[time-nuts] Averaging Location for Position Hold

EB4APL eb4apl at cembreros.jazztel.es
Thu Sep 15 22:54:40 UTC 2011


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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