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20010813: converting radar data to ascii data




Will,

The NEXRAD data on the NOAAPORT broadcast are NEXRAD Level III
products. That is, the data have already been discretized into
data ranges and have had sampling algorithms applied.
For raw data, you would want LEVEL II archived data from NCDC.

The documentation on NEXRAD Level III data is available from the
National weather service interface control documents:
http://www.roc.noaa.gov/ssb/cm/icd_downloads.asp
a synopsis of the radar data header from the RDA->RPG control
document above is: http://www.ofa.noaa.gov/~amd/1-90019-ex5.pdf

A higher level description of the data is in our online email archives:
http://www.unidata.ucar.edu/projects/coohl/mhonarc/MailArchives/datastream/msg01569.html


All the packages (GEMPAK, McIDAS, and WXP) have routines which can display
the data products, but do not readily provide a method for interrogating
the data since the products have already been processed into the radial or 
raster displays at the radar PUP.

Unidata does not provide any access to WXP. You would have to contact
Unisys directly for that package.

GDRADR is a program I developed for GEMPAK and provided in the
releases including and since 5.6.C.1. This program allows the user to
create a gridded data set from the NEXRAD Level III (NIDS) products.
Once the data is in a grid point format, you could use any of the
GEMPAK grid programs to manipulate the data. GDLIST will list out the data
values from a gridded data set in ascii. GEMPAK provides a library of
grid functions (such as the GWFS gaussian weighted sampling function)
which can be used to manipulate the data.

When you say you want "actual numbers", you need to understand that
the NIDS data, as products, are sample groups. That is, all values
in a range are assigned a discrete integer (0 - 15). So the products
are generally 8 or 16 levels of data bins. So, a "6" may represent
precipitation from 1.5" to 2.0" for example. As I mentioned above, if
you want the actual radar value, say 1.643", you would want to use Level II
data.

Steve Chiswell
Unidata User Support
address@hidden







>From: Wil Wollheim <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200108131408.f7DE8s119785

>
>Dear Steve,
>
>We are trying to convert the radar images (NTPxxx) we receive via ldm into
>a format that we can import into a GIS system.  Among the things we are
>trying to do is:
>
>1) clip the radar image from any given radar station to relatively small
>areas (about 100 - 200 km2 areas)
>
>2) calculate average precipitation for those areas, so we would like to
>use the actual numbers for each grid cell (i.e. we would like to
>have the radar data in ascii format).
>
>My question is, how do we proceed to do this?  We've looked through
>the documentation and haven't been able to track down a method.  Did
>we miss something obvious?  Among the questions we have is 
>
>1) what format is the radar data in when we get it via the ldm
>
>2) how do we convert that format into ascii text?
>
> We are members of Unidata and have Gempak on our server.  Will your new
>program, gdradr, be able to do this?  How do we get a hold of gdradr or do
>we need to reload the new version of Gempak?  
> 
>Also, can unidata users use WXP provided by UNISYS? In particular, they
>have rad and radcvt programs that we thought we could use, but these
>require a licence.  Just wondering if unidata covers that as well.
>
>Thanks for your help!
>
>Wil Wollheim
>
>
>
>Water Systems Analysis Group
>Complex Systems Research Center
>Morse Hall
>University of New Hampshire
>Durham NH 03824
>Voice: (603) 862-0812
>FAX: (603) 862-0587
>E-mail: address@hidden
>
>
>
>