John Caron wrote:
Don Murray wrote:
John-
John Caron wrote:
Well, maybe you're right. I notice that CF currently doesnt require
vertical coords. But as a visualizing client, how do you display?
Wouldnt you prefer to insist on something?
No. We import lots of data point data stored in text files that
only has lat/lon coords. In the IDV, we plot the data at the
default Z location defined by the user if we don't have a Z dimension.
We don't always want to assume 0 for the Z value.
Just to be clear: this is not about making assumptions about where the
z value is (sorry i said that earlier), in fact its the opposite:
requiring the data provider to explicitly specify the z position of
point data, in order to be CF compliant.
It seems like the possible options are:
1. z coordinate required, with values convertible to height (eg meters),
with a vertical datum (reference surface like "mean sea level") specified.
2. z coordinate required, with values convertible to height (eg meters).
3. z coordinate required, with values in any vertical coordinate system.
4. z coordinate required, may be a "nominal" value (just a string description)
5. No z coordinate required.
I would lean towards 4, but could be convinced of 5 or 3. We should however
recommend the data provider add as much info as possible, and make sure there
is a standard way to do so.
Could I ask data providers to chime in : how much z info are you willing and
able to put in your files?
It seems like there are 2 use cases people have in mind:
1. A data provider is using this Convention to write out data. Presumably the
provider is working hard to put quality metadata into their files. I'd really
like to encourage (require?) them to put in time, z, x, and y geolocation
information. Is there really a case where that information is not available
at the raw data writing?
2. Middleware software is using this Convention as an exchange format, ie
rewriting original data into this format. Here, its quite possible that
the z coordinate in particular is missing.
I like John Greybeal's suggestion to require something, and allow some standard
"dont know" string to allow the possibility that its unknown. I realize this is
a matter of style, since the effect is the same (unknown Z). Still, I have been
watching many people use the CF-compliance checking tools to make sure they have
done the right thing. If those tools spit out "missing Z coordinate" message,
the writer may be motivated to try to put in some useful info there. We have seen
situations where providers, having been told they must use CF, just add the
global attribute :Conventions = "CF-1.0". This may pass the compliance tester,
but its not useful CF.
Currently CF-1.0 (section 1.3) says "Four types of coordinates receive special
treatment by these conventions: latitude, longitude, vertical, and time. Every
variable must have associated metadata that allows identification of each such
coordinate that is relevant." Arguably, latitude, longitude, vertical, and time
are always relevent for this kind of data, and so must exist. In practice we
dont require this, our software teases out whether there enough info to
georeference a variable. But I think we could (if we wanted) insist on this point
more.
