On 1/29/2014 9:26 AM, Jonathan Gregory wrote:
Dear Steve
In practice I do not think that
standard_name=depth and positive=up are necessarily in conflict (see
bold text below). We fairly commonly encounter ocean model outputs
in which the depths are encoded as negatives:
0
-10
-20
...
There are merits to this encoding -- especially apparent in coupled
ocean/atmosphere situations. (Also it preserves the right-handed
coordinate system.) It would be quite peculiar (and a frequent
source of error) to insist that an ocean model output file name its
Z axis as "altitude", simply because it has encoded the depth values
as negatives. The purpose of the 'positive' attribute is to inform
the application when the encoding of values and the physical
interpretation of the Z axis are reversed from one another.
Maybe it's annoying, but I can't agree with that. I would say that such depth
values are wrong. The standard_name depth is defined as "the vertical distance
below the surface". -10 m means 10 m above the surface. There's no point in
having standard names unless we use them correctly, and this is one of the
kinds of imprecision standard names can help eliminate. It's fine for the ocean
model to encode -10 m for 10 m below the surface if it wants to, but then it
should use the standard_name of height instead (or altitude, since distance
above the surface and above the geoid is approx the same thing in ocean areas).
In all the ocean model data I work with (mostly from CMIP), depth is a positive
number, as it should be.
Hi Jonathan,
Yes, respectfully, we do disagree on this.
Isn't the purpose of the standard name is to capture the semantics of a
variable -- i.e. its physical interpretation? You are suggesting that
a user of the file (or a search engine) should attach a different
meaning to the concept of "DEPTH" in an ocean model based upon whether
it is encoded as a positive or negative value. Do we attach a
different interpretation to "TIME" because it is encoded as "days"
versus "hours"?
- Steve
Best wishes
Jonathan
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