Hi list.
I'm continuing my work on "GRIB1 ImageIO plugin" / "ImageIO metadata for
multidimensional coverages" started with GSOC 2007 and now I need a little
help :)
Prior to expose my question, I would to introduce some considerations:
- We are supposing to define a single coverage for each "variable" of a
dataset. As an instance, a datasource storing Temperature/Wind
velocity/Pressure variables contains 3 coverages (1 for Temperature, 1 for
Wind velocity and 1 for Pressure).
- For each coverage, we may provide specific metadata (mainly based on OGC
GML and GML on JPEG2000 for Geographic Imagery specification) which
basically allow to know:
--- name and description of the coverage
--- the 4D spatio-temporal domain expressed in terms of 2D Envelope +
Vertical Extent + Temporal Extent
--- the sample dimension and its bands (as an instance, Wind Velocity has
2 bands: U-component and V-component).
In such a context, Vertical Extent may provide a list of values, or a
Vertical Range defined by a minimun, a maximum and, optionally, a resolution
value. Furthermore we need to specify the Vertical CRS.
Now, let's talk about GRIB1 datasets.
GRIB1 data are stored as different records each one containing data and
additional information contained within a proper GRIB1 section.
Basically, the Product Definition Section (PDS) of a GRIB1 record allows to
retrieve information about the represented variable (as an instance,
Temperature), the geographical area covered by the data, and information
about the date/time and vertical level/layer for which such a data has been
collected. This information may be used to properly set metadata describing
the 4D spatio-temporal domain of the coverage (by collecting and properly
exposing all time/vertical levels information available for several GRIB1
records referring to the same Variable).
The GRIB1 related documentation provides information about the available
types and values of vertical levels. (
http://www.nco.ncep.noaa.gov/pmb/docs/on388/table3.html )
Table 3 provides a set of definitions which allow to describe the vertical
level in terms of, as an instance, height in meters above the ground, depth
in meters below sea level, isobaric level (pressure in Pascals),...
Basically, it allows to define a wide set of numeric values for a vertical
level.
Table 3a provides instead a set of special levels which allow to define
vertical level in terms of "symbolic entities" like, as an instance, Ground
or water surface, Sea Bottom and other special elements without numeric
definition.
Now my question is:
How can I handle this special vertical level in metadata? (Since, I need to
define a Vertical CRS... which vertical datum?)
I'm excluding to threat them as bands since:
- Some Variables may have different real bands (as an instance, Wind
Velocity, having U-component and V-component)
- Some multi-bands variables may have data for different "special" vertical
levels (as an instance, Wind velocity at Cloud base level and Wind velocity
at Cloud top level).
For this reason, I think mixing "concepts" (bands/vertical levels) is not a
good solution.
Did someone already encountered a similar "problem"?
There are some tips on this topic?
Thank you very much.
--
Best Regards,
Daniele Romagnoli
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