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[THREDDS #MHJ-779129]: Fw: [support] NCEP/NCAR

This archive contains answers to questions sent to Unidata support through mid-2025. Note that the archive is no longer being updated. We provide the archive for reference; many of the answers presented here remain technically correct, even if somewhat outdated. For the most up-to-date information on the use of NSF Unidata software and data services, please consult the Software Documentation first.

  • Subject: [THREDDS #MHJ-779129]: Fw: [support] NCEP/NCAR
  • Date: Thu, 28 Jan 2016 15:15:18 -0700
Hi Muhammad,

A critical thing to understand about the OPeNDAP Dataset Access Form is that it 
gives you direct access to the underlying data arrays of each variable. Each 
array has a set of dimensions which determines its shape. For example the 
'level' variable has one dimension--also named 'level'--with length 17. You can 
find that info at the bottom of the page:
    Float32 level[level = 17];
'air', on the other hand, has four dimensions of varying length. Its 
declaration is:
    Float32 air[time = 816][level = 17][lat = 73][lon = 144];

To request a subset of a variable's data, you need to specify a range of 
indices for each of its dimensions. Let's walk through an example of subsetting 
'lon' where the study area has a longitude range of 60-78 degrees_east.

1. Click on the check box next to 'lon'. The text field below will be filled 
with '0:1:143'. That string is a section specification--our notation for 
specifying a subset of an array. We've borrowed it from Fortran 90 [1]. The 
default section spec selects all values in the entire array.
2. Click the 'Get ASCII' button at the top of the page. We get all 144 
longitude values back. However, we only want latitudes between 60 and 78. We 
need to figure out the indices that correspond to that range.
3. Indexing in OPeNDAP begins at zero. Looking at all of the longitudes that 
were returned (or doing a little math based on the 2.5 delta between values) we 
can determine that longitude value '60' is at index 24 and value '67.5' is at 
index 27.
4. Go back to the form on the previous page. This time, change the section spec 
from '0:1:143' to '24:1:27'. Click 'Get ASCII'. Now only longitudes in our 
study are are returned!

You should be able to follow that procedure to find the proper ranges of 
indices for the 'level', 'time', and 'lat' dimensions as well. When you have 
them, enter them in the appropriate text fields below 'air' and you'll have 
your subsetted data.

Cheers,
Christian

[1] 
http://h21007.www2.hp.com/portal/download/files/unprot/fortran/docs/lrm/lrm0055.htm#subs_trips

Ticket Details
===================
Ticket ID: MHJ-779129
Department: Support THREDDS
Priority: High
Status: Closed