RE: altitude reference system

As far as this goes, the simplest approach is to pick one and stick to it.  The advantage of making the decision is that there is no guessing what a particular value means - you have a standard.  Admittedly, there is the drawback that when I say I'm at -60m altitude in Sri Lanka I'm not really underwater, but that's the compromise cost.

This is not a question of better or more accurate results.  Providing a value based on the geoid (as opposed to the reference ellipsoid) is no more correct, it just matches better to user expectations.  What you are talking about is a user presentation nicety.

Ta,
Martin

> -----Original Message-----
> From: Erik Wilde [mailto:dret@berkeley.edu]
> Sent: Monday, 5 January 2009 11:27 AM
> To: Thomson, Martin; public-geolocation@w3.org
> Subject: Re: altitude reference system
> 
> hello martin.
> 
> > The definition is clear enough without resorting to discussions of
> gravitational models.  The term "ellipsoid" does not introduce any
> ambiguity; no mention is made of the geoid, which is clear enough.
> EGM96 (or EGM2008) can be applied ad hoc by those who require "sea
> level" type altitude.
> 
> isn't EGM applied to make altitudes more "real"? it is my understanding
> that EGM is useful because it makes altitudes more accurate in terms of
> how the earth is really shaped, but on the other hand it is expensive
> because it is a large datasets and not all devices have it built-in. so
> what you're saying is that altitude is *not* EGM-corrected in any way,
> right? i think it would be worth to make that explicit, if that is what
> we want to do. i thought manufacturers may build in EGM support into
> their devices to make them more accurate, and we would then force them
> to expose a non-corrected value through the API. i think this kind of
> decision would be worth to be made explicit, rather than implicitly
> being made by just mentioning the GPS ellipsoid.
> 
> > Obviously, this represents a (extremely minor) disconnect between the
> results and the actual physical environment; given the inherent
> limitations of the technology with respect to accuracy, I hardly think
> that it will have much bearing on the usability of the information.
> 
> i think the disconnect is not so minor and in some places up to 100m
> (85m minus and 105m plus, i think), which can be pretty important, for
> example in mountaineering. if a device can provide better accuracy, why
> not allow it to expose it? i guess this gets back to the "the man with
> two watches", but again, these two watches are different. if the device
> has only GPS altitude, so be it, then the client would have to do the
> correction. however, if the device does have built-in support for
> providing better measurements, why disallow exposing them?
> 
> cheers,
> 
> dret.

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Received on Monday, 5 January 2009 02:53:16 UTC