Miguel,
Google Maps data are not in any way a technical product, they are
intended to be used for the layman as a (very) useful tool for finding
places, marketing and so, but you can't give for granted any kind of
precision either positional or even temporal.
I personally have measured known (surveyed) points and found
discrepancies of several meters, nothing systematic but pure image
uncorrected distortion. These were not due to different spheroids or
geodetic reference systems, just the original data lacked accuracy
(right, any accuracy is never claimed) and the image date can have
errors of years!.
You can easily see how different images (mis)matches in the stitching
border in some places. They are improving this, but probably they are
just correcting aesthetics, a true precision border matching is very
expensive and it is not needed for their business model.
Better use a good large scale local map to determine the position of
your antenna and correct for the reference system if needed.
Ignacio, EB4APL
On 13/09/2011 15:55, Miguel Gonçalves wrote:
Hi all!
I am installing a timing GPS unit over a new location where I already have a
NMEA GPS with PPS (let's call it unit A). The NMEA GPS is logging every 16
seconds its GPGGA string.
I have a log with around 213400 samples. Each line is similar to this one
55817 48494.500 127.127.20.0
$GPGGA,132814.000,4055.2142,N,00829.6071,W,2,9,1.06,278.9,M,51.2,M,0000,0000*4E
I averaged the minutes of latitude and longitude, the MSL (278.9) and the
Geoid Separation (51.2) and got the following
40º 55.2155' N 8º 29.6044' W (MSL 268.195m and Geoid Separation 51.2m) (for
unit A)
I entered this location on Google Maps I got a position that is in the
middle of a close street (see footer for image; real location is the red
dot).
I also have another GPS unit (let's call it unit B) that is on the roof and
did the same calculations (36500 samples):
40º 55.2228' N 8º 29.6103' W (MSL 262.66m and Geoid Separation 51.7m) (for
unit B)
When I plot this with Google Maps I get the exact location of the antenna
(within 1 meter; see footer for image; real location is the red dot). I am
assuming the first unit has some kind of internal error. Or, does Google
Maps has an offset?
I need this information to enter the precise location in the Oncore UT+ unit
I will be using at this location. I will be using the location of unit A.
Cheers,
Miguel
----
The images available here: http://dl.dropbox.com/u/11210443/GPS%20A.JPG and
http://dl.dropbox.com/u/11210443/GPS%20B.JPG. I did not attach these to the
message to reduce traffic on the mailing list server.
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