Ashwin,
Regarding your first question (negative values), the data are packed,
ala NetCDF convention. See:
http://www.esrl.noaa.gov/psd/data/gridded/faq.html#2
http://www.esrl.noaa.gov/psd/data/gridded/faq.html#12
-Hoop
On 05/26/16 11:10, thredds-request@xxxxxxxxxxxxxxxx wrote:
Date: Thu, 26 May 2016 22:40:23 +0530
From: "ashwinD12 ." <winash12@xxxxxxxxx>
To: James Gallagher <jgallagher@xxxxxxxxxxx>
Cc: "support@xxxxxxxxxxx support" <support@xxxxxxxxxxx>,
thredds@xxxxxxxxxxxxxxxx
Subject: Re: [thredds] OpenDAP constraint expression
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
recorded in the Unidata inquiry tracking system and made publicly
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.
thredds mailing list
thredds@xxxxxxxxxxxxxxxx
For list information or to unsubscribe, visit:
http://www.unidata.ucar.edu/mailing_lists/
--
James Gallagher
jgallagher@xxxxxxxxxxx
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://mailman.unidata.ucar.edu/mailing_lists/archives/thredds/attachments/20160526/09a21d53/attachment.html>
End of thredds Digest, Vol 88, Issue 7
**************************************
