| NetCDF file object [message #21669] |
Wed, 06 September 2000 05:33 |
Martin Schultz
Messages: 515
Registered: August 1997
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Senior Member |
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Hi all,
I am pleased to announce a working version of my NetCDF file
object. Currently, it only supports read access, but I am
planning to add methods for adding, changing, or deleting
variables from a file, and for creating a new file with data that
you pass to the object.
Usage is fairly simple, yet quite powerful and relatively fast
as far as I can tell:
thefile = obj_new('mgs_ncdffile') ; optional keyword arguments
data = thefile->GetData(variable=<string or string array>) ;
optional keywords
obj_destroy, thefile
This is the minimum set of commands that you need. If you use
the GetData method without the AsStructure flag, the data will be
returned as mgs_basevariable objects, and you will need to call
the Get method of these objects to actually access the data --
the advantage is that you get all the metadata as well. If you
set the AsStructure keyword, GetData will return a structure with
the variable names as tags and the data as values (similar to my
ncdf_read function). Note that dimension variables that are used
by any of the requested variables are automatically included in
either the object array or the structure. If you don't want them,
set the NoDimensions keyword.
The real power of the file object is the easiness how you can
access subsets ("hyperslabs") of the data: Simply specifiy the
range of the dimensions you want to limit as keywords. If you
supply float (or double) values, these are interpreted as
dimension values, alternatively you can supply integer values
which are then interpreted as firstindex, lastindex, and stride.
The respective subset for one dimension will be applied to all
variables which use this dimension and the dimension variable
itself, so you always get a consistent set of variables.
Examples:
(1)
ozone =
thefile->GetData(var='O3',lon=,lat=,lev=30,time=,/AsStructure)
This will return a structure with (dimensions arbitrary):
o3 : fltarr
lon : fltarr
lat : fltarr
lev : 982.5
time : fltarr[12]
containing ozone over Europe (except the very west) for the 31st
model level and for 12 time points roughly at the center of each
month. Circular dimensions are currently not supported properly,
i.e. you cannot wrap the longitude around.
(2)
co =
thefile-> GetData(var=['CO','EMFLX-CO'],time=0,/AsStructure,/NoDimensi ons)
This will return a structure containing:
co : fltarr[128,64,31]
emflx_co : fltarr[128,64]
I would be happy if some of you could try it out and tell me what
you think. You will need at least the following files from my
website
http://www.mpimet.mpg.de/~schultz.martin/idl/libmgs_objects. html
mgs_ncdffile__define.pro
mgs_basefile__define.pro
mgs_container__define.pro
mgs_basevariable__define.pro
mgs_baseobject__define.pro
and from my tools library (.../libmartin_schultz.html )
chkstru.pro
open_file.pro
Cheers,
Martin
PS: Ben (as my first registered user for the container object), I
changed the Get method of MGS_Container so that it returns named
objects in the order they are requested. You can set the Sroted
keyword to revert to the old behaviour of returning them in the
order they are stored.
PPS: If you are interested in learning about OOP, you can try to
adapt the ncdffile object to read a different file format. This
should be fairly easy if you plan an interface say for HDF or
HDF-EOS, because their file structure philosophy is similar to
netcdf. Note that most of the object power is actually coded in
the basefile object, and the ncdffile object only contains
routines that are specific to the netcdf format (file size of
ncdffile is 17kB, whereas basefile has 50kB). I'll be happy to
help you and to adopt your file formats in my library ;-)
--
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[ [[[[[[[
[[ Dr. Martin Schultz Max-Planck-Institut fuer Meteorologie
[[
[[ Bundesstr. 55, 20146 Hamburg
[[
[[ phone: +49 40 41173-308
[[
[[ fax: +49 40 41173-298
[[
[[ martin.schultz@dkrz.de
[[
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[ [[[[[[[
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