On Fri, 20 Jun 2003, John Schnizlein wrote:
> Thank you for the probing questions. Clarification embedded in context:
>
> At 08:38 AM 6/20/2003, Robert Elz wrote:
> >The definitions of altitude in that draft need work.
> >
> >Metres (positive and negative) is simple enough, but relative to
> >what? Unless I missed it, I could find no definition of what is 0.
>
> Zero is defined by the datum, at least for those we know.
> To cover any cases where it is not, we propose clarifying text as follows:
> If MU = 1, an AltRes value 0 would indicate unknown altitude.
> The most precise Altitude would have an AltRes value of 30.
> Unless otherwise defined by datum, a value of 0 is with respect
> to mean-low-tide.
>
> >If the assumption is that it is to be "mean sea level" then which sea?
> >If it is to be above (+ or - below) ground level, which ground level
> >within the designated area given by the latitude and longitude (sloping
> >ground).
>
> It is our understanding that mean low tide is well defined, since
> political and personal land-ownership boundaries are based on it.
It depends on the jurisdiction. For instance, it is different in Canada
and the US - one uses mean lower low water and the other uses lowest
normal tide or something as a nautical chart datum.
Defining elevation seems to be more problematic than defining position.
If consumer-grade GPS has a horizontal resolution between 5-30 metres,
that's good enough to give a street or maybe a room in a building. But the
vertical resolution may be double that, and that's 3 floors at best, even
assuming you have ground level sorted out properly. Other navigation
techniques such as LORAN (obsolescent) or cellphone-signal based may not
give elevation at all.
On the other hand, from what I read, survey-grade differential GPS
is currently capable of excellent accuracy (15cm or better), but this is
unlikely to be available in inexpensive mobile devices.
The European GALILEO service will provide greater accuracy than GPS, and I
think I read somewhere that it may penetrate buildings and dense
vegetation better. Augmentation systems such as SBAS, WAAS, EGNOS
seem to provide enhanced accuracy to 3-5 metres, so maybe within a decade
we will have inexpensive devices able to discern which floor they're on
... http://www.magellangps.com/en/products/aboutgps/ etc.
Andrew Daviel
Vancouver Webpages
Received on Tue Jul 1 23:35:20 2003
This archive was generated by hypermail 2.1.8 : Thu Jan 22 2004 - 12:32:24 EST