Hello, (01)
GPS coordinates aren't simple lat-long, although some systems simply
calculate these from raw GPS data and discard the rest. The transmissions
from the satellites can also be used to derive a temporal measure (the UTC
time) and an altitude measure (from centre of the reference geoid). It may
be that some systems only care about the point on the surface of the earth,
and ignore the other aspects. (02)
Depending on the intended use, a system might use just the derived lat-long,
in which case it represents a line extending from the centre of the Geoid
(or reference ellipsoid) rather than a point on or above it. (03)
Even when the system has an interest in the altitude, it's still not simple.
In some exotic cases they may use specialist reference ellipsoids to
estimate the distance above the surface of the earth. If two systems use
different ellipsoids, you've got problems. (04)
There is another interpretation I've seen, which is a consequence of the
accuracy of GPS. Sometimes, the coordinates provided are intended to refer
to an area centred on the point on or above the surface of the ellipsoid
whose radius is given by the tolerance of the GPS system. That gets even
more complex when landscape is taken into account. Not only does a mountain
alter the shape of the tolerance area, but it may also block some satellite
signals and so reduce accuracy of the GPS reading, which can make the area
larger. Chances are that if the mountain is small and pointy, it won't be
very well represented by the reference ellipsoid, which is another added
complication. (05)
A third case is the combination of the above where altitude and accuracy
come together, and the GPS coordinates actually represents a cone (or more
complex shape if mountains, buildings, etc. interfere) where the pointy end
is at the centre of the Geoid. (06)
Chris Partridge has done a lot of work in this area, also looking at other
coordinate systems than GPS - e.g. UK and US grid references. (07)
So, Sean and David's comments about what the coordinates refer to is
absolutely. When dealing with coordinates, if you don't understand the frame
of reference being used, the "real-world" semantics of the coordinates
cannot be properly ascertained. None of this matters in a lab, but it can be
a bit of a problem when you're using the coordinates from different systems
(with different intended references) to de-conflict commercial airspace or
drop a 1000lb bomb. (08)
Ian (09)
-----Original Message-----
From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx
[mailto:ontolog-forum-bounces@xxxxxxxxxxxxxxxx] On Behalf Of David Leal
Sent: 25 October 2010 22:24
To: [ontolog-forum]
Subject: Re: [ontolog-forum] Fw: GPS coordinates in an ontology? (010)
Dear Sean, (011)
I agree with you when you say:
> It may be worth considering separating out the concept of position from
the
>representation of position, to allow for position to be represented using
>multiple co-ordinate systems or geographic regions. (012)
We must remember that WGS84 does two things - it defines a geometric
space (within which the earth is more or less at rest), and it
defines a coordinate system for identifying points within it. (013)
TAI (International Atomic Time) is analogous - it defines a temporal
space (defined by a set of clocks at particular positions within the
earths gravitational field), and it identifies instants within it. (014)
Best regards,
David (015)
At 20:22 25/10/2010, you wrote: (016)
> The problem is not just to record co-ordinates, but to do something with
>them. This means identifying services that can evaluate poistion functions,
>such as computing proximity, comparing regions, converting between
>co-ordinates and place names, or following through the hierarchy of
>administrative districts.
>
> Also, given that there are a large number of co-ordinate systems actually
>in use (often based on different shapes for the Earth), there
> will also be a requirement for services to convert between different
>co-ordinate systems.
>
> It may be worth considering separating out the concept of position from
the
>representation of position, to allow for position to be represented using
>multiple co-ordinate systems or geographic regions.
>
>
> Sean Barker
>Bristol, UK
>
>
> > -----Original Message-----
> > From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx
> > [mailto:ontolog-forum-bounces@xxxxxxxxxxxxxxxx] On Behalf Of John F.
> > Sowa
> > Sent: 25 October 2010 14:20
> > To: ontolog-forum@xxxxxxxxxxxxxxxx
> > Subject: Re: [ontolog-forum] GPS coordinates in an ontology?
> >
> >
> > *** WARNING ***
> >
> > This message has originated outside your organisation,
> > either from an external partner or the Global Internet.
> > Keep this in mind if you answer this message.
> >
> >
> > On 10/25/2010 8:48 AM, Joshua Lieberman wrote:
> >> Depending on the required precision, GPS coordinates may or may not be
> >
> >> something to enshrine in a static ontology, and a GPS point may or may
> >
> >> not be the appropriate geometric representation.
> >
> > I strongly agree.
> >
> > Every application is likely to need other domain-dependent
> > representations, such as a street address. And it is also likely to
> > need various transformations, such as deriving speed down those roads
> > and streets.
> >
> > All the units of measure are interrelated, and the ontology should
> > include all of them.
> >
> > John
> >
>
>
>
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