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[aprssig] Re GPS off track

Keith VE7GDH ve7gdh at rac.ca
Wed Apr 2 23:25:36 UTC 2008


Phillip ZL2TZE wrote...

> The GPSmart is manually set to WGS84 and the Globalsat is
> default WGS84 Both are SIRF powered both give the same
> amount of error I am wondering if there is an error in the chipset .
> www.fortuna.com.tw/gpsmart.htm GPSmart
> www.globalsat.com.tw/eng/product_detail_00000044.htm Globalsat BU-353

Have you tried connecting both of them to a terminal program, or better
yet to something like VisualGPS and looking at the coordinates that it
is spitting out? I just did the same and created a "placemark" in Google
Earth. I confirmed that Google uses WGS84 and that the data that I was
looking at from the GPS was also WGS84. The "placemark" was very close
to my house where the GPS was located, but not exact. It was about 10
metres off to the SE of where the GPS receiver was located. Not too bad.
I know that at my location, there is about 100 M difference east-west
between NAD27 and WGS84, and about 200 M difference north-south. If I
was using a paper map, I would set the GPS to NAD27. Looking at Google
Earth (or for APRS) I would leave it at WGS84. If you go to
http://earth.google.com/userguide/v4/ug_importdata.html#note there is
some explanation about map datums and projects and the errors that can
occur with certain projections. With everything considered, I think the
coordinates shown in Google Earth are not too bad... at my location. The
error in projection could be more extreme at your location. It says...
"In some situations, the transformation might not work properly. In that
case, you can use a third-party tool to transform your data from its
original coordinate system into the one used by Google Earth." but I
didn't pursue this.

73 es cul - Keith VE7GDH
--
"I may be lost, but I know exactly where I am!"






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