Re: [thredds] OpenDAP constraint expression

  • To: "ashwinD12 ." <winash12@xxxxxxxxx>
  • Subject: Re: [thredds] OpenDAP constraint expression
  • From: Ryan May <rmay@xxxxxxxx>
  • Date: Thu, 26 May 2016 12:32:18 -0600
Regarding #2, the URL from thredds to just download the file would be:

http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis2/pressure/air.2014.nc.ascii?air[480:603][2][20:34][26:40]

You can find that in the catalog here:

http://www.esrl.noaa.gov/psd/thredds/catalog/Datasets/ncep.reanalysis2/pressure/catalog.html?dataset=Datasets/ncep.reanalysis2/pressure/air.2014.nc

(The HTTPServer link.)

Ryan


On Thu, May 26, 2016 at 11:10 AM, ashwinD12 . <winash12@xxxxxxxxx> wrote:

> James,
>            What an OpenDap primer from the person who invented OpenDap !
> Thank you very much indeed. I absorbed all of that information.
>
> I had three more follow up questions(maybe related)
> 1) I presume this URL relates to air temperatures -
> http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis2/pressure/air.2014.nc.ascii?air[480:603][2][20:34][26:40]
>
> Why are the values negative ?
>
> 2) I presume I can download this file as a  netcdf file in Python or Java ?
>
> 3) Finally is there a way in the constraint expression that I can add what
> time of day I want(00,06,12,18) ?
>
> Regards,
> Ashwin.
>
> On Thu, May 26, 2016 at 8:41 PM, James Gallagher <jgallagher@xxxxxxxxxxx>
> wrote:
>
>>
>>
>> On May 26, 2016 at 07:06:38, ashwinD12 . (winash12@xxxxxxxxx) wrote:
>>
>> Hello,
>>            I am not sure whether I will get any help for asking such a
>> basic question on OpenDAP constraint expression but if this is not the
>> forum for getting clarifications on OpenDAP constraint expression please
>> let me know the appropriate forum.
>>
>>
>> I I have this URL -
>> http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis2/pressure/air.2014.nc.ascii?air[408:603][2][20:34][26:40]
>> '
>>
>>
>>
>> This is the output of a program that fetches data from a THREDDS server.
>> I am wanting to replicate the functionality by coming up with that URL
>> myself.
>>
>> Can somebody explain to me what the values in parenthesis are ?
>>
>> This is the input I give
>>
>> variable='air',level=850,
>> months.minmax=c(5,5), years.minmax=c(2014,2014),
>> lat.southnorth=c(5, 40), lon.westeast=c(65, 100),
>>
>> I am wanting data for month of May 2014, latitude between 5 N and 40 N
>> and longitude 65 E and 100 E for the pressure level 850 hPa.
>>
>> How does that input get translated to the above URL ?
>>
>> Here’s how the URL breaks down: air.2014.nc .ascii ? air [408:603 ] [2]
>> [20:34] [26:40]
>> <http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis2/pressure/air.2014.nc.ascii?air[408:603][2][20:34][26:40]>
>>
>> The dataset (which is a file in this case) is ‘air.2014.nc’. You’re
>> asking the server to subset that dataset and return just the variable ‘air’
>> and to translate the result into ASCII (text). When the variable ‘air’ is
>> extracted from the data set its a four-dimensional array and you’re asking
>> to have those dimensions ‘sliced’ (or subset) so that, for the first
>> dimension, you see only elements 408 to 603, for the second dim only
>> element 2, for the third elements 20 to 34 and elements 26 to 40 for the
>> fourth.
>>
>> You can look at the dataset to see how your request (level=850, …)
>> translated into those array indices by looking at the datasets’s metadata.
>> To do that, removed the .ascii and replace it with .dds, .das and/or .info
>> and don’t include the query string (the part after the ‘?’). Like this:
>>
>>
>> http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis2/pressure/air.2014.nc.dds
>>
>> Dataset {
>>     Float32 level[level = 17];
>>     Float32 lat[lat = 73];
>>     Float32 lon[lon = 144];
>>     Float64 time[time = 1460];
>>     Grid {
>>      ARRAY:
>>         Int16 air[time = 1460][level = 17][lat = 73][lon = 144];
>>      MAPS:
>>         Float64 time[time = 1460];
>>         Float32 level[level = 17];
>>         Float32 lat[lat = 73];
>>         Float32 lon[lon = 144];
>>     } air;
>> } Datasets/ncep.reanalysis2/pressure/air.2014.nc;
>>
>> One thing you’ll see is that ‘air’ is a Grid - so it’s not really a
>> simple array, but rather a collection of arrays: air, time, level, lat and
>> lon. The ‘air’ array holds the data while the other four hold what could be
>> described as the independent variables (or dimensions).
>>
>> You can see more information about the variables by looking at ‘semantic
>> metadata’ like this:
>>
>>
>> http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis2/pressure/air.2014.nc.das
>>
>> Attributes {
>>     level {
>>         String units "millibar";
>>         Float32 actual_range 1000.0, 10.0;
>>         String long_name "Level";
>>         String positive "down";
>>
>> …
>>
>> Hope this helps,
>> James
>>
>>
>>
>>
>> Regards,
>> Ashwin.
>> _______________________________________________
>> NOTE: All exchanges posted to Unidata maintained email lists are
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>>
>>
>> --
>> James Gallagher
>> jgallagher@xxxxxxxxxxx
>>
>
>
> _______________________________________________
> NOTE: All exchanges posted to Unidata maintained email lists are
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> available through the web.  Users who post to any of the lists we
> maintain are reminded to remove any personal information that they
> do not want to be made public.
>
>
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-- 
Ryan May, Ph.D.
Software Engineer
UCAR/Unidata
Boulder, CO
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