[cf-pointobsconvention] Should Z Coordinates be required?



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.