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Kat Rabun wrote on Mar 27, 2010:
>> 2. If you were changing altitude (hill side), it could be that you were
>> not only subject to masking but multipath as well.
>> not only subject to masking but multipath as well.
>
> OK. I understand that the earth's "masking" of satellites on the other side
> are making the elevation accuracy about twice the 15 foot horizontal
> accuracy ... but ... why all the bouncing around?
>
> Does the masking (and multipath) also explain why the elevation number
> essentially never settles down (even after 3 to 5 minutes sitting still in
> the same spot)?
> OK. I understand that the earth's "masking" of satellites on the other side
> are making the elevation accuracy about twice the 15 foot horizontal
> accuracy ... but ... why all the bouncing around?
>
> Does the masking (and multipath) also explain why the elevation number
> essentially never settles down (even after 3 to 5 minutes sitting still in
> the same spot)?
Actually the horizontal position also 'bounces around' just like the
vertical. You won't normally see this on a Nuvi because (since it is
primarily an automotive unit) it locks the horizontal position to the nearest
road. On some handheld units you can turn the lock off, I don't know if you
can with the Nuvi. If you can turn it off and go to the map page, zoom in to
the maximum with the unit stationary you will see the position icon doing a
'drunkards walk' within an approximate circle around your actual position.
Secondly the elevation is not just based on the distance from the earth's
centre, but the distance above a notional 'mean sea level' which is the level
the sea would be at that point if it were in equilibrium (i.e. no tidal or
any other variations). The MSL (however it is calculated) depends on the
horizontal position with the result that any changes in either horizontal or
vertical coordinates also cause a change in the elevation reading.
Since all three coordinates vary slightly every time a position is
calculated, even though the unit is stationary, the elevation will bounce
around even more than the horizontal does. With some handheld units you can
get around this by 'averaging' a location so that the position and elevation
will eventually settle down to a more accurate value than the first spot
reading.
--
Mike Lane
UK North Yorkshire
email: mike_lane at mac dot com
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