Re: [galeon] Features and Coverages

• To: Jon Blower <jdb@xxxxxxxxxxxxxxxxxxxx>
• Subject: Re: [galeon] Features and Coverages
• From: Peter Baumann <p.baumann@xxxxxxxxxxxxxxxxxxxx>
• Date: Tue, 07 Oct 2008 18:59:05 +0200

Hi Jon,

my issue with anotion of "a feature is everything" is the lack of
discrimination.
Can a spoon be a feature, or is there no spoon? ;-)
I guess you want to talk about objects (to use that nomenclature -
essentially, something that has its own existence) which have some
geographic information associated, essentially: coordinates (point,
line, area, ...).
OK, then "feature" might be an "object that has at least one of the
attributes x, y, z, t, ... with their canonical meaning and methods".
Among these methods might be "give me the center of gravity in WGS84 and
ISO 8601", just to have some example.
Actually, points, lines, and areas might be subtypes of feature. I
remember TIGER being an early GIS with a strictly object-oriented
approach. Unfortunately it didn't succeed in the market.
Next step: Coverages might be a subtype of a feature, one of its
subtypes in turn being "raster". The above method still works. But what
about the other methods? How would you bring in imaging ops into this
inheritance hierarchy? Seems there is no answer out there as for today.

In the end, it is an enjoyable scientific discussion (as Ron mentioned,
it pops up again and again within OGC), but I see not a single system
following this approach.

All technology that I can find knows about vector and raster, and this
is the world we have to model in a standard. Hence, WFS and WCS coexist
on the same level. This notwithstanding that a coverage may be modelled
as a special feature type = WFS has a link into WCS.

This is the world as I understand it ;-)
-Peter



Jon Blower wrote:

Hi all,

I'm following on from previous conversations under the subject line
"WCS CF-netCDF profile document", which was getting rather overloaded,
largely my fault.  It seems to be important to get a handle on what is
a feature and what is a coverage.  We've seen various viewpoints, so I
thought I'd give my view based on a number of conversations that are
slowly crystallizing in my head (thanks go to Andrew Woolf here).

I think a "feature" is just a geographic "thing".  The term is not
meant to be discriminative - it's very closely analogous to an Object
in Java, C++ or any number of other programming languages.  It's
simply a collection of attributes and methods (operations).  A
"Feature Type" is the definition of the feature, analogous to a Class
in OO programming.

A "coverage" is a data structure.  It can be used to express the value
of a particular attribute of a feature.  For example, a Feature that
expresses a timeseries of temperature at a point might have attributes
encoding the geographic position of the measurement, plus all kinds of
other metadata like the name of the station and so forth.  The values
themselves might be encoded as a coverage, which is the "value"
attribute of the feature.  (People who are familiar with CSML will
recognise this model.)

So the relationship is that a Feature "has a" Coverage.  I think.  ;-)

Do others agree or have I got it wrong?

So, if everything is a feature, then perhaps in future WFS will be the
uber-specification that can serve everything?  Well, maybe...  I think
things will get very (i.e. even more) complicated when we start
thinking about how to subset the Coverage(s) that belong to a Feature,
in the very general case.  I can't quite grasp this myself yet.  Do we
embed a WCS-like subsetting query inside a WFS query?

What does this mean for the future of WCS?

Cheers, Jon

• References:
  • [galeon] Features and Coverages
    • From: Jon Blower