# [CF-metadata] point observation data in CF 1.4

Jonathan Gregory j.m.gregory at reading.ac.uk
Wed Oct 13 05:56:27 MDT 2010

```Dear Rich

Thanks for this explanation:

> Typical producers of this kind of data are numerical particle tracking
> models.  These codes step through time, following the (x,y,t) or
> (x,y,z,t) trajectories of individual particles.  At each time step,
> more particles may be introduced to be tracked, while other particles
> stop being tracked because they leave the domain, hit the boundary, or
> whatever.

This kind of data could be described by the trajectory feature type, but each
trajectory would be entirely independent, so they'd all have separate times,
whereas as you describe it the time coord is common to all trajectories (that
exist at a particular time). To arrange this, an indirection could be used on
the time dimension:
data(i,o)	x(i,o) y(i,o) z(i,o) t(tindex(i,o))
where i is the instance (which of the trajectories), o is the point along that
trajectory, t is the coordinate vector of common times, and tindex is an index
to t. For example, we might have these two trajectories (x,t) (omitting y and
z for simplicity)
(0,10) (1,11) (2,12)
(3,11) (2,12) (1,13) (0,14)
Then t would be [10,11,12,13,14] (all the times). For the first trajectory
x=[0,1,2] tindex=[0,1,2]
and for the second
x=[3,2,1,0] tindex=[1,2,3,4]
Is that right? Perhaps/probably there's a neater or more natural way to do it.

Cheers

Jonathan

```

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