[ntp:questions] Sure Electronics GPS board: Amazing performance. :-)
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Null at BlackList.Anitech-Systems.invalid
Thu Mar 31 21:49:01 UTC 2011
> 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?)
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.
> 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
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.
<nstb.tc.faa.gov/DisplayArchive.htm> has some information
about recent Dilution of Precision measurements and GPS
accuracy records, ...
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