Dear Steve,
Thanks for your feedback. I will have a look at using your approach,
hopefully it should solve the issue.
Kind regards,
Ag
> -----Original Message-----
> From: Steve Hankin [mailto:Steven.C.Hankin@xxxxxxxx]
> Sent: 05 June 2003 16:05
> To: Stephens, A (Ag)
> Cc: netcdfgroup@xxxxxxxxxxxxxxxx
> Subject: Re: Multiple altitude axes in one NetCDF file
>
>
> Hi Ag,
>
> Do the two vertical modes of measurement each have the same
> number of points on
> the axes? Or if they do not, then would it be acceptable to
> utilize a single
> vertical dimension of size equal to the larger of the two and
> to allow missing
> value flags for the unused space on the shorter vertical axis?
>
> If the answer to either of these is yes, then the problem
> reduces to the "sigma
> coordinate problem". To encode sigma coordinate data in
> netCDF one creates an
> additional *dependent* variable (not a standard netCDF
> coordinate variable)
> whose vertical dimension is the length above and whose values are the
> (time-varying) heights (or depths). I believe that you'll
> find this is
> documented in the CF conventions -- together with the
> conventions that link the
> measured variables to these time-varying (or space-varying) Z
> coordinates.
>
> Hope this helps.
>
> - steve
>
> P.S. There is a specific function in the Ferret program
> called ZAXIS_REPLACE to
> allow you to shift between sigma coordinates and fixed
> coordinates when working
> with the data. I'm sure there are equivalent techniques in
> IDL and Matlab and I
> presume GrADS, too.
>
> ===================================================
>
> "Stephens, A (Ag)" wrote:
>
> > Dear all,
> >
> > I have been trying to convert a colleague to using NetCDF
> and conforming to
> > the Climate and Forecasts Metadata Convention. However, she
> has some sodar
> > data that has an interesting quirk to it.
> >
> > The instrument sits on the ground and looks vertically up,
> measuring the
> > profile of wind speed and direction. It therefore has two coordinate
> > variables: time and altitude.
> >
> > There are two modes in which the instrument can measure
> which vary the
> > altitude coordinates so during the course of a day the
> sodar may be run in
> > each mode for different periods. This produces output with
> two altitude
> > dimensions.
> >
> > The easiest option for NetCDF would be to output to a new
> file each time the
> > mode changes. However, it is useful to be able to plot the
> entire day onto
> > one graph. Does NetCDF have a suitable solution to this problem?
> >
> > My hunch would be to create two altitude dimensions, say
> 'altitude1' and
> > 'altitude2' and two lots of wind parameters that vary with
> each altitude.
> > Alternatively, you can define the altitudes for each time
> step throughout
> > the measurement period (but this is inefficient).
> >
> > I would hope there was a more elegant solution which would
> allow plotting
> > packages to view the entire dataset as continuous rather
> than separate
> > variables defined on separate dimensions.
> >
> > Any ideas?
> >
> > Thanks,
> >
> > Ag
> > -------------------------------------------------------------
> > Ag Stephens Ph : +44 (0)1235 446220
> > Data Scientist, Fax: +44 (0)1235 446314
> > British Atmospheric Data Centre,
> > Rutherford Appleton Laboratory,
> > Chilton, Didcot, OX11 0QX, U.K.
> > Email: A.Stephens@xxxxxxxx Web: http://badc.nerc.ac.uk
> > -------------------------------------------------------------
>
> --
>
> Steve Hankin, NOAA/PMEL -- Steven.C.Hankin@xxxxxxxx
> 7600 Sand Point Way NE, Seattle, WA 98115-0070
> ph. (206) 526-6080, FAX (206) 526-6744
>
>
