Walking in the air with a Clip-On?

[CheshireCat]

The tracks recorded by Memory Map 2004 running on an iPAQ connected to a Fortuna Clip-On show altitudes about 50m higher than the associated terrain. The difference is not consistent but varies between 40ish and 70ish metres. Subjectively, the discrepancy is lower when in XT than in ST mode.
Without an alternative GPS device to check against, it's difficult to tell if this is a hardware or software issue or perhaps connected with the difference between GPS and OS altitudes (46m?).
If you are using a Clip-On, perhaps you could check the altitiude data you see and report back. Or perhaps there's another explanation?

[DavidW]

GPS altitudes will not match those on Ordnance Survey maps, as the WGS84 geiod is different to that underlying Ordnance Survey mapping.

It is possible, with some fairly horrendous mathematics, to convert between the two.


I suspect the XT figures may just be more inaccurate - but the error brings them nearer to Ordnance Survey figures. Of all the parameters on a GPS, altitude is the most inaccurate. I would suspect ST mode with SBAS on would be the most accurate - but there's currently no usable SBAS signal in Europe. EGNOS will become available again some time next year.


There's more information on this at http://www.gps.gov.uk, particularly in this document.



David

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