Re: [thredds] [REQUEST] VertCoord on multiple variables

• To: "Lacoste Pierre (DAVIDSON)" <PLacoste@xxxxxx>
• Subject: Re: [thredds] [REQUEST] VertCoord on multiple variables
• From: Sean Arms <sarms@xxxxxxxx>
• Date: Fri, 26 May 2017 09:56:38 -0600

Greetings Pierre,

If you do not specify vertCoord in the request, then the data on all
vertical levels are returned and the TDS does not need to look at the
vertical coordinate at all. As you've found, the vertCoord subsetting
only works when the variables requests have the same vertical
coordinate variable. This is a limitation of the TDS.

Ideally the server should treat vertCoord like lat/lon or time, in
that the "closest" match to the coordinate value requested in the
subset should be returned. It's a bit tricky, though, as different
vertical coordinates can have different units, and so choosing the
closest "value" alone (which is what is supplied to vertCoord) would
not work. The TDS handles lat/lon and time a bit differently because
it knows about the units of lat/lon and time supplied by the request,
as those are determined by the web API (fore example, times are
spcified as W3C dates, lat/lon as degrees north, degree east):

http://www.unidata.ucar.edu/software/thredds/current/tds/reference/NetcdfSubsetServiceReference.html

There is no implied unit in vertCoord, so the assumption is made that
all variables have the same vertical coordinate variable when
vertCoord is used.

Perhaps a useful enhancement would be to allow a user to supply
vertCoord and vertCoordUnit independent of what vertical coordinate
variables are in the file, but then things again get tricky. Say a
user asks for a subset for the 600 hPa level, but the data are stored
on geopotential height levels. Should we assume an ideal atmospheric
profile to compute an equivalent height? While ideal profile? What
about a request of, say 36,000 ft, but the vertical coordinates are on
sigma levels? I suspect these kinds of questions are why vertCoord is
as limited as it is at the moment.

As an end user, how would you like to see vertCoord become more
useful? I think enhancements are possible, but I am not sure what the
appropriate assumptions are to make things work.

Sean


On Tue, May 16, 2017 at 9:13 AM, Lacoste Pierre (DAVIDSON)
<PLacoste@xxxxxx> wrote:
> Hello,
>
>
>
> I’m a user of Thredds and I try to execute a request with the parameter
> “VertCoord”.
>
> When I did it, I got the following error:
>
> “The variables requested: […] have different vertical levels. Grid requests
> with vertCoord must have variables with same vertical levels.”
>
> The variable list contains variable with different dimensions
>
> When I execute the request without “VertCoord”, I didn’t get the error.
>
> Why thredds does the difference between a request which contains variables
> with different dimensions without VertCoord specified and a request which
> contains variables with different dimension with a VertCoord specified
>
>
>
> Thanks,
>
>
>
> Pierre
>
>
>
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• References:
  • [thredds] [REQUEST] VertCoord on multiple variables
    • From: Lacoste Pierre (DAVIDSON)