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

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Thu Mar 31 21:49:01 UTC 2011

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?)

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
 more accurately.

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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