should take speed data from speed tags if available

[Ian Hincks]

The GPX 1.1 specification (http://www.topografix.com/gpx/1/1/) does not specify a speed tag. However, the GPX 1.0 specification lists it as an optional parameter (http://www.topografix.com/gpx_manual.asp).

Many modules use doppler methods to compute speed accurate to about 0.3km/h, depending on conditions. I'm not sure what the margin of error is on a calculation from latitude/longitude interpolation based on, say, a 1Hz sample rate, but if it's larger than 0.3km/h, it would be worth while to check if speed tags are present before doing a speed calculation.

[Qball Cow (qball-qballcow)]

Can you provide me with an example gpx file (via mail preferable) that has speed tags. I think I would prefer tags from the gps reciever over calculated values.

Currently speed for point X+1 is calculated using the distance between point X and X+1 (using great circular distance method (elevation not taken into account, as it was highly incorrect on my garmin) and the time difference between the 2 points.
This equal to how gpsbabel does it.

[Ian Hincks (ian-hincks)]

I emailed to you the same one that is attached.

Note that this GPX file was generated by my homemade GPS logger, which consists of this guy: (http://www.sparkfun.com/commerce/product_info.php?products_id=465); connected to a microcontroller, which then reads in the NMEA, parses it, and writes GPX files to an SD card. I write this in case you are wondering what kind of GPS device is writing speed information into the GPX files.

Right now my unit is quite disassembled, so I can't make a better file for you for a while. The specification says to use units of m/s. My file has units km/h, which I will change once I reassemble it. But this shouldn't matter for testing.