[Geowanking] Specifying units of the RDF geo:alt property

[Kjetil Kjernsmo]

Hi all!

I'm new to both the geowanking list and the semantic-web list, but not 
new to either topic, allthough I cannot claim to be experienced either.

The first thing I'd like to discuss is the alt property of the 
http://www.w3.org/2003/01/geo/wgs84_pos# namespace.

Currently, it is undefined what it is, it could be anything: It could be 
the distance from the global ellipsoid focus, or it could be the 
distance above the local ellipsoid, and everything in between. And it 
could be measured in any unit, whether meter or feet or ancient 
sumerian cubits. 

In a recent chat on #swig on irc.freenode.net, Dan Brickley suggested I 
should take the issue to these list, and he would implement any 
consensus that is formed here.

RDF provides data types and also mechanisms for specifying units, but I 
think the geo:alt property is allready in widespread use. It is also 
suggested in the RDF Primer, 
http://www.w3.org/TR/rdf-primer/#properties
that one may in certain cases specify the unit of a property. 

I would personally tend to favour that the geo:alt property is specified 
as "The altitude above the local WGS 84 ellipsoid in meters", but 
clearly, this is problematic if there is allready a lot of data in the 
wild where it is given in e.g. feet. Then, I guess, the only hope is to 
use rdf:value, as seen in the example 
http://www.w3.org/TR/rdf-primer/#rdfvalue
but I can't see that really making the data that's allready out there 
useful, this means that we can't assume anything about it. Or?

I'm a physicist, and I generally dislike seeing quantities with a 
dimension as a number without a unit, but in this case, I think we 
might be better off just defining it in terms of the meter. If anybody 
used feet, they will have to update.

I feel that it is also rather important that the property reflect what 
most people would be likely to enter. I suspect that means "the number 
you read on the display on your GPS". Without having a lot of practical 
experience with GPSes, wouldn't that be meters above the local 
ellipsoid? Thus my suggestion.

Now, what's yours?

Cheers,

Kjetil
-- 
Kjetil Kjernsmo
Programmer / Astrophysicist / Ski-orienteer / Orienteer / Mountaineer
kjetil at kjernsmo.net   
Homepage: http://www.kjetil.kjernsmo.net/     OpenPGP KeyID: 6A6A0BBC