Re: [gpsd-users] Status of my patches.

[Sven Geggus]

Gary E. Miller <address@hidden> wrote:

>> AIS accuracy is usually better than the provided 4 decimal places,
>> thus as I already wrote, I think having %.6f would be better than
>> %.4f.
> 
> What would that be in meters or feet?

Worst resolution is near the equator so lets estimate it there (using
40000km as the circumference of earth).

The currunt code uses %.4f which means roughly 11.11meters
(0.0001*40000000/360.0) so we would need at least one more decimal place to
represent the _current_ accouracy of GPS.

This said I strongly feel that the reference here should not be the
accouracy of current GPS which may well get better in future but the full
resolution the encoding (NMEA) actually provides.

This is the rationale behind using 6 decimal places (%.6f) in my patch.

According to the documentation (http://catb.org/gpsd/AIVDM.html) NMEA
encoding in AIS messages provides a resolution of minutes/10000.

Converted to degrees we will thus need 6 decimal places to represent
this as 1 digit represents 0.00000166 degrees.

Conclusion: To represent GPS accuracy use at least %.45f to represent
resolution of NMEA rencoding use %.6f

Regards

Sven

-- 
Unix is simple and coherent, but it takes a genius – or at any rate a
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/me is address@hidden, http://sven.gegg.us/ on the Web