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.
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> --
> James Gallagher
> jgallagher@xxxxxxxxxxx
>
