[ntp:questions] Sure Electronics GPS board: Amazing performance. :-)

unruh unruh at wormhole.physics.ubc.ca
Fri Apr 1 06:12:56 UTC 2011


On 2011-03-31, E-Mail Sent to this address will be added to the BlackLists <Null at BlackList.Anitech-Systems.invalid> wrote:
> unruh wrote:
>> My only concern is that the unit places its location 12m
>>  away from its actual location on Googleearth-- ie, either
>>  googleearth is 12 m out in its alignment to lat/long grid
>>  at my house, or the unit is misreporting its position.
>> Note that the scatter has only a standard deviation of
>>  about 1.5 m ( coming from its use of WAAS I suspect),
>>  so it is not that that 12 m is just part of the natural
>>  variation in gps.
>>
>> (Does anyone know of anywhere where the alignment errors
>>   of google earth to the grid is discussed?)
>
><google.com/support/forum/p/maps/thread?tid=5c0d81f1629963c3&hl=en>
><BlockQuote>
> Maps Guide Adam, Google Employee
> You should *not* be using the imagery displayed in Google Earth
>   for this purpose.
>  If you have a project which requires a certain threshold of accuracy
>   and geometric precision you will need to utilize the appropriate
>   photogrammetric methodologies and tools.
>  The coordinates, elevations, distances, and measurements provided
>    by Google are approximations only.
>   Google makes no claims as to the accuracy of these measurements.
></BlockQuote>

Those are weasel words (apparently google was sued by someone who relied
on googleearth and got injured by walking along a highway instead of a
path). Google tries to make their images accurate. The question I have
is, what is the accuracy of the googleearth maps in their geolocations. 
I am sure it varies. Probably better in Vancouver say than on the
atacama in Chile.

 
>
>
>> A brief 10 min test at a survey marker put the unit about
>>  3 m away from that marker on googleearth-- but that was
>>  at a location 6km from my home.
>
> If you don't have a clear view of the sky,
>  e.g. only out a window on one side of a building
>  the positions certainly are skewed a bit (in my experence).
>
> Assuming you have a clear view of the sky in all directions,
>  run something like google.com/search?q=SA+Watch for an
>  extended period of time to determine a _fixed_ position
>  more accurately.

In my home it is out a window in the front of the house, and the
position is 12m in that direction out (if I believe googleearth).
 
>
>
> Off the top of my head (IIRC);
>
>  In general, for vehicle street / map navigation you need
>   a HDOP < 4 (~<52' / 16m)
>
>  With a CEP 95% probability, the actual position is within
>   66ft / 20m diameter circle of the position a GPS returns.
>
>  Without several GPS satellites, plus a SBAS satellite
>    (or some other form of DGPS)
>   I think you are very unlikely to continuously stay under
>    3m accuracy, on short time stationary fixes, or mobile
>    fixes with a consumer GPS.

The CEP (1 sigma) over 15 hrs was about 1.5m
It is claiming it is using DGPS (WAAS?)



>
>
><nstb.tc.faa.gov/DisplayArchive.htm> has some information
>  about recent Dilution of Precision measurements and GPS
>  accuracy records, ...
>
>




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