to. I have also observed that the
maps are generally more accurate than the error circle would lead you
to think. Garmin may be being conservative by assigning a relatively
large error to the map --- after all they do not produce the maps
themselves, but buy then from another source (NAVTEQ primarily).
Matt
>> How can the receiver know about errors in the maps?
>>
Each map you upload has a quoted accuracy, the receiver simply uses that
if it's greater than the GPS positional accuracy.
Thibaud Taudin Chabot
>>> How can the receiver know about errors in the maps?
>>>
>
> Each map you upload has a quoted accuracy, the receiver simply uses that
> if it's greater than the GPS positional accuracy.
that accuracy is?
Matt
I'm not certain of the details, but take a look at mkgmap's man pages,
and the reverse-engineered garmin img file format:
Thibaud Taudin Chabot
there is nothing mentioned about a quoted accuracy. Actualoly I can not
find anything about accuracy at all...........
So the strange situation is still not explained: 5m accuracy in the
satelite view and on my map view a blue circle of accuracy of 20m radius.
Mirek
Thibaud Taudin Chabot wrote:
> So the strange situation is still not explained: 5m accuracy in the
> satelite view and on my map view a blue circle of accuracy of 20m radius.
They are calculated at different statistical confindence bounds 50% vs 99%
:)