So I think what is confusing for me is the conflation of directionality with a non-geospatial parameter. While I understand that pressure is a very close proxy for depth in this context, it isn't really a geospatial variable. (This is a narrow perspective, obviously; in casual use, a lot of people will make the leap just fine.)
So referring to pressure as being 'positive down' seems like referring to light transmission as 'positive up' under the water -- yes, there is a relationship, it is mostly but not entirely consistent depending on the particular deployment, but it is a relationship not an equivalence. Lacking an appreciation of the interesting situation where the values have been made negative for some reason, some of us would see that and say "what does that mean in this context?"
It makes more sense to me to say 'positive increasing', as in, 'more positive values represent increasing pressure'. (Although in an example where values are simply negated, the negation would make the standard variable not pressure at all, but negative pressure. You wouldn't want people to take your 'pressure' values that are expressed in negative numbers as actual pressure, and I wouldn't expect people or computers to look for a 'positive' attribute to see whether an inversion might have been applied. Unless this practice is way more common than I'm aware.)
John
On Jul 20, 2011, at 03:13, Comiskey, Glenn wrote:
> 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
John Graybeal <mailto:jgraybeal at ucsd.edu>
phone: 858-534-2162
Product Manager
Ocean Observatories Initiative Cyberinfrastructure Project:
http://ci.oceanobservatories.org
Marine Metadata Interoperability Project:
http://marinemetadata.org
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