Thanks for the reply John.
The need to add an attribute stems from a requirement passed to me to
add an attribute to denote what the meaning of positive or negative
values represent. This is due to the fact that in addition to the
pressure gauge reading, which I concede would report a greater
pressure/depth as an increasing value, it also contains data variables
are computed values and therefore may be represented as a negative
value, i.e. "positive" would be "up" rather than "down".
While in certain circumstances, as you highlight regarding pressure,
increasing values infer a "positive" attribute value, I don't see what
questions are likely to be raised by explicitly stating a "positive"
attribute value providing it correctly stated. What kind of questions
were you thinking would arise from the use of the "positive" attribute?
Regards,
Glenn
________________________________
From: John Graybeal [mailto:jbgraybeal at mindspring.com]
Sent: 20 July 2011 05:52
To: Comiskey, Glenn
Cc: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] COARDS - "positive" attribute
Do you really need an attribute here? Can you construct a scenario in
which increasing values consistently represent a _lesser_ pressure/depth
of water?
Since the variable really is representing pressure, not depth of water,
I think it's safe to say that increasing values of course represent
increasing pressure. If that's fair, then adding the positive attribute
raises questions rather than resolving them.
John
On Jul 18, 2011, at 02:42, Comiskey, Glenn wrote:
Hi,
The COARDS covention mandates the use of the "positive"
attribute for a vertical axis that is not a unit of pressure, i.e. if
using metres to denote altitude. It further qualifies use of the
"positive" attribute as a means of discovering vertical coordinate
variables, i.e. the convention states "A vertical coordinate variable
will be identifiable by: units of pressure; or the presence of the
positive attribute..." Does this, therefore, restrict use of the
"positive" attribute to vertical coordinate variables only? NOTE: CF
convention (v1.5, Appendix A: Attributes, page 50) reports use of the
"positive" attribute as relating to coordinate data only.
I ask as I have a number of datasets in which the vertical
coordinate variable is present, named "altitude" albeit it represents
sea level with a value of 0 (zero), unit value of metres and has a
declared "positive" attribute of "up", but also contains tidal data
reporting the height of the water column above a pressure gauge and an
attribute is wanted to be added to denote that increasing values
represent a greater depth of water, i.e. "positive" is down. Would use
of the "positive" attribute for data variables interfere with the
discovery of vertical coordinates as defined by COARDS?
Kind regards,
Glenn Comiskey
Data System Administrator
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