[IDD #AEG-690130]: Decoding the LDM Level-II format

> Hello,

Hello Satpreet, 

> I am a developer at the University of Iowa IIHR and am writing to you
> for support on decoding the Level-II (super-res) NEXRAD data we receive
> through LDM

Great, pleased to meet you and apologies for the tardy response, I had some 
medical issues that have kept me away from my machine. We usually respond 
within an hour or at least the same day.

> 
> We have an application here which uses NEXRAD Level-II (super-res) data
> from the NCDC Archive (obtained using the standard HAS request
> procedure from this url: http://www.ncdc.noaa.gov/nexradinv/ and
> uncompressing the .gz files). I am trying to now use NEXRAD Level-II
> (super-res) directly obtained from LDM and it has been mentioned here:
> 
> http://www.roc.noaa.gov/NWS_Level_2/Web_FAQsRev3.pdf

Yes, this document is almost 4 years old

> that the LDM files are packaged in a specific format using BZIP
> compressed chunks:

True, and broken into 100 radial quadrants with header info (24 bit) only 
attached to the first quadrant.

> 
> * "How Are The Level II Data Prepared For Distribution?
> 
> Response: The Level II data sent to users differs from that used by the RPG
> in the following ways:
> 
> (1) The data are compressed using BZIP2 software,\

Yes

> (2) A header record containing metadata is added at the start of
> each volume scan, and

Yes, but not each quadrant...only the first.

> (3) Unidata Local Data Manager software is used to route the data"

Yes
 
> * However, it seems like there are "decoders" bundled with LDM for
> unpacking/reading specific data-types.  E.g. mentioned here:
> 
> http://weather.jsums.edu/~coamps/LDM_html/chapter2.html<http://weather.jsums.edu/%7Ecoamps/LDM_html/chapter2.html>


This refers to COAMPS data, which is model output.


Please see:

http://www.unidata.ucar.edu/software/ldm/

for information regarding the LDM and the use of decoders in the LDM 
environment.

> 
> I would like to know you could direct me to any "LDM NEXRAD Level-II file"
> decoder that I could use for this purpose.


Well, we have a JAVA decoder that we use and have shared with NCDC and is used 
in our 3D visualization package the IDV:

http://www.unidata.ucar.edu/software/idv/

GEMPAK and McIDAS do not decode the data for visualization, they read the BZIP2 
file natively.


> 
> * Alternatively, *I would like to write my own decoder/converter, but wish to
> get more details on the format. Is there any sample code available which
> does this? To clarify, I wish to write a "ldm2ncdc" program that will take
> in a LDM-packaged NEXRAD Level-II (super-res) file and give me a NEXRAD
> Level-II (super-res) file in the same format as available through the NCDC
> request.
 

I am not sure, but suspect our JAVA IDV level II reader would do that, or we 
could "customize" it to do so. We work w/Steve Ansari at NCDC so I am confident 
we can work this out.


> I did find some information on the Unidata mailing lists but believe this is
> too old (much before the introduction of super-res) to still hold:
> http://www.unidata.ucar.edu/support/help/MailArchives/idd/msg03649.html
> 

Yes, there is more current information, a good start is at:

http://www.roc.noaa.gov/NWS_Level_2/

and of course we can help you wade through this, we have much experience in 
dealing with this data.

> Thanks,
> Satpreet


Our pleasure, and again apoogies for the delay, but discs in my neck prevented 
me from typing for quite some time, just now starting to get feeling back to my 
hand.

Perhaps it would be wise for us to hold a conference call tomorrow or weds if 
you and Anton are available?

Cheers,

Jeff

 

Jeff Weber
Unidata User Support
http://www.unidata.ucar.edu

Ticket Details
===================
Ticket ID: AEG-690130
Department: Support IDD
Priority: Urgent
Status: Open