[CF-metadata] platform standard names

Jonathan Gregory j.m.gregory at reading.ac.uk
Sun Sep 11 01:33:14 MDT 2005

Dear all

> all variables regarding the platform are prefaced with 'platform_'

I agree that "platform" is a useful generic name for something moving from
which observations are made.

We are discussing standard names for quantities which are properties of the
platform, such as its velocity. This is different from existing standard
names, which describe observable properties of the world and say nothing about
the observer or the method of observation. The platform_ standard names are
providing information which is useful for interpreting or correcting the
measurements, I presume. In a sense it is metadata rather than data, since
once the appropriate corrections have been made and the data from different
sources assembled into some dataset that could be used as a climatology, for
example, there is no further need for the information about the platform.

> latitude, longitude and altitude are not appropriately prefaced.

I don't think we need platform_ versions of these since they are coordinates.
Practically, if we did introduce platform_latitude, I don't think people would
use it consistently instead of latitude.

> Jonathans earlier confusion where he didn't even realize [I think] that
> true_air_speed was that of the aircraft and not the wind.

Since I wasn't certain, I was suggesting that "ground speed" and "air speed",
although commonly used terms, do not have completely obvious meanings. Hence I
propose standard names of:

platform_speed_wrt_ground ("ground speed") (m s-1)
platform_speed_wrt_air ("air speed") (m s-1)
platform_speed_wrt_sea_water ("water speed") (m s-1)

Is it usual to speak of ship ground speed i.e. relative to the solid Earth,
even though no ground is evident?

I *don't* think we should have separate standard names for indicated and true
air speed if these are the same quantity with and without some correction.
While the speed of the platform is an observable quantity which some other
observer could measure, corrections of various kinds relate to instruments and
methods of measurement, and we could get into huge complexity if we try to
include that kind of metadata in standard names, I fear.  Another attribute
should be used to characterise corrections, such as long_name or comment.

Similarly, we need

platform_pitch_angle (degree)
platform_roll_angle (degree)

What about yaw (the third axis)?

> There is also potential for platform_pitch_rate...

platform_pitch_rate (degree s-1)
platform_roll_rate (degree s-1)

> Is there anything wrong with using platform_heading ... and platform_course?

The problem with "heading" is that it means direction of travel as well as
the direction you're pointing in. That's what I understood it to mean when
Michael proposed it, and why I asked if it was the same thing as course.
Looking on Google, I find that half the definitions are direction of travel.
Hence "heading" is too confusing, I'd say. How about

platform_orientation (degree) for the direction the platform is pointing in
platform_course (degree) for the direction it's travelling in?

Again, I don't think we should have names for "true" heading, quantities with
and without correction for magnetic declination, etc.

Best wishes


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