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.

Received on Monday, 5 January 2009 00:27:19 UTC