Re: Garmin Textout format discrepancy?


"Happy Trails" <undisclosed@xxxxxxxxxxx> wrote in message
news:8lnps1h94klc4g744kfh1s03o63sq6lnue@xxxxxxxxxx
> On 17 Jan 2006 01:48:43 -0800, exmaple@xxxxxxxxx wrote:
>
>>I have a Garmin (eTrex Vista) and I'm trying to get some "realtime"
>>data from it into my laptop.
>>The easiest way would be to have it output data in Textout format (or
>>so I thought...).
>>Unfortunately, I find there's a big discrepancy between the coordinates
>>it reports via the serial port, and what's shown on the screen. The
>>difference is about 10 miles or so.
>>For example, currently my position on the screen is N37.47363,
>>W122.24933.
>>The text output from the GPS shows it to be N37.30142, W122.15319.
>>
>>I'm at a total loss here. How can the GPS magically display the right
>>location and yet output something significantly different??
>
> Look closely at the actual values - are you outputting degrees &
> minutes, separated by a period, in one case, and degrees & decimals in
> the other? .47 of a degree is pretty close to 30 seconds, given the
> probable slight differences in successive readings.

I've just had a play with my own Vista and this appears to be the main
source of the discrepancy. Garmin text output is in the form
Nddmm.mmmWdddmm.mmm but with an implied decimal point and the coordinates
output are those derived from the internal WGS84 datum, irrespective of what
is selected on the receiver. Assuming WGS84 the difference between the two
points given by "exmaple" is actually 13mi if you enter them both as decimal
degrees. If the coordinates output by the receiver are entered as degrees
and decimal minutes then the discrepancy reduces to 2mi. The discrepancy
that would be caused if the receiver were set to the NAD27 (CONUS) would
only be 0.06mi so there is still some remaining error. Possibly the
receiver wasn't getting a good signal and maybe "exmaple" misread the
receiver display - since most of the remaining discrepancy is in the
Northing. Where exactly were you at the time - according to your receiver
display you would have been on Topaz Street, Redwood City (according to
Google Earth), whereas the text output locates you two miles to the north
between Laurel St and the 82 (and between Morse Blvd and Arroyo Ave).

Make sure that your Vista is set to WGS84 and hddd*mm.mmm' and try the
experiment again.

David


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