Re: [galeon] Features and Coverages

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What about the property of a road which describes the pavement roughness
as function of arc length?

R

-----Original Message-----
From: galeon-bounces@xxxxxxxxxxxxxxxx
[mailto:galeon-bounces@xxxxxxxxxxxxxxxx] On Behalf Of Wenli Yang
Sent: October 7, 2008 2:50 PM
To: Unidata GALEON
Subject: Re: [galeon] Features and Coverages

While there are many different understandings/view points/etc of feature
and coverage, I would like to point out that the main reason for
"coverage" service is to provide "GRID coverage" whose data values are
"regularly" aligned with some axes/dimensions so that one can specify
low/up limits and an interval along each axis/dimension and get an array
of (many) coverage data points.  By "regularly" I mean that the coverage
data points have the same interval along each axis/dimension.  If an
axis is a well defined coordinate axis, such as geographic longitude,
then the data points are said to be rectified along this axis.  If an
axis is something like a satellite track, the data points may still be
considered rectified along this axis, but since a satellite track
usually does not align with any geographic/projected coordinate axis,
the data points are thus usually considered un-rectified (with regard to
well defined earth coordinate reference systems).  Whether rectified or
not,  a grid coverage data is a n-D data array with all data points
having the same data type.  In case a coverage contains multiple "range
sets" such as temperature, humidity, and land surface type, a coverage
will have multiple arrays each having the same data type.  This well
maps to the variable array(s) in a netCDF file.

This spatial (or geometry) property of coverage is fundamentally
different from most feature types.  I consider a road feature a typical
feature type.  To describe a road feature, we usually use discrete pairs
of coordinates (x,y) to indicate its spatial location and use attribute
values such as pavement type, route number, and road width.  The
attributes may or may not have the same data type and a feature usually
has relatively fewer values for each attribute as compared to data point
values in a grid coverage's range set.  The data structure of a feature
is thus usually different from a grid coverage.
Due to such major differences, it is inevitable that WCS interface will
be different from WFS interface.

There are, of course, cases where feature and grid coverage are more
similar, such as John Blower's point time series profile case.  When
considered as a grid coverage, it has only 1-dimension and the intervals
of data points along this dimension may often be irregular (e.g., 50mb,
100mb, 200mb).  When considered as a feature, the data structure can be
equally efficient.  However, in general n-D (n>1) cases, I there are
significant differences among WCS (for GRID or rater type) and WFS (for
vector data type).

-- Wenli




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